CdS Quantum Dots in an

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Nondestructive Encapsulation of CdSe/CdS Quantum Dots in an Inorganic Matrix by Pulsed Laser Deposition Antoine Aubret,*,† Julien Houel,† Antonio Pereira,† Justine Baronnier,† Emmanuel Lhuillier,‡ Benoit Dubertret,‡ Christophe Dujardin,† Florian Kulzer,† and Anne Pillonnet*,† †

Institut Lumière Matière, CNRS UMR5306, Université de Lyon, Université Lyon 1, 69622 Villeurbanne Cedex, France LPEM, ESPCI-ParisTech, PSL Research University, CNRS, UPMC Paris VI, Sorbonnes Universités, 10 rue Vauquelin, 75005 Paris, France

‡

S Supporting Information *

ABSTRACT: We report the successful encapsulation of colloidal quantum dots in an inorganic matrix by pulsed laser deposition. Our technique is nondestructive and thus permits the incorporation of CdSe/CdS core/shell colloidal quantum dots in an amorphous yttrium oxide matrix (Y2O3) under full preservation of the advantageous optical properties of the nanocrystals. We find that controlling the kinetic energy of the matrix precursors by means of the oxygen pressure in the deposition chamber facilitates the survival of the encapsulated species, whose well-conserved optical properties such as emission intensity, luminescence spectrum, fluorescence lifetime, and efficiency as single-photon emitters we document in detail. Our method can be extended to different types of nanoemitters (e.g., nanorods, dots-in-rods, nanoplatelets) as well as to other matrices (oxides, semiconductors, metals), opening up new vistas for the realization of fully inorganic multilayered active devices based on colloidal nano-objects. KEYWORDS: CdSe/CdS quantum dots, encapsulation, inorganic matrix, pulsed laser deposition (PLD), single-photon emitters quantum efficiency of less than 0.1%;13 this poor performance is partially attributed to degradation of the QDs caused by the encapsulation methods.11 Developing a versatile, nondestructive encapsulation technique is therefore crucial to eliminate one of the current bottlenecks for a wider adoption of QDbased photonic devices. As a consequence, numerous approaches have been tested to achieve this goal: molecular beam epitaxy (MBE), 14,15 migration enhanced epitaxy (MEE),16 atomic layer deposition (ALD),17,18 sol−gel processing and spin coating of hybrid organic/inorganic solutions,19−21 successive ionic layer adsorption and reaction (SILAR),22,23 metal−organic chemical vapor deposition (MOCVD),24,25 and magnetron radio-frequency (RF) sputtering.13 While each of these techniques has been successful to a certain degree in the incorporation of colloidal QDs into inorganic matrices, none of them currently offer the combination of the following characteristics: full preservation of the optical properties of the QDs, reproducible and precise control of film thickness, material versatility, and time-efficient deposition. Furthermore, out of all the above-mentioned studies, only the one of Mashford et al.21 has explored the optical properties of encapsulated nanoparticles at the singleemitter level, which means that many important issues for developing single-photon devices still remain to be clarified. In this paper we present the first successful incorporation of colloidal QDs in yttrium oxide (Y2O3) grown by pulsed laser

1. INTRODUCTION Colloidal quantum dots (QDs) are a class of nanoparticles with many intriguing properties with respect to fundamental nanoscience as well as for potential applications:1 ease of synthesis in liquid or gas phase, high absorption cross sections and luminescence quantum yields emission wavelengths tunable from the visible to the mid-infrared, narrow emission line widths, nanosecond recombination lifetimes, and roomtemperature single-photon emission. Thanks to these features, QDs are now widely used in nanotechnological devices for the emission, detection, or conversion of light, such as photodetectors,2,3 all-optical spectrometers,4 solar cells,5,6 nanosensors,7 or single electron transistors.8 Light-emitting diodes (LEDs) in particular were among the first such applications,9 and many types of QD-LEDs have since been realized, based on various polymer, inorganic, or hybrid organic−inorganic matrices.10,11 A major technological challenge for QD-LEDs and other QD-based electro-optical devices lies in preserving the optical properties of these nanoparticles when integrating them into inorganic matrices with precisely defined band gaps and accurately controlled thicknesses.5,6 The principal advantage of inorganic materials is their stability in air compared to organic thin films. Metal oxides in particular are promising candidates due to their diversity, permitting their use as charge transport layers with fine-tuned bands alignment,12 as well as for forming capacitive multilayered structures.13 Furthermore, the usually high refractive index of metal oxides facilitates the design of waveguide-based devices. Previous attempts to fabricate fully inorganic diode structures that incorporate colloidal QDs have reported an electrical © XXXX American Chemical Society

Received: June 17, 2016 Accepted: August 9, 2016

A

DOI: 10.1021/acsami.6b07367 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

2.3. Microscopy and Spectroscopy of Individual QDs. The fluorescence properties of single colloidal QDs were studied using a home-built confocal microscope, which allowed us to excite the nanocrystals with a continuous-wave argon laser (λ = 475 nm) or a pulsed laser diode (λ = 444 nm, pulse duration 80 ps, repetition rate 2 MHz). The collected QD luminescence, centered around 597 nm, could either be sent to a spectrograph with a CCD camera to record luminescence spectra or to two single-photon counting avalanche photodiodes (SPADs). A custom-made Mathematica program26 was used to quantify the photoluminescence intensity of single QDs in intensity images recorded with a SPAD under uniform CW excitation. Fluorescence lifetime studies were carried out under pulsed excitation by recording the photon arrival times relative to the excitation pulses with time-correlated single-photon counting electronics (internal resolution 40 ps, overall instrumental response function 700 ps). Fluorescence antibunching measurements were conducted by constructing short-time photon coincidence histograms using a 50/ 50 beamsplitter and the two SPADs in a standard Hanbury Brown and Twiss configuration. (Further experimental details can be found in the Supporting Information.)

deposition (PLD). We chose yttrium oxide as a proof-ofconcept material because it has the following advantageous properties: a large band gap, which ensures optical transparency and quantum confinement of the charge carriers generated in the QDs, an ultralow intrinsic charge carrier density, which reduces luminescence quenching due to trap states in the vicinity of the QDs, and chemical passivity as far as QD constituents are concerned. We show that all pertinent optical properties of individual nanoparticlestheir photoluminescence intensity as well as the spectrum, the dynamics, and the single-photon character of their emissionare preserved for a carefully chosen set of process parameters for the deposition of the Y2O3 matrix, and we thus highlight the nondestructive character of the PLD technique, meaning that it can satisfy all of the above-mentioned technical requirements for device fabrication.

2. EXPERIMENTAL DETAILS 2.1. Encapsulation of QDs by Pulsed Laser Deposition. Colloidal CdSe/CdS core−shell semiconductor quantum dots with emission wavelengths centered at 597 nm were synthesized as described in ref 26. The QDs were deposited on a BK7 substrate by spin-coating a 100 μL drop of a solution of QDs in hexane at 6000 rpm; the concentration of QDs was chosen sufficiently low to obtain 90% of optically resolvable single QDs on the sample surface. The ablation target was synthesized from compressed Y2O3 powder that was annealed in air at 1400 °C for 12 h. The Y2O3 target and the substrate with the QDs were then installed in a high-vacuum deposition chamber, which was prepumped to P = 10−7 mbar and then placed under an oxygen atmosphere, whose pressure was varied between 10−3 and 10−1 mbar for different deposition runs. (The minimum oxygen pressure necessary to maintain the stoichiometry of Y2O3 is 10−4 mbar.) The substrate−target distance was held constant at 5 cm for all experiments. The rotating Y2O3 target was ablated by an argon−fluorine (ArF) excimer laser (Lambda-Physik LPX-100 ArF, λ = 193 nm, pulse duration 17 ns, repetition rate of 10 Hz, fluence ≈2.5 J cm−2), which was focused on the target at an incident angle of 45° with respect to the rotation axis of the target. (A more detailed description of the PLD apparatus can be found in ref 27.) Three types of samples were produced: (i) Calibration samples consisted of Y2O3 films deposited on empty BK7 substrates. These QD-free films with thicknesses between 200 and 1000 nm were studied by M-line spectroscopy (see below) to measure their refractive index and to determine the deposition rate, i.e., the film thickness as a function of the number of ablation pulses, which enabled us to adapt the number of pulses to the desired thickness of the Y2O3 layer in subsequent deposition runs. (ii) Asymmetrically encapsulated QDs, for which the Y2O3 film was deposited on top of a QD-covered BK7 substrate. (iii) Symmetrically encapsulated QDs, in which case an Y2O3 film was first deposited on an empty BK7 substrate, followed by spincoating of QDs from solution and finally a second Y2O3 deposition, so that an Y2O3−QDs−Y2O3 sandwich structure was formed. The Y2O3 deposition only covered part of the BK7 substrates, meaning that reference measurements on QDs outside of the deposition zone could be conducted for all samples of encapsulated QDs. 2.2. Matrix Characterization. The refractive index and the thickness of Y2O3 films deposited under different oxygen partial pressures were studied with M-line spectroscopy,28 which allowed us to determine the refractive index with an absolute precision of 10−2 and the film thicknesses with a relative precision of 2%. Our setup for M-line spectroscopy used a 594 nm He−Ne laser and a LaSF35 prism; results were interpreted under the assumption of a step index profile29 or by using leaky modes for low-refractive-index films.30 The morphology of the Y2O3 films was evaluated by atomic force microscopy (AFM, Nanosurf scanHead, FlexAFM), using dynamic force mode and a standard silicon-tipped cantilever (BudgetSensors, TAP190AL-G).

3. RESULTS AND DISCUSSION 3.1. Deposition Pressure and QD Survival. Figures 1a and 1b show two typical photoluminescence (PL) intensity

Figure 1. Photoluminescence intensity of single QDs as a function of the oxygen pressure P(O2) during encapsulation in Y2O3 by PLD. (a, b) 20 × 20 μm2 confocal images of samples grown at (a) P(O2) = 10−3 mbar and (b) 10−1 mbar, respectively. (c) 20 × 20 μm2 confocal image of a reference sample of colloidal QDs spin-coated on a BK7 substrate without subsequent Y2O3 encapsulation. The three images were recorded under identical conditions: excitation intensity 40 W/cm2, integration time 10 ms per pixel, pixel size 100 nm, scale bars represent 4 μm. To facilitate the visual comparison of the number of QDs, the maxima of the false-color intensity scales (specified in counts per 10 ms) have been chosen such that the QD spots are oversaturated. (d, e) Intensity time traces and corresponding count histograms of (d) a single QD encapsulated in the P(O2) = 10−1 mbar sample and (e) a free QD in the reference sample, respectively; the two traces were recorded at the same excitation intensity of 40 W/cm2.

images of 20 × 20 μm2 areas on samples in which QDs were covered with Y2O3 films (thickness 60 nm) grown by PLD at oxygen pressures of P(O2) = 10−3 mbar and P(O2) = 10−1 mbar, respectively. No QD luminescence spots are visible in the film deposited at the lower oxygen pressure, while numerous single-QD signals can be discerned in the P(O2) = 10−1 mbar sample. In fact, the QD surface density and average PL intensity in the latter case are comparable to those of a reference sample of nonencapsulated QDs on BK7 substrates (identical spincoating parameters) (see Figure 1c). Furthermore, when comparing intensity time traces of QDs under continuous excitation, we found the signatures of two well-defined emission B


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10−2 mbar. The average luminescence count rate was found to be Ienc = 76 ± 12 kHz for QDs encapsulated at pressures above Pthr(O2) and Ifree = 83 ± 10 kHz for the reference sample of free-standing QDs, leading to an intensity ratio of Ienc/Ifree = 0.92. During the PLD encapsulation process, the pressure in the deposition chamber modulates the kinetic energy of the vaporized fragments that impinge on the substrate and selforganize to form the encapsulation matrix; the higher the pressure, the lower this kinetic energy will be.32,33 For example, time-of-flight measurements realized during Au ablation33 have shown that the velocity of the matrix precursors can be reduced by a factor of around 6 (from 9 to 1.5 km/s) when the O2 pressure in the process chamber is increased from 10−2 to 10−1 mbar. As far as our QD encapsulation experiments are concerned, we therefore speculate that the loss of photoluminescence in films grown at oxygen pressures below Pthr is caused by surpassing a threshold kinetic energy above which the deposition process deteriorates the QD surface to a degree that leads to the appearance of additional nonradiative decay channels. The luminescence quenching effects of such surface defects are well-known, which is why the surface of core/shell QDs is covered with organic ligands that passivate surface defects and thus suppress the nonradiative decay channels. The two transitions visible in Figure 2 can be understood as follows: Pthr(O2) ≈ 7.5 × 10−2 mbar marks a deposition pressure below which the kinetic energy of the matrix precursors is high enough to damage or partially remove the surface ligands, leading to a drastic reduction of the photoluminescence while the number of detectable QDs remains constant. This transition between destructive and nondestructive deposition is very abrupt, and we do not observe a more gradual variation of the PL intensity as one might expect if there were intermediate states of surface degradation. For P(O2) ≲ 10−2 mbar, all the organic ligands seem to be destroyed or ejected, and the degradation of the surface of the QD shell itself may begin, which opens up enough additional nonradiative channels to completely quench all luminescence. To ensure that the loss of PL intensity is not caused by any other effect related to the conditions of the PLD process, we have verified that the QDs outside the deposition zone retained their normal luminescence intensity in all our samples. 3.2. Characterization of the Y2O3 Encapsulation Matrix. As discussed above, the properties of PLD-grown films can be tuned by adjusting the kinetic energy of the ablated species through regulation of the oxygen pressure in the deposition chamber.32,33 Under vacuum, the kinetic energy of

states, characteristic of single quantum dot blinking,31 for both encapsulated QDs in the P(O2) = 10−1 mbar Y2O3 film and the free QDs of the reference sample, as presented in Figures 1d and 1e, respectively. We furthermore compared the surface density of QD luminescence spots in the Y2O3 film grown at P(O2) = 10−1 mbar with the surface density of free QDs residing outside of the deposition area in the same sample; we found a ratio of ρenc/ρfree = 0.72 ± 0.27, which shows that Y2O3-encapsulation by PLD at P(O2) = 10−1 mbar preserves both the number of luminescent QDs and their original emission intensity. An overview of QD survival is given in Figure 2, which shows the evolution of the surface densities and the luminescence

Figure 2. Survival of individual QDs as a function of the oxygen pressure P(O2) during encapsulation in Y2O3. (a) Average surface density of QDs, normalized to the corresponding density observed outside of the deposition zone on each sample. Each data point summarizes the analysis of eight distinct regions of size 10 × 10 μm2; the error bars represent the empirical standard deviation of each ensemble of observations. (b) Average detected photoluminescence intensities of single encapsulated QDs (data points with error bars for empirical standard deviation). The solid horizontal line and hatched area indicate the corresponding average intensity and its standard deviation found for a reference sample not subjected to any PLD treatment.

intensities of encapsulated QDs as the oxygen pressure during PLD growth of the Y2O3 film is varied from 10−3 to 10−1 mbar. Figure 2a presents the QD surface densities, in all cases normalized to the densities found for free QDs located outside of the deposition zone in the very same samples. We observed an onset of QD survival at a threshold oxygen pressure between 2.5 × 10−2 and 5.0 × 10−2 mbar, above which the observed surface density of the encapsulated QDs rises sharply from virtually zero to being comparable to the out-of-matrix density. However, as Figure 2b shows, the photoluminescence intensity of encapsulated QDs is still 1 order of magnitude smaller than that of QDs outside of the deposition zone; the oxygen threshold pressure that preserves both number and PL intensity of the Y2O3-embedded QDs was found to be Pthr(O2) ≈ 7.5 ×

Figure 3. Characterization of Y2O3 films grown on a BK7 substrate. (a, b) Typical AFM images recorded in tapping mode for samples grown at P(O2) = 10−3 and 10−1 mbar, respectively. Both images cover an area of 2 × 2 μm2 in 256 × 256 pixels with a scanning frequency of 2 Hz. The scale bars represent 400 nm. (c) Evolution of the refractive index for TE modes (filled circles) and TM modes (open circles), as well as of the surface roughness (squares with error bars), as a function of the oxygen pressure in the PLD chamber during film growth. C


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ACS Applied Materials & Interfaces the film precursors arriving at the sample surface is sufficiently high to foment diffusion, thus facilitating the formation of dense and smooth films, whose structure is known to be amorphous for the range of deposition pressures used in the present study.34 More porous films are obtained if the oxygen pressure is increased, owing to the reduced mobility of the matrix precursors that impinge on the surface of the substrate. Figures 3a and 3b show topographic AFM images of Y2O3 encapsulation films grown at P(O2) = 10−3 and 10−1 mbar, respectively. The surface roughness (root mean square of height fluctuations) of the low-pressure sample remains within our experimental resolution of 0.38 nm, while that of the P(O2) = 10−1 mbar sample reaches 1.10 ± 0.05 nm. This observation might indicate that not only the kinetic energy at deposition but also the porosity of the Y2O3 matrix may play a role in avoiding the surface degradation of the embedded QDs. Nevertheless, a comparison with the QD diameter of 11 ± 1 nm suggests that even films grown at the highest oxygen pressures do not contain a significant number of pores that would be large enough for the QDs to reside in them without actually being in contact with the Y2O3 matrix. However, the threshold kinetic energies for inhibiting surface diffusion and for avoiding degradation of the QDs do seem to fall into the same pressure range: As can be seen in Figure 3c, the surface roughness of the PLD-grown films increases abruptly for P(O2) ≳ 5 × 10−2 mbar, a behavior that is similar to the sudden boost of the QD survival and the PL intensity that we observed in the same range of pressures. No significant modifications of the surface morphology were observed when comparing QD-doped samples to QD-free Y2O3 films grown at the same oxygen pressures. For deposition pressures above P(O2) = 1.0 × 10−1 mbar, highly porous matrices are obtained, which are less suitable for technological applications and were therefore not investigated further. To independently characterize the morphology of Y2O3 films as a function of the oxygen pressure during deposition, we have performed M-line spectroscopy28 to measure thicknesses and refractive indices of films grown on a BK7 substrates without embedded QDs, but under otherwise identical conditions. As can be seen in Figure 3c, a roughly linear decrease of the refractive index with increasing pressure (rate: 5.6 refractive index units per millibar) is observed for both transversal-electric (TE) and transversal-magnetic (TM) modes. We attribute this decline to the presence of a lesser-index material (air) in the Y2O3 film, whose volume fraction grows with increasing oxygen pressure, because it is well-known that the porosity of PLDgrown Y2O3 films increases with P(O2).32 Combining the refractive indices measured by M-Line spectroscopy with a model based on the Bruggeman effective medium theory,35 we can estimate the volume fraction of air in the films (see below for further details); we thus find values of about 36% and 52% for P(O2) = 7.5 × 10−2 and 1.0 × 10−1 mbar, respectively, assuming 0% (i.e., a compact Y2O3 matrix) at P(O2) = 1.0 × 10−3 mbar. 3.3. Luminescence Spectra, Exciton Lifetimes, and Single-Photon Emission. After having shown that CdSe/CdS QDs can be encapsulated into Y2O3 under preservation of their photoluminescence intensity, we now present further experiments to verify the structural integrity of the encapsulated QDs. An easily accessible indicator is the luminescence spectrum, which is known to become blue-shifted during encapsulation processes that affect QD structure.15 Figure 4a compares a representative spectrum of a single nonencapsulated QD

Figure 4. Optical characterization of colloidal QDs. (a) Emission spectra of a single free QD from the reference sample (green circles) and of two individual QDs encapsulated in Y2O3 films grown at P(O2) = 1.0 × 10−1 mbar (blue squares) and P(O2) = 7.5 × 10−2 mbar (red triangles), respectively. The solid lines are Gaussian fits to the data to determine the maximum emission wavelengths (see Table 1). The dashed line represents the ensemble spectrum of the QDs when dissolved in hexane. (b) Luminescence decay curves of individual QDs, one free QD from the reference sample (green circles), one QD embedded in an Y2O3 film grown at P(O2) = 1.0 × 10−1 mbar (blue squares), and one from a 7.5 × 10−2 mbar film (red triangles). The solid black lines represent MLE fits according to eq 1.

(reference sample) with two spectra of individual QDs that were encapsulated in Y2O3 at P(O2) = 1.0 × 10−1 mbar and P(O2) = 7.5 × 10−2 mbar, respectively. We found that the central emission wavelength (around 597 nm) and the homogeneous line width (22 nm) remain unaffected by the encapsulation process; all single-QD spectra remain in spectral range defined by the ensemble emission spectrum of QDs in solution that is also shown in Figure 4a. As summarized in Table 1, we did not encounter significant differences between the distributions of the central luminescence wavelength of the ensembles of 30 different QDs that were studied in each of the three samples. This preservation of the emission spectrum makes it unlikely that the encapsulation process has caused significant modifications of the QDs as far as chemical composition, crystal structure, and the endowment with surface-passivating ligands are concerned. We have furthermore conducted fluorescence lifetime measurements to asses how the encapsulation process influences the exciton emission dynamics of the QDs. To this end, we have spin-coated QDs onto a previously deposed Y2O3 layer, which was subsequently covered with a second deposition of Y2O3 to encapsulate the nanoparticles. Both Y2O3 layers had thickness of 300 nm and were grown under identical PLD conditions for all our samples to ensure that the matrix around the embedded QDs was as homogeneous as possible. Figure 4b shows three single-QD luminescence decay curves that are representative for the exciton emission dynamics that we observed for the reference sample of free QDs and for two geometries of QDs sandwiched between two Y2O3 layers grown at P(O2) = 1.0 × 10−1 mbar and P(O2) = 7.5 × 10−2 mbar, respectively. All fluorescence decay curves I(t) were analyzed using a maximum-likelihood estimator (MLE) for identifying the parameters that best reproduced the data according to

I(t ) = A e−t/ τ + B(t )

(1)

where A is the decay amplitude and τ the exciton lifetime. The function B(t) represents a time-dependent background contribution, weak but noticeable, arising from the Y2O3 matrix itself; this corrective term was determined by measuring decay curves at QD-free positions of the matrix under conditions that were identical to those used for recording the corresponding QD decay curves. Thirty QDs were studied on each sample, D


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Table 1. Optical Properties of Free Colloidal Quantum Dots on BK7 (Reference Sample) and of QDs Encapsulated in Y2O3 Matrices at Two Different Oxygen Pressures P(O2)a sample type free QDs encapsulated QDs

P(O2) [mbar]

λ [nm]

σλ [nm]

γ [ns−1]

σγ [ns−1]

neff

n.a. 1.0 × 10−1 7.5 × 10−2

598 595 598

5 5 6

0.017 0.022 0.027

0.004 0.007 0.008

1.25 1.43 1.59

The table lists the average emission wavelength λ with its standard deviation σλ as well as the mean luminescence decay rate γ with standard deviation σγ (30 QDs were measured for each parameter and sample). The effective refractive index neff was calculated according to the Bruggemann effective medium theory, based on the refractive indices of the Y2O3 films as determined by M-line spectroscopy. a

and all time-resolved experiments were performed at excitation intensities well below saturation to minimize photobleaching. The general trend observed for the three samples is summarized in Table 1: Going from free nanoparticles to QDs encapsulated in the more compact matrix, the average decay rate γ = 1/τ increases from 0.017 to 0.027 ns−1. We do not attribute this variation to the presence of additional nonradiative relaxation channels (which would reduce the emission quantum yield), but rather to the expected sensitivity36 of the exciton lifetime to the refractive index of the surrounding medium. Local-field models predict a monotonic rise of the radiative decay rate with increasing refractive index if an emitter is embedded in a homogeneous, linear, and nonabsorptive medium.37 In fact, we find that a variation in the effective refractive index neff from 1.25 to 1.59 is consistent with the observed increase of the decay rate according to the Bruggemann effective medium theory,38 and the values of neff thus obtained for the two Y2O3 samples are in agreement with their macroscopic refractive indices as measured by M-line spectroscopy (see previous section). We do note that the empirical standard deviation of the decay rates doubles for the encapsulated QDs as compared to the reference sample, which we attribute to local inhomogeneities in the Y2O3 films. Previous investigations have shown that QDs are sensible to the refractive index of the surrounding medium over a length scale of about 100 nm;36 consequently, local variations in the density of the Y2O3 films will be reflected in concomitant variations of effective refractive index and fluorescence decay rate. Overall we can conclude that the observed changes in the exciton lifetime justify the assumption that structurally intact colloidal QDs are reacting to the proximity of high-index matrix material. Finally, we tested the suitability of the encapsulated colloidal QDs as single photon sources, which is an important verification as it has been shown that modifying the immediate environment of single QDs can degrade the single photon characteristics of their emission.39,40 We have performed a photon coincidence measurement in a Hanbury Brown and Twiss interferometer, whose results are presented in Figure 5 in the form of normalized short-time coincidence histograms of the photons emitted by two individual QDs, one from the reference sample and the other one covered by a 60 nm film of Y2O3 grown at P(O2) = 1.0 × 10−1 mbar. Both coincidence histograms show a clear dip at zero delay, which reaches 0.05 (free-standing QD) and 0.20 (encapsulated QD), respectively; both curves thus descend well below the limiting value of 0.5 that is unambiguous proof for a single-photon emitter.41 The antibunching dip does not fully descend to zero due to the nonnegligible probability of coincidences with background photons; this fact can be taken into account by applying the correction42

Figure 5. Normalized coincidences recorded with the HBT setup for a single QD in reference sample C (circles) and from sample B (P(O2) = 1.0 × 10−1 mbar, squares). The solid lines are fits to the data following eq 2.

C(τ ) = ρ2 g(2)(τ ) + (1 − ρ2 )

(2)

where C(τ) is the measured coincidence histogram and g(2)(τ) = 1 − e−γτ is the background-free second-order intensity correlation function at short times in the limit of low excitation power. The correction term ρ = S/(S + B) is the ratio between the single-emitter luminescence signal S and the total detected intensity including background, S + B. Equation 2 was adjusted to the measured coincidence histograms by maximum-likelihood estimation with the decay rate γ and the signal fraction ρ as fit parameters. We thus found decay rates γ = 0.025 ± 0.001 ns−1 for the data presented in Figure 5, which is compatible with the distributions of lifetimes measured in analogous samples (Table 1). The corresponding signal fractions, ρ = 0.97 for the free-standing QD and ρ = 0.87 for the encapsulated one, are likewise consistent with the range of signal-to-background ratios that we observed in confocal images and single-QD time traces in the same samples, S/B ≳ 20 for naked QDs and S/B ≈ 5−10 after encapsulation in Y2O3. All our results thus support the hypothesis that the encapsulated QDs fully retain their efficiency as single-photon emitters. 3.4. Comparison of Encapsulation Techniques. We can now compare the advantages of different encapsulation techniques with respect to industrial-scale fabrication of QDbased devices (see Table 2). The sol−gel technique can incorporate colloidal QDs in ZrO2, SiO2, ZnS, or ZnO matrices under preservation of measurable QD luminescence;19,21 however, while this technique is well adapted to thick planar waveguides with high refractive index, it is less suited for reproducibly manufacturing the thin layers (few nanometers) needed for industrial LED fabrication, and its material versatility is limited. Trials of MEE16 and MBE15 encapsulation of colloidal QDs in ZnSe have reported a significant drop of QD quantum yield, which was attributed to the formation of deep traps during ZnSe deposition, surface oxidation, and the general detrimental influence of the relatively high process temperature (550 K). Moreover, while MEE and MBE allow exquisite thickness control and reproducibility of thin film E


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4. CONCLUSION We have reported the first successful utilization of pulsed laser deposition (PLD) for the encapsulation of single colloidal QDs in an inorganic matrix under full preservation of their optical properties. We show that the key factor for preserving the structural integrity of the nanoemitters is controlling the kinetic energy of the precursors of an Y2O3 matrix by adjusting the oxygen pressure in the deposition chamber. We find that P(O2) ≥ 7.5 × 10−2 mbar leads to nondestructive encapsulation, while lower pressures cause nearly complete loss of luminescence; the transition between these two regimes is very abrupt. In samples grown under nondestructive conditions, we have verified that the blinking behavior, resistance to photobleaching, luminescence spectra, and fluorescence lifetimes of single QDs remain free from any effects that would suggest a degradation of the structural quality of the nanocrystals. We have furthermore characterized the structure of the obtained Y2O3 films by atomic force microscopy and M-line spectroscopy, and we have shown that the encapsulated QDs preserve their efficiency as single-photon emitters. We have thus demonstrated that PLD is a valuable addition to the catalog of techniques used for the encapsulation of nanoemitters, given that PLD is precise, scalable, and versatile in its choice of materials, for example, for other interesting candidate matrices such as Al2O3 and ZrO2. Furthermore, the method can be applied to other colloidal nano-objects such as nanorods, dot-in-rods, rods-in-rods, or nanoplatets and thus may find applications for fabricating novel electro-optical devices based on fully inorganic structures and/ or the combination with nanoantennas. Promising devices that are currently attracting interest are transport-layer-free diode such as those reported by Bozyigit et al.,13 for which one can expect a boost in efficiency if these structures are fabricated with a technique that fully preserves the luminescent properties of embedded nanoemitters.

Table 2. Comparison of Encapsulation Techniques in Terms of Process Parameters That Are Relevant for Industrial Fabrication as Well as with Respect to the Preservation of QD Luminescence technique

precision

versatility

speed

scalability

lumin

sol−gel19,21 MBE/MEE/ALD14−18 MOCVD25 SILAR22,23 sputtering13 PLD

− + + + + +

− + − − + +

+ − + − + +

+ + + − + +

± ± − + − +

deposition, these methods are not readily scalable for timeefficient industrial fabrication. Kim et al. have reported the encapsulation of CdSe/ZnS in a ZnO matrix using ALD17 but did not address the issue of maintaining the luminescence quantum yield. Lambert et al.18 managed to embed CdSe/ZnS QDs in Al2O3 under conservation of ≈70% of the luminescence quantum yield using thermal ALD; however, they observed a complete loss of luminescence when same method was applied to CdSe/CdS QDs. Mueller et al.25 have incorporated CdSe/ ZnS nanocrystals in a GaN matrix by MOCVD but do not discuss the optical properties of the QD-doped thin film, as they focus on transport properties. Khon et al.23 have used a pore-filling technique to incorporate CdSe cores into a CdS matrix, obtaining thin films doped with colloidal QDs whose quantum yields reached up to 52%. Unfortunately, the porefilling technique is achieved through SILAR, which becomes cumbersome and time-inefficient for matrix thicknesses above a few tens of nanometers and for multilayered structures. Furthermore, the versatility of the method is limited, and its feasibility has so far only been confirmed for the specific couples CdSe/CdS and PbS/CdS.22,23 Bozyigit et al.13 have embedded CdSe/ZnS QDs in Al2O3 by sputtering at room temperature, which is a time-efficient and reproducible growth method and has a large versatility with respect to the materials that can be deposited. However, the process conditions turn out to be rather severe, which is reflected in the low quantum efficiency of the resulting device, partly due to the QDs being damaged during sputtering. We have shown in this work that the PLD technique is a valuable addition to the canon of techniques for encapsulating colloidal quantum dots in inorganic matrices, as it can be carried out under conditions that are sufficiently mild to preserve all pertinent optical properties of the luminescent nanoparticles, while also offering versatility with a wide range of materials, process speed, and scalability, as well as access to multilayered structures with nanometer accuracy.27,43 The present work was concerned with QDs embedded at low concentrations in a benchmark oxide, which was motivated by the goal of verifying the conservation of photophysical properties at the level of single QDs. The logical next step will be studies of various inorganic materials doped with higher concentrations of QDs to elucidate how such concentrations might affect the structural integrity of the host matrices. Regarding this perspective, we note that the PLD technique can readily be combined with other QD deposition techniques that are suitable for large-scale fabrication, such as dip-coating, inkjet printing, phase separation approaches, contact printing, or assembly by depletion forces.11,44,45

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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/acsami.6b07367. Further experimental details on confocal microscopy and M-line spectroscopy (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (A.A.). *E-mail [email protected] (A.P.). Present Addresses

A.A.: Physics Department, University of California, San Diego (UCSD), 9500 Gilman Dr., La Jolla, CA 92093-0319. E.L.: Institut des Nanosciences de Paris, UPMC-CNRS UMR ̂ courrier 840, 75252 Paris cedex 05, 7588, 4 place Jussieu, boite France. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank N. Abdellaoui and B. Mahler for discussions, and we gratefully acknowledge technical support by J.-F. Sivignon, Y. Guillin, and the Lyon Center for Nano-Opto Technologies (NanOpTec). This work was supported by the Fédération de Recherche André Marie Ampère (FRAMA) and F


Research Article

ACS Applied Materials & Interfaces by the Programme Avenir Lyon Saint-Étienne (ANR-11-IDEX0007) of Université de Lyon, within the program “Investissements d’Avenir” operated by the French National Research Agency (ANR). This work was performed in the context of the European COST Action MP1302 Nanospectroscopy.

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CdS Quantum Dots in an

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