Cathodoluminescence in a Scanning Transmission Electron

Letter pubs.acs.org/JPCL

Cathodoluminescence in a Scanning Transmission Electron Microscope: A Nanometer-Scale Counterpart of Photoluminescence for the Study of II−VI Quantum Dots Zackaria Mahfoud,† Arjen T. Dijksman,¶ Clémentine Javaux,¶ Pierre Bassoul,¶ Anne-Laure Baudrion,‡ Jérôme Plain,‡ Benoît Dubertret,¶ and Mathieu Kociak*,† †

Laboratoire de Physique des Solides, CNRS/UMR8502, Université Paris-Sud, Bâtiment 510, Orsay 91405, France Laboratoire de Nanotechnologie et d’Instrumentation Optique, Institut Charles Delaunay, UMR CNRS 6279, Université de Technologie de Troyes, France ¶ Laboratoire de Physique et d’Etude des Matériaux, UMR 8213, Ecole Supérieure de Physique et de Chimie Industrielles, ParisTech, 10 rue Vauquelin, 75005 Paris, France ‡

ABSTRACT: We report on nanometer-scale cathodoluminescence (nanoCL) experiments in a scanning transmission electron microscope on individual core−shell CdSe/ CdS quantum dots (QDs). By performing combined photoluminescence (PL) and nanoCL experiments of the same individual QDs, we first show that both spectroscopies can be used equally well to probe the spectral properties of QDs. We then demonstrate that the spatial resolution of the nanoCL is only limited by the size of the QDs themselves by performing nanoCL experiments on QDs lying side by side. Finally, we show how nanoCL can be advantageous with respect to PL as it can rapidly and efficiently characterize the optical properties of a large set of individual QDs. These results contrast with pioneering CL works on II−VI QDs and pave the way to the characterization of any II−VI quantum-confined structure at the relevant scale. SECTION: Plasmonics, Optical Materials, and Hard Matter

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microscope, in particular, in scanning EM (SEM-CL), should in principle be an excellent candidate. Indeed, individual III−V QDs have been successfully studied by means of SEM-CL.8−11 However, the spatial resolution was larger than the QD size by approximatively an order of magnitude. The reasons for the loss of spatial resolution are numerous11 but are generally related to the very strong interaction of the electron beam with the sample at the relatively small accelerating voltages used in SEM-CL (see below). This precludes the extension of the technique to the study of packed sets of QDs or to individual QD internal structures. It is worth stressing also that in these works,8−10 the relationship between CL and PL spectra was not studied. The situation for II−VI QDs is even worse. First, it is worth noting that the higher ionicity of the bonds in II−VI compounds makes them much more prone to damage through electron beam irradiation than, for example, III−V compounds. However, even discarding this issue that is predominately important when using high current densities such as needed for getting nanometer resolutions, other limitations exist. Indeed, in their pioneering works, Rodriguez-Viejo et al.12 performed a comparison of the optical properties of films of CdSe/ZnS QDs as probed by PL and CL in a SEM. The CL was performed

he study of quantum-confined structures, in particular, those composed of II−VI compounds, has exploded in the past 20 years. Indeed, the advances in synthesis of quantum wells,1 quantum rods,2 and quantum dots (QDs)3 make it possible now to manufacture 100% quantum efficiency, nonblinking QDs.4 The long predicted uses of such QDs, from biomarkers to quantum cryptography, are now at hand. Such objects are, for obvious physical reasons, much smaller than the optical diffraction limit. The mandatory optical investigations, usually done via absorption or photoluminescence (PL) spectroscopies are thus limited to sets of QDs or well-separated individual QDs.5 This makes it impossible to retrieve individual QD optical properties in a compact environment or to quickly assess the optical properties of a large set of QDs and disentangle, for example, homogeneous and inhomogenous broadenings. Such an analysis is however crucial for the study of real devices.6 Alternatives to purely optical methods are thus needed. In a pioneering work,7 Erni et al. have used electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM). Despite the impressive proved benefit of the nanometric spatial resolution offered by electron-based spectroscopies, fundamental and technical issues led unfortunately to inadequate spectral resolutions. Owing to its potentially similar high spatial resolution but much better spectral resolution, cathodoluminescence (CL) in an electron © XXXX American Chemical Society

Received: October 16, 2013 Accepted: November 15, 2013

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Figure 1. Comparison between PL and CL spectra of individual CdSe/CdS QDs. (a) Scheme of the PL- and nanoCL-compatible substrate (all made up of Si for the presented example). (b) HAADF image of the QD, the PL/CL spectroscopies of which are shown in (c). (c) CL and PL spectra of the same QD. The integration time is 1 min for PL and 50 ms for CL. The energy shift can be explained by a temperature difference between the measurements (see text). (Inset, left) Scheme of the nanoCL/PL experiment. (right) PL time correlation curves for the same QD.

beam current, single-photon detection from nitrogen vacancy centers in nanodiamond could be observed,15 implying the possibility of addressing individual II−VI QDs in a linear regime with the same setup. In the present experiments, typical conditions were a probe size around 1 nm, accelerating voltage of 60 keV, and a current around 1 nA. Note that accelerating voltages from 40 to 100 keV were also used without detectable differences in the results. Experiments were performed in the spectral imaging mode. In this mode, the beam is scanned over the region of interest. At each scan position, a spectrum is recorded, along with the highangle annular dark field (HAADF) signal. At the end of the scan, both a CL spectrum image (SI), that is, an image with a spectrum per pixel, and a HAADF image are available. From the latter, the local thickness can be determined. In the following, spectra and maps are directly extracted from such a SI. We have used CdSe/CdS QDs with a core between 3 and 5 nm in diameter and with a relatively thick shell (10 nm). These QDs have been shown to have a bright emission and almost no blinking.16 Such optimized QDs were important for the success of the experiments, which were dedicated to study the spectral properties of the QDs and to perform combined PL/CL measurements. Spectral measurements usually require long exposure times and high signal levels because the detection is parallel and one has to overcome losses in the detection path (imperfect CCD camera quantum efficiency, lenses and gratings losses, etc.). Detection schemes using a simplified optical path with fast, high-sensitivity serial detectors (such as avalanche photodiodes) could be used to study less robust QDs. Indeed, we performed experiments on commercial QDs (Evident Technology) with thin shells. Although spectra could be obtained from large aggregates of QDs, individual QD signals could not be obtained due to a very fast vanishing of the signal; their study would benefit from a faster detection setup. The QDs were deposited on commercial Si or Si3N4 membranes that were thin enough (15 nm) to be electrontransparent (see Figure 1a). In the case of correlated PL/CL measurements, before deposition, the membrane was patterned by e-beam lithography with reference crosses that can be

without spatial resolution. Although spectra were dominated by a single peak in both cases, major differences were observed. The CL peak was red-shifted by around 6 nm, and the peak full width at half-maximum (fwhm) was much broader (40−50 as compared to 30−40 nm for the PL). The former effect was attributed to nonlinearities, possibly related to the quantum confined Stark effect (QCSE)13 and the latter to the effect of electron beam heating. Quenching of the emission was observed over time. Finally, a huge secondary electron emission (SEE) triggering further delocalized CL emission was diagnosed. All of these results, the absence of spatial resolution, nonlinearities, heating, SEE, and so forth, were related to the huge interaction of the electron beam with the sample in a SEM and the high electron beam current used. In this Letter, we will present combined PL and STEM-CL (nanoCL) measurements of spectra from individual core−shell CdSe/CdS QDs. We will show that the PL and CL spectra are equivalent and thus that nanoCL can be used as a nanometerscale counterpart to PL for the study of II−VI QDs. We will then present nanometer-scale nanoCL experiments showing how individual QD optical properties can be obtained even when they are close-packed and how a very large number of QDs can be investigated at once. PL experiments were performed at room temperature with a confocal microscope (MicroTime, PicoQuant) and a Hanbury−Brown and Twiss setup based on two avalanche photodiodes and a single photon counting module (HydraHarp 400, Picoquant). The individual QDs were excited with a pulsed diode emitting at 402 nm, and the emitted light was collected with an air objective (100×, NA = 0.9). All or part of the emitted light was directed toward a spectrometer with a 150 l/mm grating and a resolution of ∼0.8 meV (Shemrock 750, Andor Technology). CL experiments were performed at 170 ± 20 K in a VG HB 501 STEM, fitted with a homemade nanoCL system.14 In former works, this setup allowed us to disentangle the spectral signatures of individual III−V, 1−5 nm thick, quantum wells separated by less than 4 nm,14 with a minimum QCSE. Also, thanks to a high-efficiency design that collects most of the cathodo-generated photons and thus minimizes the electron 4091

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Figure 2. High spatial resolution CL mapping of the QDs. (a) HAADF image of a set of six QDs. (b) HAADF of the same set acquired in parallel to a SI. (c) Spectra extracted from the SI on areas indicated on (b). (d) CL intensity map. Only three QDs are emitting. The scale bars in (c) and (d) are 20 nm.

detected both in the photon and electron microscopes;17 clearly separated individual QDs can thus be localized and observed by both techniques. Note that if the QDs are exposed to air for too long (i.e., few days), both PL and CL signals vanish. Compared CL/PL experiments were thus performed within the same week. Numerous QDs were studied, hundreds by individual PL or nanoCL experiments and more than 20 in correlated CL/PL experiments. In one case, we performed a PL/CL/PL series showing no spectral shift between both PL experiments but a decrease in lifetime. This implies a specific, but minor, deleterious effect of the e-beam on the optical properties. However, in both PL and CL, excessively long observations led to a bleaching of the QDs. In most cases, the bleaching in CL is associated with a morphological change in the QDs. Figure 1 presents the comparison between a PL and a CL experiment performed on the same QD. The HAADF (Figure 1b) exhibits the typical diamond-like shape of the QDs. The fact that the observed object is truly a QD is ascertained by the PL time correlation function showing a zero delay peak smaller than 0.5 (inset of Figure 1c). In the three investigated QDs showing CL and PL emissions, CL and PL have very similar spectra, made up of a single peak at around 620 nm, as expected given the 3 nm core diameter. The CL is however systematically blue-shifted by 10−16 nm with respect to the PL. The CL peak fwhms, on the order of 14 ± 5 nm (around 28 meV), are also systematically smaller than those in PL by a factor of 1.2−1.5. The former effect is well-explained by the difference in temperature between the experiments; a blue shift of around 14 nm is expected between 300 and 170 K.18 The latter is most probably related to the spectral diffusion19 as the

fwhms in both spectroscopies are already much larger than the temperature spreading. In the PL experiment, the integration time was 1 min, and that of the CL experiments was only 50 ms, which would explain the larger broadening for the former spectroscopy. This clearly shows that nanoCL in a STEM and PL provides almost the same spectral information on individual QDs, contrary to the results of previous SEM-CL experiments,12 with a minimal introduction of nonlinear and heating effects due to the electron beam. Figure 2 presents SI mapping of a set of six QDs packed together. Again, the diamond-like shape of the QDs is clearly visible in the HAADF image (Figure 2a). The HAADF taken along with a SI is shown in Figure 2b. In this case, the emission spectrum taken on different QDs (Figure 2c) is almost the same, and no variation of spectral shape is seen within a single QD. Also, the fact that the same QD can be probed several times during the scan is a good indication that the nanoCL is not inducing rapid damages and cannot be responsible for the absence of emission of the neighboring nonemitting QDs. By comparing the HAADF image (Figure 2b) and the intensity map extracted from the SI acquired in parallel (Figure 2d), one notes that within individual QDs, the intensity increases roughly monotonically with the local thickness. More strikingly, it is obvious that only three QDs are emitting. Such information would have been unavailable from a PL experiment. Interestingly enough, no correlation with a specific feature in the QD (orientation, presence of defects, and so forth) is observed here or in any other nanoCL experiment to explain the presence or absence of emission. 4092

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Figure 3. Batch measurement on a large set of QDs. (a) HAADF image of the set of QDs. Individual QDs can be seen. (b) Selected spectra extracted from areas circled on (a). (c) HAADF image acquired during a SI on the set. (d) Intensity; (e) wavelength; (f) fwhm maps extracted from the SI. Scale bars in (c−f) are 200 nm.

This example allows us to set a limit in the spatial resolution. Usually, four main parameters affect the spatial resolution of a CL experiment. First is the size of the electron probe, which is here typically less than 1 nm and thus not relevant. Second is the SEE, triggering photon emission from the neighborhood of the impact area.12 This effect is here clearly ruled out by the absence of detected CL signal when the beam is hitting the dark QDs. If the SEE were retriggering photon emission in the neighborhood, an emission would have been measured at these points. Third is the broadening of the emission location due to multiple deflections and interactions of the incident electron before it is absorbed. This broadening is known as the “excitation pear”. Here, the effect is shown to be negligible as a variation of the emitted intensity within individual QDs can be detected (see Figure 2d). This rules out the existence of a broad excitation pear. The absence of an excitation pear is indeed expected for a 30 (QD) + 15 (substrate) nm thick sample at 60 keV. Fourth is that the charge carriers created by the incoming electron at a given point may diffuse in the sample and recombine far away from the point where they have been created. This effect, known as “charge carrier diffusion”, might again degrade the spatial resolution as the emitted light might come from a place far from the excitation point. As mentioned above, the signal intensity within a QD seems to scale with its thickness. No wavelength or fwhm changes can be detected within an individual QD. From the point of view of the electron/matter interaction, this means that either the diffusion length is larger than the QD size or the QD responds as a whole to the electronic excitation. Either way, the absence of measurable spectroscopic change within an individual QD means that the effective spatial resolution is set by the QD size itself. We now show how it is possible to explore the statistics in the samples by analyzing large SI.

Figure 3a presents a HAADF image of a large set of QDs, in which the emission changes from point to point (Figure 3b). On this set, we have acquired a SI in which the spatial sampling has been set approximatively to the size of a single QD, as seen on the HAADF image acquired in parallel (Figure 3c). The acquisition time of the SI is around 10 min. As in the previous case, comparing the HAADF and the SI results, we observe that some of the QDs do not emit, as clearly seen on the intensity map Figure 3d. Also, the intensity variations follow the thickness variations. The wavelength dispersion from QD to QD (Figure 3b and e) is on the order of 40 nm, and no correlation could be found between the wavelength changes and the thickness variations. The fwhm spatial variations shown in Figure 3f do not follow any clear trend. From the fact that neither the wavelength nor the fwhm follows the thickness, we can deduce that no obvious heating effects are seen in nanoCL, even when several QDs may be stacked along the electron beam direction. The number of pixels containing at least one QD is larger than 2000, with approximatively 600 that are emitting. This shows how quickly information on the optical properties of a dense ensemble of QDs can be obtained. The fact that a large number of QDs are not emitting is related to the imperfect yield of this new synthesis of thick-shell QDs, leading to the likely presence of nonradiative centers at the surface of the QDs. We have shown that nanoCL spectroscopy can be used to study individual II−VI QD optical properties. By comparing with PL spectroscopy from the very same individual QDs, we have shown that both spectroscopies offer essentially the same spectral information. Common sources of heating and nonlinearities previously attributed to electronic excitation12 are here shown to be irrelevant in a STEM. While PL is limited by the optical diffraction limit and regular (low-voltage) SEM on thick samples is limited by the excitation pear and SE 4093

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Dots by Low-Temperature Cathodoluminescence. Phys. E 2005, 26, 203−206. (10) Nogues, G.; Merotto, Q.; Bachelier, G.; Lee, E. H.; Dong Song, J. Fabrication and Tuning of Plasmonic Optical Nanoantennas around Droplet Epitaxy Quantum Dots by Cathodoluminescence. App. Phys. Lett. 2013, 102, 231112. (11) Yacobi, B.; Holt, D. Cathodoluminescence Microscopy of Inorganic Solids; Springer: New York, 1990. (12) Rodriguez-Viejo, J.; Jensen, K. F.; Mattoussi, H.; Michel, J.; Dabbousi, B. O.; Bawendi, M. G. Cathodoluminescence and Photoluminescence of Highly Luminescent CdSe/ZnS Quantum Dot Composites. Appl. Phys. Lett. 1997, 70, 2132−2134. (13) Miller, D. A. B.; Chemla, D. S.; Damen, T. C.; Gossard, A. C.; Wiegmann, W.; Wood, T. H.; Burrus, C. A. Band-Edge Electroabsorption In Quantum Well Structures  The Quantum- Confined Stark-Effect. Phys. Rev. Lett. 1984, 53, 2173−2176. (14) Zagonel, L. F.; Mazzucco, S.; Tence, M.; March, K.; Bernard, R.; Laslier, B.; Jacopin, G.; Tchernycheva, M.; Rigutti, L.; Julien, F. H.; Songmuang, R.; Kociak, M. Nanometer Scale Spectral Imaging of Quantum Emitters in Nanowires and Its Correlation to Their Atomically Resolved Structure. Nano Lett 2011, 11, 568−573. (15) Tizei, L. H. G.; Kociak, M. Spatially Resolved Quantum NanoOptics of Single Photons Using an Electron Microscope. Phys. Rev. Lett. 2013, 110, 153604. (16) Javaux, C.; Mahler, B.; Dubertret, B.; Shabaev, A.; Rodina, A. V.; Efros, A. L.; Yakovlev, D. R.; Liu, F.; Bayer, M.; Camps, G.; Biadala, L.; Buil, S.; Quelin, X.; Hermier, J. P. Thermal Activation of NonRadiative Auger Recombination in Charged Colloidal Nanocrystals. Nat. Nanotechnol. 2013, 8, 206−212. (17) Koberling, F.; Mews, A.; Philipp, G.; Kolb, U.; Potapova, I.; Burghard, M.; Basche, T. Fluorescence Spectroscopy and Transmission Electron Microscopy of the Same Isolated Semiconductor Nanocrystals. Appl. Phys. Lett. 2002, 81, 1116−1118. (18) Al Salman, A.; Tortschanoff, A.; Mohamed, M. B.; Tonti, D.; van Mourik, F.; Chergui, M. Temperature Effects on the Spectral Properties of Colloidal CdSe Nanodots, Nanorods, And Tetrapods. Appl. Phys. Lett. 2007, 90, 093104. (19) Fernee, M. J.; Plakhotnik, T.; Louyer, Y.; Littleton, B. N.; Potzner, C.; Tamarat, P.; Mulvaney, P.; Lounis, B. Spontaneous Spectral Diffusion in CdSe Quantum Dots. J. Phys. Chem. Lett. 2012, 3, 1716−1720. (20) Spinicelli, P.; Buil, S.; Quelin, X.; Mahler, B.; Dubertret, B.; Hermier, J. P. Bright and Grey States in CdSe−CdS Nanocrystals Exhibiting Strongly Reduced Blinking. Phys. Rev. Lett. 2009, 102, 136801.

reemission, nanoCL is only limited by the object size, which itself defines the necessary spatial resolution. We have shown how this gain in resolution can be used to rapidly characterize the emission properties of a given set of individual QDs. We focused in this work on the spectral information, which requires relatively long acquisition times. It might be interesting to upgrade the setup with fast, high-sensitivity, low-loss serial detection. This would allow us to address with the improved spatial resolution other issues like QD blinking.20 This work paves the way toward the systematic study of II−VI and III−V QDs at the single QD scale. Also, it offers the possibility of studying plasmon/II−VI QD coupling at the nanometer scale, much in the same way as has been done for III−V systems.10

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.K. thanks M. G. Walls for his careful reading of the manuscript. This work has received support from the French National Agency for Research under the program of future investment TEMPOS-CHROMATEM with the reference ANR-10-EQPX-50 and the HYNNA project. The research leading to these results has received funding from the European Union Seventh Framework Programme [No. FP7/2007- 2013] under Grant Agreement No. n312483 (ESTEEM2). The authors acknowledge the Région Champagne-Ardennes, the Conseil général de l’Aube, and the FEDER funds through their support of the regional platform Nano’mat.

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REFERENCES

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