Ultramicroscopy 174 (2017) 50–69

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Ultramicroscopy

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Cathodoluminescence in the scanning transmission electron microscope MARK ⁎ M. Kociaka, , L.F. Zagonelb a Laboratoire de Physique des Solides, Université Paris-SudParis-Sud, CNRS-UMR 8502, Orsay 91405, France b “Gleb Wataghin” Institute of Physics University of Campinas - UNICAMP, 13083-859 Campinas, São Paulo, Brazil

ARTICLE INFO ABSTRACT

Keywords: Cathodoluminescence (CL) is a powerful tool for the investigation of optical properties of materials. In recent Cathodoluminescence years, its combination with scanning transmission electron microscopy (STEM) has demonstrated great success STEM in unveiling new physics in the field of plasmonics and quantum emitters. Most of these results were not Nano-optics imaginable even twenty years ago, due to conceptual and technical limitations. The purpose of this review is to Plasmonics present the recent advances that broke these limitations, and the new possibilities offered by the modern STEM- Quantum emitters CL technique. We first introduce the different STEM-CL operating modes and the technical specificities in Band gap measurements STEM-CL instrumentation. Two main classes of optical excitations, namely the coherent one (typically plasmons) and the incoherent one (typically light emission from quantum emitters) are investigated with STEM-CL. For these two main classes, we describe both the physics of light production under electron beam irradiation and the physical basis for interpreting STEM-CL experiments. We then compare STEM-CL with its better known sister techniques: scanning electron microscope CL, photoluminescence, and electron energy-loss spectroscopy. We finish by comprehensively reviewing recent STEM-CL applications.

1. Introduction mixture of charge density waves and photons form at the surface of nanoparticles when their sizes become comparable to the Cathodoluminescence (CL) i. e. the emission of light from a wavelength of light [2]. These are surface plasmons (SPs). Their material upon interaction with an electron, is a well-known phenom- resonance frequency depends on the size and geometry of the enon. As a regular characterisation technique, it has been applied with nanoparticles, and can thus be tuned by changing these parameters; great success in geological sciences and for the characterisation of it also depends on the local dielectric environment of the nanopar- semiconductors [1]. In the past fifteen years it has undergone a major ticle. Because they are resonant, these excitations dominate the rebirth related to the development of optically active nanomaterials optical spectrum of small metallic nanoparticles. Finally, SPs make and nanostructured materials. This rebirth is certainly linked to the it possible to concentrate the electromagnetic field at the nanometer fact that an electron beam can be made arbitrarily small compared to scale. These properties - size and shape resonance-energy depen- the nanometer scale at which the three main phenomena that are dance, local environment sensitivity, electromagnetic energy focus- driving the optical behaviours of nanomaterials or nanostructured ing - indicate a bright future for SP applications such as sensing or materials occur. In other words, such phenomena happen at scales that cancer therapy. are hardly reachable with conventional far-field diffraction limited • Band gap variations at the nanometer scale (Fig. 1b): the energy optical techniques. In contrast, modern Scanning Electron Microscope band-gap in a semiconductor largely determines its absorption (SEM) or scanning transmission electron microscopes (STEM) can properties, and, if it is luminescent, its luminescence properties. nowadays form extremely small electron probes (<1nm) and benefit In semiconductors or insulators, luminescence arises through the from optimised light detection schemes. Although such small probes creation of charged carriers (electron-hole pairs in case of CL or can lead to very small excitation volumes, they do not lead necessarily photoluminescence, PL) that subsequently recombine radiatively. In to high spatial resolutions. However, we will see that under certain the absence of deep recombination centres (see later), most of the circumstances they do, which justifies in part the writing of this review. recombinations arise close to the band gap energy. Minor differ- The three main phenomena under discussion (see Fig. 1) are: ences between the emitted light and the band gap energies may be due to the presence of shallow defects close to the valence or • Plasmon confinement (Fig. 1a): standing waves made up of a conduction band to/from which electron-hole recombination occur

⁎ Corresponding author. E-mail address: [email protected] (M. Kociak). http://dx.doi.org/10.1016/j.ultramic.2016.11.018 Received 29 July 2016; Received in revised form 16 November 2016; Accepted 18 November 2016 Available online 19 December 2016 0304-3991/ © 2016 Elsevier B.V. All rights reserved. M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69

Fig. 1. Summary of the main physical properties that are efficiently investigated in STEM-CL (a–c), and how different information can be obtained by this technique (d–o), as described in detail in this review. (a). Plasmons emission arises from the deexcitation of plasmonic waves, which are essentially charge density waves. The figure shows the surface charge distribution for a triangular prism for one of the degenerate dipolar modes and for the hexapolar mode. After [3], reprinted with permission. (b). Local optical transition energy (optical band gap) variations in semiconductors. The optical property variations might arise from a local change in composition or stress, changing the “bulk” energy band gap, or from quantum confinement. In either case, electron-hole pairs will recombine where locally the energy (E) of the excited states (here schematised as the energy difference between the conduction band CB and the valence band VB) is the lowest (here along an arbitrary axis x). (c). Point defects and related atomic-like defects. Point defects or atom inclusions in a relatively high band gap material can behave as an idealised two levels system lying in the gap of the host material. (d). HAADF image of a 360 nm wide silver triangle. (e). CL polychromatic maps (see text) showing the spatio-spectral behaviour of plasmon modes; this can be directly correlated to the morphological information in (d). (f). Two typical spectra have been extracted at the tip (orange) and the side (green) of the prism. (e). (g). Angle-resolved CL spectroscopy of a plasmonic band gap material consisting in a series of silver pillars. The periodicity induces a band gap that is directly detected in STEM-CL. (h). HAADF image of a series of GaN quantum discs (QDisk) (bright contrast) embedded in a AlN (dark contrast) shell. (i). Related polychromatic map, showing individual QDisk colour variation. (j): Spectra extracted on two different QDisks. (k). Time correlation functions taken at the centre of each disc allowing to determine the QDisks lifetimes (indicated close to the corresponding peak). (l). HAADF of an h-BN flake. Because the flake is very thin, its HAADF contrast is essentially null, to be compared with the CL contrast in (m). (m). Monochromatic image filtered at the energy of an individual defect, the signature of which is shown in (n). (o). Time correlation function displaying a dip indicating single photon emission. (d-f) adapted from [4], (g) from [5],(h–k) from [6] and (l–o) from [7]. Reprinted with permission.

(in the case of near band edge, NBE) and larger ones to the material embedded in a higher energy gap semiconductor has at occurrence of excitonic recombinations or shallow defect recombi- least one of its dimension smaller than a typical exciton Bohr radius, nations. There are two main reasons for band gap modulation at the which varies from tens of nanometers to tens of ångströms. The two nanometer scale. The first reason might be a change of the material effects, of course, can compete to change the local emission composition or characteristics - for example, a desired or undesired wavelength. Such band gap monitoring and engineering is of the

change of the value of x in an InGaxx1− Nmaterial, or a change of the utmost interest in many fields: from solid state lighting (Light stress states that modify the local energy gap. The second reason Emitting Diodes, LEDs), bioimaging (for example using II-VI might be some quantum confinement, i. e. the fact that the electron quantum dots), photovoltaics, etc. and hole wavefunctions can be confined when a semiconductor • Structural defects (Fig. 1c): Structural defects have a range of effects

51 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69

on luminescence properties. For example, some dislocations can act as non-radiative centres, quenching the emission of the object of interest, but others can be radiative. Colour centres can be highly absorbing and/or emitting, dominating the optical properties of the host matrix in the visible range - consider for example the case of a ruby stone. Colour centres are of particular interest, as they are a solid-state realisation of a two-levels system (TLS). Among their other properties, TLSs can be used as single photon emitters (SPEs), with applications in quantum computing and quantum cryptogra- phy.

Because the optical properties of nanomaterials are determined largely at the nanometer scale or below, and are intimately linked to the morphology or even the atomic structure or chemistry of the object of interest, SEM-CL and STEM-CL are clearly techniques of choice. STEM-CL has ultimately a better spatial resolution (a few nanometers Fig. 3. Mapping quantum emitters with STEM-CL. Top: HAADF image of a series of proven now [8,9]) for probing optical properties of semiconducting GaN quantum discs (bright) embedded in AlN (dark). Bottom: Multicolour CL image obtained on a stack of GaN quantum discs (QDisks) embedded in an AlN nanowire. This materials, it can be less intrusive (no heating, no non-linearities), has a image has been obtained by colouring each wavelength-filtered image of a CL SI with the higher sensitivity for plasmons in very small nanoparticles [10,11], false colour corresponding to the energy (see bottom scale), and then overlapping each of and can be easily coupled to ultra-high resolution imaging and them without any other data treatment. Note that despite the high contrast in the CL chemical imaging techniques. STEM-CL was pioneered in the late image, a much more thourough information extraction process must be applied to the 70's and early 80's [12–15]. However, it appears that up to the spectral image to isolate individual QDisks' emission wavelength, see text. A rectangular beginning of the 2000's, technological (mirror machining, optical area is displayed to emphasise the link between morphology (HAADF) and optical properties (CL). Accelerating voltage is 60 keV. Adapted from [8]. fibres, CCD cameras…) as well as conceptual reasons (such as the difficulty of believing that CL could be employed for plasmonics or that tions, see Section 3.1) has been covered in a reference book [1]. the electron-hole diffusion could be small enough to address individual Applications of SEM-CL to quantum confined semiconductors can be quantum-confined structures) slowed down the emergence of disrup- found in a comprehensive review [23]. F. J. Garcia de Abajo has tive applications of STEM-CL. The situation changed with the first covered S(T)EM-CL theory for coherent excitations, mainly, in his mapping of plasmons by Yamamoto [10] shown in Fig. 2 in the early excellent and dense review article on nanophotonics with electrons 2000's and the spectral-mapping of quantum-confined GaN structures [24]. Tutorial papers on plasmon mapping with fast electrons (includ- embedded in an AlN nanowire [8] (Fig. 3) ten years later. Both ing CL) presenting simplified theories [2,25] and detailed instrumenta- demonstrated that STEM-CL enables the investigation of the nano- tion explanations [2] have been recently published. A more generic optical properties of material at the relevant scale: that of the plasmon review describing electron-based plasmon spectromicroscopy with an spatial modulation or quantum confinement lengths. emphasis on time-resolved techniques has been given [26]. Recent The still nascent field of STEM-CL will thus be studied here; it is reviews on SEM-CL applied to coherent [27] and incoherent [20] worth noting that the parallel revival of SEM-CL -also formerly excitations have been published. A review of the early stages of STEM- neglected for similar reasons- has led to impressive successes in the CL for incoherent excitations has been published a decade ago [28];a past years [16–22]. It is by no means our goal to artificially promote more recent review of applications of STEM-CL has been given [29]. STEM-CL. The two techniques are extremely similar, share many The general field of STEM-CL and its applications has however not interests and concepts, and have their pros and cons defining niche been reviewed in recent years, especially concerning incoherent applications, which will be discussed later in the paper. excitations. Our goal is thus to present a technique which has a SEM-CL (mostly excluding plasmons and other coherent excita- tremendous potential for nanooptical applications, and that has not been subject to comprehensive books, review articles or lectures over the past fifteen years, in which major steps have been made.

2. Instrumentation

2.1. The scanning transmission electron microscope, the spectral imaging and other related CL acquisition schemes

2.1.1. scanning transmission electron microscope and accessories A scheme of a modern STEM is presented in Fig. 4; a detailed discussion of all elements may be found in any textbook on STEM (the interested reader may find useful a more comprehensive description relevant to CL in [2]). An electron beam is focused onto an object of interest forming a very narrow probe. Given the high energy of the electrons (typically from 60 keV to 300 keV) and the subsequent picometer range of their wavelength, the ultimate resolution is limited mainly by the geometric aberrations of the microscope. Nowadays, an ångström-sized electron probe carrying a hundred pA is easily attain- able, although many applications in STEM-CL are likely to need only a Fig. 2. Mapping plasmons with CL in a STEM. Left: Series of CL spectra taken at various few tens of ångströms and tens of pA. Neither values are requiring electron beam positions on a 140 nm diameter silver nanoparticle. Right: a) Secondary electron image of a 140 nm silver nanoparticle. b) c) d) e) filtered maps of the dipolar and aberration-corrected microscopes, but a high brightness gun is cer- quadrupolar modes as seen along two different polarisation directions. Electron beam tainly necessary, see e.g. [6]. After interaction with the sample, the acceleration voltage is 200 keV. After [10], reprinted with permission. electron may be scattered at high angle into a high angle annular dark

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Fig. 4. (a). Scheme of a STEM fitted with CL and EELS detectors. Cond. Lens: condenser lenses; SC: scanning coils; Obj. Lens: objective lens. The light collected by the mirror can be analysed in multiple ways, described on the adjacent panels: (b). the photon beam can be directed to the entrance of a spectrometer via a collecting optics (here schematised by a convergent lens, but other means, typically optical fibres, can be used as well). A spectrum can then be acquired on a CCD camera. When the beam is scanned, one spectrum can be acquired per pixel, leading eventually to a spectrum image, the most widely discussed mode in this paper. Alternatively, a photo-multiplier (PM) can be used instead of a camera. In such a case, a slit can be used to wavelength-filter the light before hitting the PM. Upon scanning, an energy (wavelength) filtered map can be acquired. (c). Alternatively, angle-resolved emission patterns (for a given beam position) can be obtained by imaging the mirror onto a camera. The light can be optionally energy filtered and/or polarised. (d). Another possibility for angle-resolved pattern acquisition is to scan a pinhole in a plane where the light is collimated, and to send the remaining light onto a spectrometer. In this way angle-resolved spectra can be acquired. (e). Another possibility consists in sending the collected light to an intensity interferometer. It is made up of a beam splitter (BS) with one photon detector (PM or avalanche photodiodes, APD) attached to each of the two paths. The photon detection electronic signal is then sent to the required correlation electronics in charge of reconstructing a correlation function.

field (HAADF) detector: the heavier the atoms under the probe are, the sending it to a spectrometer. Getting a single spectrum at a given larger the HAADF signal. The electrons that have not been deflected position is of course not sufficient for an in-depth analysis of the optical can be collected by a bright field (BF) detector (not shown), giving an properties of nanomaterials, therefore spectral-images (SI) have to be image which is to first order complementary to that of the HAADF. By acquired. loosing energy, the primary electrons may trigger several events. In a STEM, HAADF, BF or SE images are acquired by scanning the Secondary electrons (SE) can be generated and then collected. The beam onto the sample and acquiring the respective scalar signal at each primary electrons' energy losses can be measured by energy-dispersing point of the scan, so that a scalar image can be eventually recorded. the electrons in an electron spectrometer, giving information on the Similarly, spectral-images, i.e. images with a spectrum at each pixel, chemistry of the material, or its optical absorption (see Section 3.2.3). can be acquired for example for CL spectra (see Fig. 4b). This gives This is known as electron energy-loss spectroscopy (EELS). More high-precision access to the spatial variations of given spectral features. importantly in the context of this review, the energy given by the Moreover, several signals can be recorded at once (for example, electron to the material can be radiated back into the far-field. If this HAADF, BF, EELS and CL signals), making it possible to compare, happens in the infra-red/visible/ultra-violet (IR/Vis/UV) range, it can pixel-by-pixel, morphological, chemical and optical information [6]. be gathered by a CL collection and detection system; such a system is This correlation is of prime importance in understanding the physics of described at some length in [2] and its optimisation will be discussed optical phenomena at the nanometer scale. shortly in the following sections. Finally, emission can happen in the X- Note that a SI includes a huge amount of information. Extracting Ray range, and Energy Dispersive X Ray spectroscopy (EDS), not this information requires specific tools, from the simplest (spectra shown here, can provide chemical information on the sample. extraction or image filtering) to the more involved (multivariate analysis [30–32]) through sequential peak-fitting of the whole dataset 2.1.2. Spectrum measurement and spectral imaging [8,33].InFig. 1 (e) and (i) we have used a particular compression Fig. 4b–e present different CL acquisition set-ups. Probably the scheme, presented in details in [6]. Although a SI is typically acquired most widely used detection scheme is the one described in Fig. 4b. It spectrum-by-spectrum, it can also be analysed as a stack of energy- consists in simply collecting the light coming out of the CL mirror and filtered images. With this in mind, one can attribute a different colour

53 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69 to each different energy-filtered image, following a predetermined 2.1.4. Intensity interferometry colourscale. We note that in Fig. 1e, the colourscale roughly corre- Very generally, the beam coming out of the CL collection system sponds to the absorption colours, while in Fig. 1i it is arbitrary, the (basically, a mirror) can be analysed with any conventional optical tool, energy being above the human visible spectrum. In each energy-filtered although the whole SEM-CL and almost all the STEM-CL literature map, the intensity of a given pixel is used to weight the red-green-blue concentrates on the above-mentioned modes, which only make use of (RGB) encoding, meaning that within each map the colour is preserved the detected CL intensity. However, more complex quantities, such as but is more or less intense. Each energy-filtered map therefore results the time-correlation function g(2), which reveals second order informa- in an RGB image. All the RGB images corresponding to all the filtered tion in the CL intensity, can be measured. This function is given by: images are finally summed to provide a final, compressed, RGB gτ(2)()=⟨()(+ItIt τ )⟩, and gives the probability of detecting one photon at Iτ()2 representation of the SI. By no means are these representations time τ when already one has been detected at time 0. comprehensive, but they are relatively fair in the sense that they do gτ(2)( ) functions can be measured thanks to an intensity interferometer fi not rely on a speci c data analysis treatment. Of course, quantitative as depicted in Fig. 4e. Such an interferometer, sometimes called the fi analysis is required, as already said. Therefore, speci c tools have been “Hanbury-Brown and Twiss” (HBT) interferometer [37], is simple. The developed to analyse spectral-images, in particular the open-source beam is sent onto a beam splitter. Each split beam is sent onto a suite Hyperspy [34]. The spectral-imaging mode will be extensively different detector (usually a photomultiplier (PM) or a avalanche illustrated in this review. photodiode). The two signals are correlated using specifically designed In certain occasions, a serial detector, such as a photomultiplier electronics to produce a g(2) measurement [38]. One of the two (PM) may be positioned after the spectrometer in place of the camera detectors signals is usually delayed to allow for symmetric correlation (not shown in Fig. 4b). Then, maps of the CL intensity (instead of a full measurement. fi SI) can be formed. These maps may be ltered, if a slit is provided after Applications of gτ(2)( ) measurements will be given in Sections 3.4.1 the spectrometer, or panchromatic if there is no slit. This mode, once and 3.4.2. widely used [28], is however essentially obsolete now thanks to the high sensitivity and increasing readout speed of the modern CCD or CMOS cameras. Also, it is likely that other parallel detectors will be 2.2. STEM-CL technical constraints used in the future. For example, parallel PM arrays would give the extreme speed of the PMs together with the wealth of information A long discussion about the requirements for optimisation of a provided by parallel detection. STEM-CL detector can be found in [2], and a more general description Finally, a polariser might be placed in the path of the photon beam for CL irrespective of the type of microscope used [20] has also been (not shown in Fig. 4b). As described in the next section, the use of a given. Here, we want to recall just the essential ideas when designing or polariser together with high numerical aperture (NA) optic has to be using a CL system. As we will see in Section 3, the signal can be handled with care. Indeed, a high NA optic will change the local extremely weak compared to the case of the SEM-CL, where both the polarisation of off-axis light rays in a complicated manner. In the interaction cross-section and the incident current can be increased general case, the original polarisation cannot be retrieved without the dramatically. Collection and detection optimisation are thus critical knowledge of the light rays intensity distribution, a knowledge which is here. It is more involved in STEM-CL than in SEM-CL. Indeed, besides basically integrated in the configuration described in Fig. 4b). This other practical details such as higher magnetic field in the pole-piece problem can be avoided by restricting the detection angles to a narrow region and higher vacuum, the main restriction is the lack of room in a range centred on the optical axis. Then both polarisations, parallel and STEM pole piece. This constrains the size of the mirror to around 2 mm perpendicular to the sample plane, may be roughly discerned. Doing thickness in a typical Cs-corrected STEM, while this value can easily be this of course induces a loss of the detected signal. increased to centimetre or more in a SEM. Given these constraints, in STEM-CL, we want to gather as many photons as possible, and we want to disperse them through an optical 2.1.3. Angle-resolved measurement spectrometer onto a parallel detector (a CCD typically) ideally with the As described in Fig. 4c and d, angular analysis can be performed on highest spectral resolution and of course with the smallest loss of the photon beam exiting from the mirror. There is indeed a one to one photons from the time they are collected to the time they hit the correspondence between the position of a pixel in the far field image of detector. Thus, obviously, all optical elements have to be optimised for the mirror, and the direction of propagation of the photon beam out of minimising losses, which includes matching all NAs along the photon the sample. The emission pattern can thus be retrieved from an image beam path. The detector should obviously also be as efficient as of the mirror through a mathematical transformation [35]. There are possible. two major ways of acquiring this image. The first one, depicted in Moreover, one can show that the condition of having the highest Fig. 4c consists in projecting the image onto a CCD. When needed, the spectral resolution together with the highest collection angle leads to a beam can be filtered in wavelength, to isolate mode-specific patterns contradiction, so that in general extreme care must be taken in design [5,19]. The full angular and spectral information could then be and mechanical alignment. Some trade-off between field of view, theoretically reconstructed by using different filters sequentially. collection efficiency and spectral resolution has to be found. Due to However, the second form of angular distribution measurement is the design of optical spectrometers, a high spectral resolution is more efficient if the full spectral information is to be analysed. As obtained by following two rules: having the smallest spot and smallest depicted in Fig. 4d, it consists in selecting a given photon path (thus a angle (smallest numerical aperture for the spectrometer) at the given direction of propagation) using a pinhole, and analysing it entrance of the spectrometer. The spot formed on the sample appears spectrally through a spectrometer. Then, by scanning the pinhole, a as a magnified spot at the spectrometer entrance. So, if we want to full angular and spectral mapping can be performed. An application of match the numerical aperture of the collection optics (NAc) to that of this mode is given in Section 3.2.3. the spectrometer (NAs), we need an optical system having a magnifica- In both modes, a polariser can be used along the beam path. The tion equal to NAcs/ NA . We see here the paradoxical requirement of relation between the signal measured after polarisation and the having a high NAc and a high spectral resolution, as this leads to a large polarisation of the emitted beam is not straightforward due to the spot at the entrance of the spectrometer when a small one is required. high numerical aperture of the collecting optics. Nevertheless, the full Therefore, if a smart trade-off is not found, one is obliged to use a small polarisation information can be retrieved using dedicated schemes slit at the spectrometer' entrance, at the price of a loss in intensity. [36]. Also, as shown in Fig. 5, this restricts the field of view attainable for a

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Fig. 5. Basic constraints on a spectrometer for CL spectral imaging. For the sake of simplicity, the photon beam path has been straightened and aligned with the electron beam path. The collection optic has to collect the largest signal (high numerical aperture), Fig. 6. Coherent and incoherent excitation in CL. (a). Coherent excitation. ① An electron while at the same time the spectrometer has a small numerical aperture. This results in a impinges on a nano-object (here a metallic nanorod) and ② polarises it (here, only the large magnification of the source (O1) at the entrance plane of the spectrometer (here, the longitudinal dipolar plasmon mode is shown). ③ The excited mode freely oscillates which magnification has been considerably reduced for the sake of the representation). Also, the may lead to radiation in the far-field. (b). Incoherent excitation. ① An electron impinges size of the light spot at the entrance of the spectrometer defines its spectral resolution. on a thin piece of material. ② When the incoming electron is inside the material, it Any shift of the source (O1 to O2) with respect to the focal point of the collection optics creates a bulk plasmon (among others but much less probable excitations), that ③ quickly (either due to a misalignment or a large scanned area) is magnified by the collecting- de-excites in the form of a small number of electron-hole pairs. ④ the charge carriers can propagating optics (I1 to I2), which leads rapidly to a vanishing of signal reaching the then drift for about a typical diffusion length distance (note that the charges are not spectrometer. necessarily bound together, contrary to this example), before ⑤ de-exciting either non- radiatively (black stars) or radiatively (yellow one). given spectral resolution given by the entrance spot size, as a slight 3. Physical principles and applications misalignment between the electronic and optical paths may result in a photon beam shifted away from the entrance of the spectrometer. 3.1. Coherent and incoherent excitation These limitations stem from a general physical principle, namely the conservation of the etendue (see e.g. [20]), which is basically the There exists essentially two ways of creating luminescence from a consequence of Liouville's theorem. Applied to the present situation, it sample with a fast electron beam, as shown in Fig. 6,acoherent and an states that [20]1 incoherent way [24]. The coherent way concerns the excitations of coherent electromagnetic waves, such as surface plasmons or guided dNAs modes within nanoobjects (it is illustrated in the case of plasmons in FOV = fi 2 NAc (1) Fig. 6): the electromagnetic eld following the electron polarises the nanoparticle and thus creates a plasmon or a polariton, that may decay where FOV is the field of view in the image plane, d is the slit width of in the form of a photon. This scheme is said to be coherent because the the spectrometer or entrance spot size, whichever is the smallest. Note optionally emitted photon has a memory of the way it has been created; that the effect of a misalignment due to a shift can be accounted for by for example, an electron impinging on the same point from different assuming that the FOV is equal to the misalignment error. Therefore, angles may lead to different types of emissions. this rule must be obeyed to keep both spectral resolution and intensity. The incoherent way concerns the creation of a bulk plasmon, that A slit of 100 μm and a typical NAs =1/10 are typical values for a very rapidly decays in the form of electron-hole (e–h) pairs. It is said to NAc reasonable spectral resolution and collection efficiency. This means be incoherent because the e-h pairs do not have any memory of the that a precision at least better than 5μm in all the three spatial incoming electron. Once created, the e-h pairs may diffuse into the directions must be realised to get both high collection and high spectral material, and find energy minima where they can recombine either resolution without intensity loss. In practice, the alignment precision radiatively or non-radiatively. might even need to be better, as a typical mirror (parabolic or elliptical) This difference is very clear in at least one case. If one considers the is, in terms of imaging capability, extremely aberrated as soon as the emission pattern, the situations prevailing in the two excitation modes source point is displaced from the focal point. are totally different. In the incoherent case, the emission pattern only The consequences for practical systems are that 1: alignment depends on the object characteristics: electronic band structure details, between the mirror and the sample has to be better than a few microns shape of the nano-objects, etc. Whatever a particular excitation, say, an in the three dimensions and 2: the sample has to be movable exciton, is created, the emission pattern will be the same. The situation independently of the mirror, otherwise only areas a few tens of microns is opposite in the case of coherent emission. For example, a plasmonic square can be studied on each sample. Finally, for the same reason, the nanoantenna excited on one tip will emit light in the direction of the larger the field of view, the poorer the spectral resolution. Indeed, when other tip: the light emission pattern depends both on the geometry of scanning large regions, one expects the spot at the entrance of the the object and on the interaction geometry. spectrometer to move (and thus the spectra to shift on the camera) and As a final note, one can generally say that the spectra of coherent possibly not to enter the spectrometer. Edwards et al. have been excitations are best interpreted using the analysis of the Maxwell discussing several workarounds for this situation, the conceptually equations and related boundary conditions, while the spectra of simplest one consisting in scanning the sample instead of the beam incoherent excitations are best interpreted using the analysis of the [20]; another more flexible possibility is to use a bundle of optical fibres Schrodinger equation and related boundary conditions. that is optimised to conserve as much as possible intensity and spectral resolution, as discussed in [39]. 3.2. Coherent excitation

3.2.1. Transition radiation and cerenkov emission The most simple form of coherent excitations produced by an electron is probably Cerenkov emission. The light cone of Cerenkov 1 A similar equation can be found in [20]. The difference here is that our expression is radiation is always aligned on the electron beam path, as a clear valid for arbitrarly large NAs. demonstration that the Cerenkov emission is coherent.

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In vacuum, an electron plane wave and a photon plane wave cannot semi-analytically solvable systems to guide the intuition, see [2] for a exchange energy, because their momenta do not match for any energy: discussion on this point. there is no Cerenkov emission in vacuum. However, energy and A general description, taking advantage of the very intuitive momenta matching may arise in an insulating infinite medium, and formalism developed for Near-Field optics has however been given in this is the reason for Cerenkov emission. For plasmons, there is also the [53]. In this paper, it was shown that the signal measured in EELS, as a need for the energy and momentum to be conserved. In this case, the function of position and energy, was very close3 to a quantity known as momentum matching arises from the fact that their related electro- the electromagnetic local density of states (EMLDOS), projected along magnetic field is evanescent and therefore contains a large distribution the electron beam path (zEMLDOS). In a nutshell, for a given structure, of plane wave momenta, see e.g. [24]. Note that in the next section, we the zEMLDOS is a 3D quantity that corresponds spatially to the will use a different formalism better adapted to plasmons (the modulation of the square of the electrical eigenfield (projected along electromagnetic local density of states). The emission of Cerenkov z), and spectrally peaks at the eigenenergies of the structure. As the radiation depends on the electron speed and the medium's optical zEMLDOS is proportional to the square of the eigenelectrical field, it index, the speed of the electron needing to be larger than the speed of also tells directly how the density of electromagnetic energy varies light in the medium for the momentum matching to be possible. locally. In the quasistatic case, i.e for objects small enough so that the If the medium possesses an interface, another type of coherent Maxwell equations reduce to the Poisson equation, the interpretation emission arises: transition radiation (TR). There are different possible can be made even simpler: the zEMLDOS gives the spatial distribution interpretations for the mechanism leading to light emission in TR of the eigenpotential of the structure. [24,40]. One is that when the electron traverses the surface, it However, the interpretation for CL is slightly more involved. experiences a sudden change of optical indices, and thus a sudden Indeed, for non-dissipative modes, such as Bloch waves in a photonic deceleration inducing then light emission. Another interpretation band gap, all the energy lost by an electron is transformed into consists in evoking the image charge that the electron creates in the radiation. In these cases, EELS and CL are exactly identical, and the medium when it is outside. The image charge and the charge itself form former interpretation applies [53]. However, in the more common case a dipole of varying amplitude as the electron and its image travel of dissipative excitations, such as plasmons, part of the energy given by towards each other and eventually collapse, and therefore produce the incoming electron to the medium is dissipated in the form of heat, radiation. Whatever the interpretation, the TR produces a wide band and therefore not re-emitted. By analogy with the EELS case, and spectrum that can be analytically deduced [24], and which only understanding that CL probes only the radiative modes, it is tempting depends on the dielectric constant of the medium and of the electron to identify CL with a different EMLDOS [55], specifically the radiative speed. TR was first explored in the context of CL by Yamamoto, using a EMLDOS (rEMLDOS), which precisely describes the density of radia- STEM [41]. Recently, Brenny et al. [40] used the robustness and almost tive states [56]. This analogy has been made rigorous recently [57]. The universal character of the TR radiation emission to calibrate the CL EELS is still related to the zEMLDOS, while the CL is related to the spectral acquisition chain. zrEMLDOS. This later quantity has the same spatial properties as the In most of the applications of STEM-CL, the TR are relatively weak full zEMLDOS. Therefore, modes are mapped spatially the same way in and uniform in energy. They usually have no real influence on the EELS and CL. This is demonstrated experimentally in Fig. 7a, where detection of peaks and can be safely ignored when using SI, or maps of the eigenmodes of a silver triangle are almost identical for the corrected for by subtraction from a spectrum taken outside the region two first eigenmodes. of interest. If panchromatic imaging is to be used, then TR might be an However, two differences emerge between the two quantities, as well issue for weakly emitting objects [42]. as between EELS and CL. First, of course, only radiative modes Finally, it is worth noting that the frontier between Cerenkov and participate in the rEMLDOS. More precisely, the more radiative a mode, TR emission blurs in objects with boundaries [43], and that Cerenkov the higher its weight in the radiative EMLDOS. This is exemplified in the emission can be used as a probe of photonic devices [44,45]. case of a gold nanoprism small enough so that only one of its modes is radiative, see Fig. 7, left. This mode, peaked at the tip of the triangle, is 3.2.2. Theoretical foundations for the interpretation of coherent well known to be dipolar. Now, in the quasistatic limit, only dipolar excitations in STEM-CL modes are radiative. Therefore this dipolar mode is the only visible mode The theoretical description of the interaction of an electron beam with CL, while all modes are seen in EELS. Second, the spectral response with coherent electromagnetic modes, such as surface plasmons or is slightly different between the full and the radiative zEMLDOS. guided modes is in principle relatively simple. A common and very Consequently, EELS and CL spectra are shifted, see Fig. 7, right. robust assumption2 is to assume that locally the electromagnetic Although it might sound counterintuitive that the resonance energy response of a material is well described by the dielectric constant of value of an eigenmode depends on the technique used to measure it, it is the corresponding bulk material. Within this so-called local continuum a well-known property of damped excitations [58]. dielectric theory [46], the interaction of an electron and a material and Two well-established macroscopic spectroscopic optical quantities, therefore the EELS and CL signals, can be directly computed by solving namely the extinction and the scattering cross-sections exhibit the the Maxwell equations [24]. The required boundary conditions are same shift. The extinction cross-section quantifies the energy taken out given by distinguishing regions of different dielectric constants, and the of an incoming photon beam by absorption or scattering due to its electron beam is supposed to be a point charge travelling at constant interaction with a nanoparticle. The scattering cross-section quantifies speed and acting as a source term in the Maxwell equations. the energy transferred through the scattering alone. It is possible to Many numerical solutions exist to this problem, most of them now formally link the spectral properties of EELS to the extinction, and the openly available, including the boundary element method (BEM [47– that of the CL to scattering [11,57], explaining the close similarities in 49]), the discrete dipole approximation [46,50,25], the green dyadic the shifts. One of the consequences of this correspondence is that the method [51] or time domain finite difference [52]. Nevertheless, they EELS intensity4 scales as the volume of the object of interest, while the give very little physical intuition on how to interpret STEM-CL maps in simple terms. Unfortunately, there are also only very few analytically or 3 The correspondence is not exact. There exists a Fourier transform along the electron beam direction to be done, that makes regular EELS and CL spatial variation difficult to 2 We note that objects emitting light through coherent excitation (therefore objects of interpret quantitatively. In the case of EELS, 3D reconstruction techniques can make the interest for CL) are all sufficiently large for the classical assumption to work perfectly. correspondence almost quantitative, see e.g. [54]. This isn't the case for very small (less than 5nm) objects, for which quantum corrections 4 Note that we are here dealing with surface excitations. Volume excitations, like bulk to the classical assumptions are necessary [24]. plasmons, will have cross-section values scaling as the thickness of the object.

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Fig. 8. Polarised spectrally filtered CL maps of a silver nanorods. One can see the oscillations of the plasmons waves, with different localizations for each polarisation. A Fig. 7. Comparison of EELS and CL maps and spectra. Left panel: EELS (left) and CL typical cross-section is schematised at the top, showing the symmetry of the modes (right) mapping of the two first plasmon modes (dipole first two lines, hexapole next revealed by both polarisations. After [62], reprinted with permission. lines) for a relatively small gold nanoprism (first lines of dipole and hexapole) and relatively larger silver prism (second lines). EELS and CL have almost the same spatial showing the impressive ability of electron-based spectroscopy for distributions. However, the hexapolar mode does not show up in CL for the smaller plasmon mapping [10], and since then demonstrated to be a well prisms, as these modes are non-radiative in small particles. Right: EELS and CL spectra adapted tool in this respect [60–62,11,4,63]. A stereotypical case where acquired on two different beam positions of the larger silver prism. For both the dipolar and hexapolar modes, one sees an energy shift. Adapted from [4,11]. STEM-CL has an obvious interest in understanding plasmon properties has been given in [62]. In this work, large (100 's of nanometers in CL intensity scales as the square of the volume. This is exactly what length, tens of nanometers in diameters) silver nanorods were studied happens for the extinction cross-section, scaling as the volume, and the by spatially resolved STEM-CL, and a polariser was used in conjunc- fi scattering cross-section, scaling as the volume squared. tion. In Fig. 8, polarised spectrally ltered maps of two silver nanorods To end up this description, it is worth mentioning that combined are displayed. Both exhibit the expected spatial modulation, which fl EELS and CL experiments are quite involved, while independent EELS re ects the modulation of the zEMLDOS. The use of a polariser helps to or CL measurements are more straightforward and give very similar identify the nature of the modes, as they don't have the same symmetry information as soon as the particles are large enough. Therefore, a along the perimeter of the nanorod: one possesses a uniform charge combined experiment should probably only be performed when trying distribution along the cylindrical shape of the rod, therefore having an to understand the differences between the absorption and radiative almost homogenous intensity distribution in the transverse direction, properties of modes [11,57,59], or if for example the exact shape and while the other has a charge sign inversion along the section, translat- energy position [57] of the peaks are worth investigating. A special care ing in a intensity distribution peaked on the sides of the cylinder. In a has to be taken in calibrating the two different techniques' spectro- recent study, this kind of analysis has also been used to unravel the meters. Indeed, the expected shifts between both (of the order of tens physics of coupled nanoholes [63]. to one or two hundred of meV) can be rapidly smaller than a typical Alternative techniques are however worth considering. SEM-CL has EELS calibration error. Also, the optical spectrometer response is proved to be extremely powerful for single plasmonic particle analysis – usually non-uniform. In particular, it is peaked around the so-called [18,64 66,52]. However, theory predicts a higher cross section at fi “blazing” wavelength of the particular grating used with an asymmetric higher accelerating voltage, as con rmed already in the pioneering response on both sides of the peak; also, the camera used usually has a work of Yamamoto [10], and quite logically, the smallest nanoparticles non-uniform quantum efficiency. For experiments concerned with have been measured by STEM-CL [11], see Fig. 7, rather than SEM-CL. short wavelength intervals and sharp peaks (see for example Section Probably, at the moment, the advantages of SEM-CL, in particular the 3.3), these non-uniformity are causing minor effects such as small much higher versatility of a SEM with respect to a STEM and the changes in the peak intensity ratios. However, in the case of plasmons increased available currents, explain the obvious practical success of or other excitations with wide peaks, this can change their shape and the SEM-CL compared to STEM-CL for plasmonics applications. induce energy shifts, and if several peaks span a wide energy region, STEM-EELS is also extremely suitable for plasmon mapping, this induces large errors in the estimation of peak intensity ratios. especially for very small nanoparticles, see e.g. [24,2,29,67,25]. This Also, the conditions for working in EELS and CL can be quite different is because the ratio of the cross-sections of CL to EELS scales roughly (acceleration voltages when working in two different microscopes [59], as the volume of the nanoparticle [2,24], so, for particles smaller than difference in electron beam current used for the two techniques, that can typically a hundred nm, EELS is worth considering. One limitation, span orders of magnitude [2,11], potential local changes due to hydro- however, is the poor spectral resolution of EELS in most of the electron carbon or water contamination during the experiment or between microscopes (at best 100 meV, even if new monochromation techni- different microscopes…) and care has to be taken to disentangle ques are now providing 10 meV [68]). This is larger or of the order of a differences which would happen due to the different physics probed by typical plasmon full width at half maximum (FWHM). In contrast, any – EELS and CL or different technical details that could be different between well-made STEM-CL system can easily deliver a 10 30 meV spectral the two spectroscopies but not related to any relevant physics. resolution without loss of intensity. Also, well-resolved EELS micro- scopes and spectrometers are rather expensive, and EELS SI analysis may require extensive alignments and treatments [2] whereas STEM- 3.2.3. Plasmons CL is much cheaper, can fit most of the older generation microscopes Single particles. Historically, STEM-CL was the first technique and does not require extensive data treatments.

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The EELS stability is also rather poor for obvious reasons. Indeed, a Fig. 6. Therefore, there is no surprise that the way CL mapping has given feature at energy δE in an energy loss spectrum comes from an to be interpreted is very different in the case of coherent excitations. In electron having an energy of E0 − δE, where E0 is the nominal energy of particular, the concept of EMLDOS mentioned in Section 3.2.2 istotally the electron. Therefore, measuring an energy loss at δE =1eVwith an irrelevant. 5 electron beam nominal energy of E0 =10eVrequires a precision of As briefly stated before (see Fig. 6), the processes of light emission 105; measuring a FWHM of 100 meV a precision of 106. These are can be decomposed into several steps that we rediscuss in a bit more extremely stringent conditions, partly explaining the long development details here. We first consider a very thin sample. By “very thin”,we time of high resolution EELS. On the other hand, CL relies on the mean that the thickness is much smaller than the mean free path λe of detection of photons, whose properties do not essentially rely on the the electron in the material. λe depends on the material and on the primary electron energy. Also, STEM-CL or SEM-CL can give informa- accelerating voltage, with λe increasing with increasing acceleration tion on the emission directionality and full polarimetric information, voltage. For electrons of high energy, such as those used in a TEM, the which is not yet available in STEM-EELS. STEM-CL is thus a very electrons are losing energy essentially through excitations of bulk interesting alternative to STEM-EELS for quick characterisation of plasmons. The probability for an electron to experience n such inelastic tλn nanoparticles plasmonic properties. Finally, it would be tempting in scattering events when traversing a thickness t is Pe= (/e ) −/tλe. Thus, n n ! this comparison to point out the fact that STEM-CL can be used for for tλ/⪡e 1, the incoming electron will experience zero inelastic event temporal investigations of SPs (lifetime measurements, measurement with probability Ptλ0 =1− / e and 1 inelastic event (one bulk plasmon … of autocorrelation functions ). Unfortunately, plasmon lifetimes are creation) with probability Ptλ1 =/e. For the few electrons losing energy, much too short (typically tens of fs) to be reached by STEM-CL the energy given by the electron to the material will be very limited and common measurements tools and most of the PL techniques. will roughly correspond to the bulk plasmon energy (typically 20– Therefore, alternative techniques have to be found [26]. 30 eV, depending on the materials). The bulk plasmons have very short Plasmonic crystals. STEM-CL has been extensively used to study lifetimes (a few fs), and thus de-excite in the form of hot electron-hole plasmonic crystals [69–71] with the set-ups shown in Fig. 4 c and d. An pairs [72]. Due to energy and momentum conservation, typically only a example is given in Fig. 9. In this paper, the authors have studied few electron-hole pairs [73–75] can be generated.5 The hot e-h rapidly plasmonics band gap materials made up of regularly spaced silver (few ps) thermalise to the local energy minimum. They then diffuse nanorods. For a constant diameter, an energy band gap forms as seen within the material before meeting a radiative or non-radiative centre from the dispersion relation in Fig. 9a. However, the situation changes in a few tens of ps. If the e-h excite a radiative centre, it will emit light when a defect is introduced, in this case a series of nanorods with with a time lapse equal to its lifetime τe. If we assume that the lifetime different diameters (Fig. 9c). They induce new states in the band gap of the emitters is larger than any of the above-mentioned timescales, (see Fig. 9b), that can be imaged in real space as well, see Fig. 9d,e,f. several excited states can be excited at the same time. This is the case for many emitters of interest, which have lifetimes larger than a few hundred of ps. All these excited states will then re-emit in a typical time

3.3. Incoherent excitation window equal to τe. The resulting photons will then be emitted in synchronisation, as was recently demonstrated in the case of radiative 3.3.1. Physical principle of light generation by incoherent excitations point defects in nanodiamond and h-BN nanoparticles by measuring As already mentioned, there is a large difference in the way light the time autocorrelation function (see Section 2.1.4) of CL emission, emission is triggered for coherent and incoherent excitations, see see Fig. 10. Note that the effect depends on the incoming electron current: at higher current, the photon bunching decreases, because the statistics of the emitted photons start to be dominated by those of the incoming electrons [75]. Applications of this physical effect to measure lifetimes at high spatial resolution will be given later, in Section 3.4.2. When the sample thickness increases, several effects arise, which are represented in Fig. 11. The simulations represented in this figure were performed using CASINO v2.4.8.1 with Rutherford cross-sec- tions, 105 incident electrons and considering a 1 nm wide electron probe [76]. The GaN density was set to 6.15 g. cm−3. Fig. 11 shows the lateral spread of the deposited energy, which is related to the region in which carriers that will lead to CL signal are generated [77]. This does not take into account any carrier diffusion. For moderate thicknesses (tλ/<1≈e 1), an increasing number of energy losses arise, each eventually responsible for additional electron-hole (e-h) pair creation.

However, in this limit (tλ/≈e 1) the beam is very weakly broadened by interaction with the sample.6 For increasingly thicker samples, or decreasing acceleration voltages, one has to rapidly take into account the deflection of the electron due to the interaction with the sample. One sees in Fig. 11a) (low energy electrons as used in a SEM) that the net result of the interaction is a spread of the electron onto a volume sometimes called the “interaction pear”. The interaction pear

Fig. 9. Plasmonic crystal optical properties in the reciprocal and real space. The 2D plasmonics crystal is composed of silver pillars 100 nm high and has a period of 600 nm. 5 Note that even if the bulk plasmon would de-excite in the form of the lowest energy e- The nominal diameter of the pillars is 250 nm; a. Measured dispersion relation for a h pair (i.e having an energy equal to the energy band gap Eg), the number of e-h pairs perfectly periodic crystal. An energy band gap is observed. b. Measured dispersion would be equal to NEE≈/gpwhich is typically less than ten and places an upper limit on relation for an imperfect crystal where rows have been substituted with 400 nm the total number of e-h pairs created by a fast electron in a thin sample diameters pillars, as shown on the panchromatic image in c. Two states appear in the 6 Of course, using an ångström-wide electron beam, such a broadening would be band gap. d, e,f: real space mapping at different energies showing the localised modes at already large enough to prevent atomic resolution. However, considering the nanometer the origin of the band gap states. d, e were acquired with two different polarisation states, spatial resolution discussed for CL applications, the broadening can be considered f is an unpolarised measurement. Reproduced from [5] with permission. negligible.

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Fig. 10. Synchronised emission of photons coming from nitrogen-vacancy centres upon electron excitation. CL g(2)(τ) measurement (continuous lines) for intensity values I ranging from 1.2 to 11 pA for an individual nanodiamond, 60 nm thick. Each measure- ment lasted 5–20 min. The data were fitted with the Monte Carlo simulations (dashed line). The lifetime was retrieved using an exponential fit of the bunching curves, leading to τe ≈26±1ns. All the parameters of the Monte Carlo simulation were kept fixed except the current, which was changed until a good agreement between the experiment and the simulation was found. The resulting fitted currents are written in parenthesis next to the measured experimental values. Inset, left: PL g(2)(τ) measurements performed at two different laser excitation powers on another ND from the same batch. In contrast with the case of CL, the function is totally flat. After [75].

essentially dissapear for higher voltages, such as used in STEMs (Fig. 11b)). Within this volume, the electrons may interact inelastically. Excited e-h pairs may thus be generated in a volume much broader than that given by the probe size or probe geometrical broadening. Together with the further e-h diffusion, this certainly broadens the resolution one can expect from the sole consideration of the probe beam size. As a rule of thumb, the higher the electron energy and the thinner the sample (see Fig. 11c), the lower the interaction, the less the number of hot e-h pairs created, the less the energy lost by the electron, and finally the higher the spatial resolution. An important point has to be made here. In CL, we only control the electron beam, but we don't know where the emission comes from. Therefore, information extracted from CL maps will rely on assump- tions about the diffusion mechanism, recombination, etc… One of the advantages of working in a STEM is to get additional information (morphology, chemistry, etc…) that can help in guiding these assump- tions. Another approach is to use a near-field set-up to detect the light emitted at some place when triggered by the electrons at another [78]. Such an approach is very difficult to implement in a STEM, but would provide extremely interesting physical insights.

3.3.2. Comparison to EELS In contrast with the case of coherent excitations, where the EELS Fig. 11. Result of Monte Carlo simulations. The contour lines show the projected lateral and CL signals have a very close resemblance, it turns out that in the spread of the deposited energy (which is proportional to CL signal) on a GaN film with a general case STEM-CL and STEM-EELS signals are quite different and thickness of 50 nm hit with an electron beam with 5 keV in (a) and with 60 keV in (b). The contour lines indicate the fraction of energy deposited inside each line (the 95% the former is much more suitable than STEM-EELS for the study of contour line indicates that 95% of the energy is deposited inside this line). In (c) the optical properties of semiconductors close to or above the band gap. deposited energy spreads up to 99% of the total value for 10, 50, 100 and 1000 nm thick There is a fundamental reason for this, aside experimental issues, GaN samples for several electron beam energies ranging from 1 to 200 keV. especially the lack of spectral resolution of EELS, which will however sooner or later be improved [68]. In an EELS experiment on a other hand is a two-step process. First, some energy is transferred from semicondutor, one measures the whole energy transferred from the the electron to the material (absorption), and then converted to hot e-h electron to the material. Therefore, EELS spectrum features close to or pairs, which will populate excited states in the material, possibly within the band gap will share a large resemblance with optical emitting light (emission). Therefore, the cross-section for CL is absorption. In particular, EELS will exhibit after the gap a large proportional to that of the energy-summed EELS cross section. The absorption band. The position of the gap is in practice difficult to energy available for the creation of e-h pairs can be extremely large, as define mainly because of the tail of this rather intense band. CL on the it includes the volume plasmon. This boosts the CL cross section

59 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69 dramatically. Moreover, schematically, the CL spectrum is very peaky - (Fig. 12) to the ca. 30 nm resolution in case of an isolated GaN possibly including a peak at the band gap energy for example. Thus, all QDisks7 in Fig. 13. In the two cases, of course, the resolution is the previously collected energy, which was spread over a large energy enough to get the relevant information. The same things happen for range in the EELS spectrum, may now be experimentally present in a the emission of an individual nitrogen-vacancy defect in diamond, peak, which significantly increases the SNR, and helps to determine the CL intensity signature of which extends over roughly 70 nm, or precisely the optical band gap position. in individual CdSe/CdS quantum dots for which the spatial resolu- tion is precisely equal to their size (Fig. 13). 3.3.3. Comparison to SEM-CL and PL Concerning the deleterious effect of the electron beam on the SEM-CL and PL are so well adapted to the study of luminescent objects of interest, it is worth noting a paradoxical advantage of using semiconducting materials that one may wonder why it would be a fast electron beam. Of course, such a beam can disrupt the atomic attractive to switch to STEM-CL, which is burdened with all the structure examined, and radiation damage is likely to kill the lumines- annoyances of TEM techniques: sample preparation is difficult - for cence very quickly [87]. However, if one is able to find an appropriate example it is absolutely impossible to study directly samples grown acceleration voltage at which the damage is not happening rapidly, on a substrate and it is more costly and less versatile than the two then the transfer of energy between the electron beam and the sample techniques. On the other hand, it also has all the advantages of can be extremely weak. TEM techniques: in addition to the optical signal, morphology and In a typical SEM situation (thick sample, low voltage) each electron structure can be determined with possibly atomic resolution. With interacts with the sample, and all the energy of each electron is respect to PL, STEM-CL has the same advantages as with SEM-CL: transferred to the sample. This induces a large density of charge a spatial resolution which is not diffraction-limited (i.e sub-200 nm carriers, possibly leading to non-linearities. Also, this can easily heat up resolution is theoretically possible) and an easy access to the the sample. In a SEM, a typical number of charge carriers created per properties of deep blue or UV luminescence, which is always incoming electron is given by [1] NEE=/30 g where E0 is the incoming difficult and expensive for pure optical techniques. We note also electron energy and Eg the energy gap of the material. that very high spectral resolution can be obtained with SEM-CL Thus typically, for GaN in a SEM experiment at 30 keV, 3000 e-h [79–81], although, as we will see below, not with the ultimate pairs will be created per electron. In a STEM, for a thin sample, a large spatial resolution. Now, SEM-CL has two main limitations. Firstly, number of incoming electrons may even not interact inelastically with despite impressive sub-diffraction resolution [82,83,79,21,84], the sample, thus not transferring energy to the sample. For the SEM-CL has not demonstrated the capacity to resolve the lumines- electrons interacting inelastically, the energy transfer will be small cence at the relevant scale (that of a confinement length-few (of the order of the bulk plasmon energy), leading to the creation of a ångströms or nanometers) for some of the most technologically small number (say, around 3) of e-h pairs [72,74]. This makes the relevant materials (such as III-V and in particular III-N). This STEM-CL a potentially less invasive technique. limits its use to the study of well-separated emitters, or leads to This is demonstrated in Fig. 14, where the luminescence emitted by ambiguities in the assignment of emission wavelengths. Secondly, the same CdSe/CdS Quantum dot has been recorded by STEM-CL and the strong interaction of the electrons with the material, which PL [89]. Firstly, it makes obvious from the figure that a single II-VI increases very rapidly when decreasing the accelerating voltage Quantum Dot can be measured by CL, in contradiction to early (see Fig. 11), leads to heating of the sample and non-linearities, statements based on SEM-CL experiments [85]. One can see that the ultimately leading to energy shifts and broadening when trying to emission line wavelength is almost the same (the shift is due to access individual quantum-confined objects [82,85]. different measurement temperatures in the PL and CL experiments). STEM-CL partly addresses the two issues. The spatial resolution More importantly, the FWHM of the peak is larger for the PL in CL is limited by whichever is the larger quantity between the measurement than for the STEM-CL. This surprising result points to probe size, the excitation pear radius and the diffusion length of the the fact that STEM-CL can be even less disruptive than PL for charge carriers. In the case of a STEM, the probe size is negligibly measurements on sensitive luminescent nano-objects. Beyond the high small. However, the main interest is that, due to the high electron spatial resolution offered by STEM-CL, the possibility of working in a speed, the excitation pear size is also extremely small for sample linear regime even when addressing a single, atomic-like emitter and with thicknesses compatible with (S)TEM imaging (say a few tens yet getting enough signal is certainly an advantage, whose exploitation to a hundred nanometers); the broadening due to the electron is discussed in the following sections. interaction with the sample is thus negligible at the scale of a confinement length. Thus, the diffusion length is the quantity defining the spatial resolution in STEM-CL. However, as most of 3.3.4. Applications of incoherent excitations the objects of interest (quantum dots, quantum wells…)areefficient Note that in the next section, we will use a different formalism charge-carrier traps, the diffusion length and thus the spatial better adapted to plasmons (the electromagnetic local density of resolution are essentially determined by the size of the object of states). High resolution mapping of the spatial variation of the optical interest, and thus it is not relevant to get a better spatial resolution. transition energy and intensity. This is exemplified in Figs. 3 and 12, where the spectral imaging The first obvious application is the study of sets of quantum- has been performed on a stack of GaN Quantum Discs (QDisks) confined objects in their environment. Fig. 12 presents the study of a embedded in AlN. The strong confinement makes the diffusion set of individual GaN QDisks embedded in AlN, within a nanowire. As length as small as a few nanometers [9]. However, if the QDisks are already explained, it is possible, through a detailed analysis of the separated by less than the diffusion length, emission from the whole SI [8,9], to assign a given emission line to a given QDisk. At the nearest neighbours can overlap. In this case, as explained in same time, it is possible to get the size of every QDisk with monolayer Fig. 12, a rigorous analysis of the SI data allows one to assign (ML) precision, and also to know exactly where each QDisk is within specific emission lines to a given quantum confined structure [8]. the nanowire. This makes it possible to analyse the emission wave- Indeed, it is clear that the signature of each QDisk is well isolated in length as a function of the ML number, as shown in Fig. 12c. At first the combined spatial/spectral space, and can be directly correlated sight, this dispersion seems unremarkable. This is the dispersion to the maxima in the HAADF image [8,9,29]. We note that the diffusion length is dependent both on the type of materials analysed 7 Note also that the CL intensity signal is even not symmetric with respect to the and how they are combined into nanostructures. One can compare QDisk, due to the fact that the way the charge carriers are driven to the QDisk depends on the resolution of a few nanometers in a stack of GaN QDisks the position of the electron probe with respect to the QDisk, see [86].

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Fig. 12. Spatial and spectral identification of individual QDisks stacked in a nanowire. (a). HAADF of a nanowire containing a stack of GaN QDisks embedded in an AlN shell. GaN appears bright, AlN dark. The growth direction is from left to right. Scale bar is 20 nm. (b). 2D plot of a one-dimensional SI. Wavelength versus electron probe location is shown. On top is the HAADF profile acquired during the one-dimensional SI. Inset: magnification of part of the plot showing that individual QDisk signatures consist in well separated 2D maxima in the wavelength/position space. Note also how these maxima correspond to those in the HAADF, i.e. to the presence of GaN. (c). Dispersion relation for the QDisks of two nanowires. (d). Wavelength versus disc position for an individual nanowire (crosses are simulations). The Qdisk index increases along the growth direction. Note the redshift for a given QDisk thickness as the QDisk shell thickness decreases. After [8].

Fig. 13. Sample-dependent spatial resolution. HAADF Images of (a). A nanodiamond, (b). A set of CdSe/CdS quantum dots and (c). a GaN quantum disc within an AlN nanowire. The corresponding CL image (d). Fitted on the nitrogen vacancy emission line [88], (e). Fitted on the CdSe/CdS emission lines [89] and (f). Filtered on the GaN quantum disc [86]. Note the asymmetry of the CL intensity. relation for a textbook “particle in a box”: the wavelength is propor- can exist within III-nitrides. These fields are so strong that they can tional to the size of the box (the QDisk’ width in this case). However, bend the bands within the QDisks; an effect called Quantum Confined confined states should naturally arise at energies larger than that of the Stark Effect (QCSE) [90]. If the bands are bent enough, then the bulk (GaN, in this case). This is not the case for some of them, see difference in energy between the lowest electron state and the highest Fig. 12c. This apparent contradiction is however solved when one hole state can be smaller than the bulk band gap, explaining the effect considers the fact that high internal (either pyro- or piezoelectric) fields seen.

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defects [105]). Quite generally, they locally change the electronic properties of the material, creating states in the band gap, and can be detected and even quantified through CL [81]. Therefore, they are likely to quench the NBE emissions of the material because e-h pairs may be captured by these states lying at lower energy than the NBE ones. The presence of defects may thus be detected either by a quenching (darkening) of the NBE emission, or by an emission at the defects energy, if they are radiative. Its imaging resolution and different imaging modes makes STEM- CL a technique of choice for studying the optical properties of 1D [102] and 2D defects [100], as there exist techniques to specifically image these defects. The case of 0D defects is more difficult, as imaging point defects requires extremely thin samples, which is generally incompa- tible with a high CL emission. A very good example of the potentiality of STEM-CL for 2D defect analysis has been given in [100]. In this experiment, a basal plane stacking fault (BSF) in a GaN nanocolumn (i.e wide nanowires) was studied. One can directly see in Fig. 16a the Fig. 14. Comparison between PL and CL spectra of individual CdSe/CdS QDs. CL (red) presence of the BSFs, identified as brighter lines in the image. The and PL (blue) spectra of the same QD. The integration time is 1 min for PL and 50 ms for panchromatic image in Fig. 16b shows that there is emission almost CL. The energy shift can be explained by a temperature difference between the everywhere in the nanocolumn. However, a spectrum-line clearly measurements. Note the higher FWHM for the PL spectrum. (Inset, left) Scheme of shows that the NBE emission at 360.6 nm is quenched when the the CL/PL experiment. (Inset, right) Scheme of the substrate used for CL/PL cross- correlation. After [89]. electron probe approaches the BSF region. Indeed, at around 150 nm from the first BSF, the BSF emission (around 361.7 nm) starts showing up. This is an interesting example of correlation between conventional The analysis also shows that, for a given QDisk thickness, known imaging and spectral imaging. In addition, it is worth noting that the with a ML resolution, there is a wide range of emission energies. This is experiments were performed at very low temperature (15 K). The use troubling, because the precision on both the emission wavelength and of the necessary liquid He stage is always a burden for TEM experi- the size of the QDisks is much better than the observed differences; ments. However, no emission for the BSF is expected at liquid nitrogen thus, the difference has to arise from a difference of environment. or room temperature, and this demonstrates the need for better Indeed, if one plots (Fig. 12d) for a given nanowire the emission as a Helium stages for performing meaningful spectroscopy experiments. function of the position of the QDisks within the nanowire, one sees a Other defects can be obviously studied by STEM-CL. Defects in systematic redshift along the growth direction. This is because the AlN diamonds are a good example [88]. Among them, NV centres in shell around the nanowire generates strain which increases the diamond are of particular interest. Indeed, such centres are well- bandgap, but the shell thickness decreases along the growth direction, known to be single photon emitters (SPE). As such, they are candidates reducing this effect, all else being equal. We thus see directly how to be used in many applications related to quantum cryptography and powerful the combined use of STEM-CL and high resolution imaging quantum computing. CL is a well-known tool in the study of such can be for disentangling the physics of quantum-confined objects. Such centres [105]. However, it is only recently that the mapping of an analysis can be nicely transposed to planar structures, such as individual NV centres has been reported [88]. The application of InGaN multilayers [91]. STEM-CL to the study of quantum-optical properties of defect centres STEM-CL finds a natural application in the III-N systems will be presented in Section 3.4.1. [92,8,9,93–97,86,98,99,91,100,101,97,102,103], but it is not re- stricted to quantum-confined systems. Indeed, STEM-CL is very useful for analysing sub-20 nm fluctuations in emission wavelength in (AlGa) 3.4. Advanced applications N alloys [95,96], likely due to alloying modulations, or variations in exciton spectral fingerprints in h-BN [97], related to folding of this 2D 3.4.1. Non-linear and quantum optics material. non-linear optics. Despite the huge interest in PL studies done with Likewise, the very high degree of localisation in InGaN allows high varying excitation power, the corresponding approach is much less resolution CL imaging [93,94,99,98,91,104]. This is interesting, be- frequent in CL. Power-dependent measurements demand the possibi- cause this is a material of high technological importance due to its lity of changing the excitation intensity by some orders of magnitude potential tunability in the visible range, which makes it appropriate for and can yield very rich information concerning for example the applications such as photovoltaics or white LEDs. Fig. 15 presents two recombination routes and non-linear response of semiconductors – fi studies concerning InGaN inclusions embedded in GaN nanowires [106 108]. In particular, for quantum-con ned nanostructures, the [93,98]. In such systems, the question is often to know wether all the density of carriers inside an individual quantum dot, well or disc, for ff inclusions are emitting, at which wavelength, and why. In Fig. 15 (left), instance, can have a profound e ect on its emission energy and ffi ff one can easily see that only two inclusions are emitting, and the e ciency. Several e ects may occur and when carrier density is ffi emission wavelength can be easily measured. The reason for one su ciently high, emitted photons can be absorbed by carriers inside ff ff 8 inclusion not emitting is not clear from this study [93]. However, the the very same quantum object, an e ect known as the Auger E ect ff absence of emission may be related to threading dislocations [98].In [109]. In the case of light emitting diodes, this e ect is highly undesired ffi Fig. 15 (right), similar experiments have been performed [98]. In these as it leads to a steep decrease in the external quantum e ciency, “ ffi ” experiments however, a high resolution image is available as well, usually called e ciency droop [110]. Recently, STEM-CL has been ff demonstrating that the emitting inclusions under consideration do not used to determine the e ects of varying excitation over 4 orders of include dislocations. This might explain the reason why no quenching magnitude on GaN QDisks embedded in AlN [111]. In this system, a fi of luminescence is observed in this case. strong internal electric eld causes a large red shift, as discussed above, Defect mapping. CL in general is quite well suited to studying defects in semiconductors, should they be 2 dimensional (stacking fault 8 This Auger effect has not to be confused with the Auger effect better known in the [100], grain boundaries [42]), 1D (dislocations [13]) or 0D (point electron microscopy community.

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Fig. 15. Spectral-mapping of InGaN inclusions in GaN. Left: InGaN inclusions inside a GaN nanowire. After [93]. a. HAADF: inclusions appear whiter. b. Spatio-spectral mapping of the nanowire (HAADF profile is also given). Only two inclusions are emitting. They clearly emit at two different wavelengths, as seen in c. Note that the maximum of emission is shifted with respect to the centre of the inclusion, see Fig. 13 and [86]. Right: InGaN inclusions within a GaN nanowire. After [98]. a. HAADF and EDX In profile. b. Bright field image. c. d. CL filtered maps at two different energies. due to the QCSE. For QDisks sufficiently thick (>2nm) and with emitted light also does. Indeed, 3D-confined systems are best repre- recombination times sufficiently long ( > 100 ns) the STEM electron sented as TLS, see Fig. 1. Such a system can only emit one photon at a probe can excite individual QDisks so fast as to generate significant time. At first sight, the quantum behaviour of a beam coming from an internal carrier densities [111,112]. In this system, the carrier density SPE is not obvious. However, a beam consisting of only one photon has two major effects: it blue-shifts the emission energy by partially means that the light is definitely a particle. In this case, the uncertainty screening the internal electric field, thus reducing the QCSE; and it on the number of photons is null, and Heisenberg principle tells us that induces an efficiency droop which can be attributed to the Auger effect. the phase is therefore totally unknown. Single photon emission is Fig. 17 (a) shows a collection of spectra acquired with the electron therefore one of the (sparse) manifestations of the quantum character beam currents indicated in the legend when the beam was right in the of light. One of the interests of having single photon states relates to middle of the QDisk (sample similar to those from Fig. 12). From 0.08 their use in quantum cryptography [38]. An observer wanting to spy on pA to about 442pA, the emission energy shifted by about 0.3 eV. a communication between two points will have to intercept single Instead of observing the shift visually on the spectra, one can plot just photons one by one, and read their states. However, if the incoming the emission energy (peak centre) with respect to the emission state is randomly generated (for example, a random possibility of being intensity (peak area). The results, shown in Fig. 17 (b), correspond to in two polarisation states), the observer will certainly project the 3089 spectra acquired on an individual QDisk and indicate how the incoming electron in generating another state by reading it, and will energy shifts continuously as a function of emission intensity. not be able to know in which state to re-emit the single photon, leading Interestingly, in the top right of the plot, the energy increases very to the possibility for the receiver to know that the line is being spied rapidly with the intensity. This indicates that, even if the carrier density upon. has increased (since the emission energy has), the intensity didn't The emergence of quantum cryptography engineering has triggered follow. Fig. 17 (c) confirms that the electron beam current has a direct numerous studies in quantum optics. Among them, the search for and effect on the emission energy, both being nearly proportional. Finally, characterisation of the source of single photons has been very intense. Fig. 17(d) shows that for currents higher than 1pA emission efficiency Recently, point defects in semiconductors have emerged as promising drops rather abruptly. This behaviour, consistent with Auger effect, candidates, following the now famous example of nitrogen vacancy shows that, for this system, exciting with high currents (or at lower centres in diamond [37]. With the expected downscaling of quantum voltages for higher excitation cross-section) could reduce the effective optics devices, analysis tools with sub-wavelength resolution which are light emission rate. not available with conventional PL tools, are highly desirable. CL has Quantum Optics. e-h pairs can be confined in 3D, just as in the case many potential advantages for such a role: it has been known for a long of QDots (see e.g. [89]). In such cases, not only the object of interest time that CL is extremely sensitive to defects [1,105], see Fig. 18 (a) presents quantum properties (like quantum confinement) but the and (b), and the ability for CL to measure luminescence from the infra-

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red to UV in principle enables it to scan across all luminescent centres efficiently. STEM-CL presents another advantage. As explained in Section 3.3.3 and demonstrated in 3.4.1, STEM-CL has the ability to generate a very small number of e-h pairs per unit of time. SPE being two-level systems, they are already saturated when excited once. The system can be re-excited only after the TLS has gone back to its

fundamental state, which happens in a typical lifetime τe timescale. Typical SPEs have lifetimes in the 1–10 ns range. Assuming an object

thin enough to be in a regime where tλ/<0.e 5 - a typical regime for STEM investigations-, one e-h pair will be created every two incoming electrons, leading to a rough ratio of 1 created eh pair per incoming electron. If the pairs are created directly at the SPE position and are able to excite it, this restricts the incoming current around 10–100 pA or less in order to remain in the non-saturated regime. This is easily achieved in a current STEMs - in practice, the excitation probability is even much less here, due in part to the diffusion neglected in our model, and we are not aware of any saturation measurement in SPE by STEM CL. In contrast, in the case of a low voltage and a relatively thick sample (i.e. typical SEM-CL situation), where thousands of e-h pairs can be created per incoming electrons, the saturation will be reached very rapidly. There is however an issue when characterising SPEs. Indeed, the only way to be certain that a photon beam contains only one photon is to use an intensity interferometer set-up, such as described in Section 2.1.4 and Fig. 4 e. Such a set-up allows one to measure the correlation function gτ(2)( ). Indeed, as described for few representative cases in Fig. 19, the gτ(2)( ) function provides information on the statistics of the emitted light. A purely random emission of light, such as produced by a laser, results in a flat gτ(2)( ), with a constant value of one (Fig. 19a): the probability of detecting one photon at time τ if a photon has already Fig. 16. Basal plane Stacking fault luminescence in GaN nanocolumn. (a) HAADF image τ of a single nanocolumn exhibiting three basal plane stacking faults in bright contrast. (b) been detected at time 0 is the same whatever the delay . The photons Corresponding panchromatic imaging. (c) Spectrum-line along the nanocolumn. emitted by a chaotic source of light such as a tungsten filament are Reprinted with permission [100]. bunched together, resulting in an increase of probability at small delay τ, see Fig. 19b. The width of the so-called “bunching” peak at zero delay

is directly related to the typical coherence time τe of the photon beam.

Fig. 17. In STEM-CL, the probe current can be changed to change the excitation power. In (a) some spectra from an individual QDisk are shown for several probe currents, from 0.08 to 442 pA. The colour of the spectra indicates the current according to the legend. The shift in peak energy can be followed in (b) as function of the spectrum intensity. In (b) whole SIs are used and the large overlap among data points shows the absence of significant damage. The spectral energy shift for several QDisks as function of the probe current is shown in (c). In this figure and in (a), we only consider the emission when the probe is exactly on top of the QDisk. The continuous shift shows that the probe current increases the carrier density across the whole range studied. Such an increase in carrier density is responsible for the drop of emission efficiency shown in (d). Emission efficiency as a function of the current. Curves are guides for the eye.

64 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69

Fig. 18. Single photon detection from point defects. a. HAADF image of a nanodiamond. b. Typical spectrum of a neutral nitrogen vacancy point defect. The shaded area represents the spectral window over which the intensity interferometric experiments have been done c. Filtered CL map of the nanodiamond shown on a. d. g2 (τ) measurements performed on the blue and red areas indicated in a and c. After [113].

On the other hand, other possibilities exist, such as other centres in diamonds, defects in SiC… In particular, reports of SPEs emitting in the UV are very sparse [114]. One of the reasons might be the relative difficulty of performing optical measurements in the UV. There are however no particular problems with of doing them in CL. As shown in Fig. 1, STEM-CL has been successfully used to detect a new type of SPE in the UV range in hexagonal boron nitride (h-BN ) [7]. It is worth noting that this defect was already known in the literature [115]. Nevertheless, before the report in [7], there was no proof of the nature Fig. 19. Three typical light statistics and their signatures. a. Poissonian statistics. The photons arrive randomly, so that their is an equal chance to detect a photon at any given of the defect. Knowing it is an SPE indicates that the defect is a point delay τ, resulting in a flat g(2)(τ). This is typically the case for a laser light. b. Super- defect, even if its structure is yet to be solved. poissonian light. If the photons are emitted in the form of bunches, the correlation function presents a so-called “bunching” peak. In the case of chaotic light, the width is 3.4.2. Lifetime measurement τ related to the correlation time of the light e, of the order of few fs. In the case of In the preceding section, a time-dependent function (the time bunching in cathodoluminescence [75], this is related to the lifetime of the emitters, autocorrelation function) was used to characterise an SPE. Although typically in the ns range. c. Sub-poissonian light. This is the case of single photon emission. In the light emitted by a SPE, only one photon exists at a time. Therefore, the the main purpose of these studies [7,113] was the detection of SPE correlation function is zero at zero delay, and presents a so-called “anti-bunching” dip quantum states, another benefit was the measurement of the lifetime of with width related to the lifetime τe of the emitter (see text). the emitter. However, this relied on a specific model for the emission from a very special type of emitters, and therefore intensity inter- It is extremely short for a chaotic source of light (of the order of fs). ferometry, as a lifetime measurement technique, seems to be of very Finally, in the case of an SPE, one expects a so-called “antibunching” narrow applicability. dip going to zero at zero delay (Fig. 19c). Nevertheless, we have seen in Section 3.3.1 that the way lumines- An exponential fit of the antibunching dip gives a measure of the cence is triggered passes trough the synchronised generation of hot e-h; SPE lifetime [37]. Note that the use of an intensity interferometer is this eventually leads to the appearance of a bunching peak in the time straightforward in the case of single photon emission. Indeed, in this correlation function. This was first demonstrated on different groups of case, only one photon exists at a time and therefore only one of the two SPEs [75], and later on quantum-confined structures [6]. Now, it can detectors can be hit at a time, leading to an obvious absence of be shown [6,75] that the lifetime of the emitter can be deduced through correlations between the two detectors at zero delay. a simple exponential fitting of the bunching peak. This opens the way The first adaptation of an intensity interferometer on a CL system is for lifetime measurements at deep subwavelength. Indeed, since the reported in [113]. In this work, the goals were: 1. to detect single spatial resolution is ultimately limited by the size of the object and/or photon emission with CL and 2. to evidence gτ2 ( ) spatial variations the diffusion length, which can be extremely small [9,116], it is sensible (implying changes in the quantum state of the emitted light). This was to think that lifetime measurement could be extended to very small done, as exemplified in Fig. 18, where antibunching was demonstrated sizes. for a NV centre, and a change in the gτ2 ( ) function as the beam moved Lifetime measurement in a SEM-CL has undergone a constant and away from the defect centre position was observed. In these experi- vigorous growth in the past few decades [117,17,22], showing amazing ments, the NV centre was chosen because it is a well-known defect spatial resolution down to a few tens of nanometers. Nevertheless, this centre in optics, known to be photostable [37] (and also electrostable is not enough to resolve individual quantum objects. We also note that [113]), bright and easy to isolate. in all these works, the excitation beam was pulsed (either with blanking

65 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69 plates or a laser). With the bunching effect discussed here, however, no coatings where loaded separately. In order to understand how NDs special gun is required. with different coatings would internalize when loaded together, T1 NDs An example of the power of the technique is shown in Fig. 1. This where coated with PAH, and T2 NDs with PEI. Therefore, by following figure shows a stack of QDisks whose widths are few tens of ångströms, the luminescence, one can track the type of coating. and which are separated by about 15 nm. As already shown in other Fig. 20a shows the principle of the measurement, where both a cell examples, the emission of each disc can be isolated. In addition, a ultrastructure (here shown in HAADF mode) and the luminescence can correlation function can be acquired for each of these QDisks. be measured at the same time. From this type of images, the content of Following an exponential fitting, the lifetime can be deduced, leading vesicles in NDs coated with PEI (ND-PEI) and PAH (ND-PAH) can be to a very high spatial resolution for lifetime determination. As is clearly deduced, together with structural information (like the vesicle dia- seen in Fig. 1, and confirmed statistically, a decrease in lifetime meter), as shown in Fig. 20b, validating STEM-CL as a promising corresponds to an increase in emission energy. This is explained in CLEM technique. the following way. For such confined systems, the higher the confine- ment, the higher the energy on one hand, and the higher the electron- 4. Conclusions and prospectives hole wavefunctions overlaps leading to a shorter lifetime on the other hand. [90]. We also note that systematic comparison of lifetimes as This review paper has focused on novel uses of STEM-CL, and its measured in PL and in CL have been performed in the same paper, advantages and drawbacks with respect to its sister techniques (SEM- showing an excellent agreement between the two techniques thus CL and PL). It is clear that STEM-CL has several well-defined areas of validating CL as a nanoscale counterpart of time-resolved PL. applications where it can be very competitive, especially in high resolution investigations of plasmonics and quantum confined materi- 3.4.3. Correlative bioimaging als. New applications, such as time-resolved STEM-CL or bio-imaging In the field of bio-imaging, optical and electronic microscopy each are most likely promised to a great future. With the availability of have both their own, complementary, interest. On the one hand, optical commercial STEM-CL systems, it is clear that all these fields will be microscopy can be used in-vivo and/or to perform fast and dynamical investigated. But beyond this, coupling STEM-CL with other techni- imaging. It usually involves the use of fluorophores that can for ques, especially those involving light injection, should lead to exciting example be functionalised to characterise a particular biological perspectives. function. On the other hand, electron microscopy can image the ultrastructure of biological material. In the past year, a large effort Acknowledgments has been made to bridge the two worlds using correlative light electron microscopy (CLEM) techniques. The present paper is the result of many interactions. We started the As the name suggests, the idea behind this technique is to combine, STEM-CL project at Orsay in 2004 with Odile Stéphan and Christian on the very same part of a sample, the information from both the Colliex, in a large part motivated by Javier Garcia de Abajo's ideas and optical and the electron microscopy by overlapping images acquired suggestions. We have been closely working in particular on STEM-CL with the two techniques. Several approaches have been applied, either together since then. Their inputs have been invaluable and their by taking images in different microscopes or by integrating an optical imprint is obvious from the many papers cited here. Naoki microscope into the electron microscope [118]. Yamamoto has been an important source of inspiration and guidance, The success of the CLEM approaches militates for a simplification since well before we even directly interacted with him. Finally, Maria of the necessary alignment of images between techniques. It is no Tchernycheva and her team (Francois Julien, Lorenzo Rigutti, Gwénolé surprise that CL was recently seen as a potential solution to this Jacopin) have early guided us in one of the most interesting field of problem. Successful proof of principle of the combined use of EM and applications for STEM-CL (III-N heterostructures). We want to thanks CL imaging has been provided in SEMs [119]. Unfortunately, although all of them. Our views on STEM-CL have been sharpened through the gain in spatial resolution was obvious, it did not yet lead to the cell constant interactions with permanent members of the team, especially ultrastructure resolution. Marcel Tencé, Luiz Tizei, Jean-Denis Blazit, Alberto Zobelli, Mickael Recently, the principle of integrated CLEM using STEM-CL was Pelloux, Laura Bocher, as well as post-docs and students involved in proven [120]. In this study, two types (T1 and T2) of luminescent STEM-CL: Sophie Meuret, Arthur Losquin, Zackaria Mahfoud, Hugo nanodiamonds (NDs) were used, each having a different spectral Lourenço-Martins, Alfredo Campos, Romain Bourrellier, Anna signatures stemming from point defects. The NDs were coated with Tararan, Stefano Mazzucco, Jérome Schindfessel, Pabitra Das, two different polymers: polyallylamine hydrochloride (PAH), and Sounderya Nagarajan, Naoki Kawazaki, Simon Hooks. We want to polyethyleneimine (PEI). When coating smaller (around 50 nm) NDs, thank them as well as Francois Treussart, Bruno Daudin, Bruno Gayral, PAH and PEI were indeed shown to trigger different internalization Pierre Lefevbre, Julien Barjon, Brigitte Sieber, Martin Albrecht, Jurgen pathways [121]. In these experiments however, the two types of Christen, James Griffiths, Takumi Sannomiya, Paul Edwards, Ulrich

Fig. 20. a. (left) Large field of view HAADF image of a cell where two different types of NDs with two different polymer coatings have been loaded. (right) magnification of a vesicle, together with energy-filtered CL maps showing the existence of two differently emitting NDs inside the same vesicle. b. Cytometric plots of internalization vesicles labeled with polymer coated fluorescent NDs. The diameter of the circular markers is proportional to the vesicle size (refer to the scale bar). Different vesicles with the same number of ND-PAH and ND-PEI but different diameters appear as concentric circles. The occurrence (i.e. the number of vesicles encompassing the same number of NDs whatever its coating is) is given by the colour.

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Hohenester, Franz Schmidt, Cleva Ow-Yang and the Attolight team [22] X. Fu, G. Jacopin, M. Shahmohammadi, R. Liu, M. Benameur, J.-D. Ganiere, J. Feng, W. Guo, Z.-M. Liao, B. Deveaud, D. Yu, Exciton drift in semiconductors (especially Samuel Sonderegger and Jean Berney) for fruitful and under uniform strain gradients application to bent zno microwires, Acs Nano 8 (4) stimulating interactions that powered the writing of this review. (2014) 3412–3420. http://dx.doi.org/10.1021/nn4062353. Finally, we want to thank C. Colliex and M. Walls for their in-depth [23] M.A. Herman, D. Bimberg, J. Christen, Heterointerfaces in quantum-wells and epitaxial-growth processes - evaluation by luminescence techniques, J. Appl. Phys. and sharp reading of this manuscript that helped making it clearer. 70 (2) (1991) R1–R52. http://dx.doi.org/10.1063/1.349613. This work has received support from the National Agency for Research [24] F.J. Garcia de Abajo, Optical excitations in electron microscopy, Rev. Mod. Phys. under the program of future investment TEMPOSCHROMATEM with 82 (1) (2010) 209–275. http://dx.doi.org/10.1103/RevModPhys.82.209. the Reference No. ANR-10-EQPX-50 and the french CNRS METSA [25] C. Cherqui, N. Thakkar, G. Li, J.P. Camden, D.J. Masiello, Characterizing localized surface plasmons using electron energy-loss spectroscopy, Annu. Rev. Phys. Research Federation FR CNRS 3507. This work has received support Chem. 67 (2016) 331–357. http://dx.doi.org/10.1146/annurev-physchem- from CNPq and from projects 2014/23399-9 and 2012/10127-5, Sao 040214-121612 (arXiv:1509.08430). Paulo Research Foundation (FAPESP). [26] A. Losquin, T.T. Lummen, Electron microscopy methods for space-, energy-, and time-resolved plasmonics, Front. Phys. 12 (1) (2017) 127301. [27] E.J.R. Vesseur, J. Aizpurua, T. Coenen, A. Reyes-Coronado, P.E. Batson, References A. Polman, Plasmonic excitation and manipulation with an electron beam, MRS Bull. 37 (8) (2012) 752–760. [28] H.P. STRUNK, M. Albrecht, H. SCHEEL, Cathodoluminescence in transmission [1] B. Yacobi, D. Holt, Cathodoluminescence Microscopy of Inorganic Solids, electron microscopy, J. Microsc. 224 (1) (2006) 79–85. Springer, US, 1990. [29] M. Kociak, O. Stéphan, A. Gloter, L. Zagonel, L.H.G. Tizei, M. Tencé, K. March, [2] M. Kociak, O. Stephan, Mapping plasmons at the nanometer scale in an electron J.D. Blazit, Z. Mahfoud, A. Losquin, S. Meuret, C. Colliex, Seeing and measuring in – microscope, Chem. Soc. Rev. 43 (2014) 3865 3883. http://dx.doi.org/10.1039/ colours electron microscopy and spectroscopies applied to nano-optics, Comptes 〈 〉 C3CS60478K (URL http://dx.doi.org/10.1039/C3CS60478K ). Rendus Phys. 15 (23) (2014) 158–175 (seeing and measuring with electrons: [3] F.P. Schmidt, H. Ditlbacher, F. Hofer, J.R. Krenn, U. Hohenester, Morphing a Transmission Electron Microscopy today and tomorrow). – plasmonic nanodisk into a nanotriangle, Nano Lett. 14 (8) (2014) 4810 4815. [30] N. Bonnet, N. Brun, C. Colliex, Extracting information from sequences of spatially http://dx.doi.org/10.1021/nl502027r. resolved {EELS} spectra using multivariate statistical analysis, Ultramicroscopy [4] N. Kawasaki, S. Meuret, R. Weil, H. Lourenço-Martins, O. Stéphan, M. Kociak, 77 (34) (1999) 97–112. Extinction and Scattering Properties of High-order Surface Plasmon Modes in [31] F. de la Peña, M.-H. Berger, J.-F. Hochepied, F. Dynys, O. Stephan, M. Walls, Silver Nanoparticles Probed by Combined Spatially Resolved Electron Energy Mapping titanium and tin oxide phases using eels an application of independent Loss Spectroscopy and Cathodoluminescence, ACS Photonics http://dx.doi.org/ component analysis, Ultramicroscopy 111 (2) (2011) 169–176. 10.1021/acsphotonics.6b00257 [32] P.R. Edwards, D. Sleith, A.W. Wark, R.W. Martin, Mapping localized surface [5] H. Saito, N. Yamamoto, Control of light emission by a plasmonic crystal cavity, plasmons within silver nanocubes using cathodoluminescence hyperspectral – Nano Lett. 15 (9) (2015) 5764 5769. http://dx.doi.org/10.1021/acs.nano- imaging, J. Phys. Chem. C 115 (29) (2011) 14031–14035. lett.5b01719. [33] J. Nelayah, O. Stephan, M. Kociak, F.J. de Abajo, L. Henrard, I. Pastoriza-Santos, [6] S. Meuret, L.H.G. Tizei, T. Auzelle, R. Songmuang, B. Daudin, B. Gayral, L.M. Liz-Marzan, C. Colliex, Mapping surface plasmons on single metallic ff M. Kociak, Lifetime measurements well below the optical di raction limit, ACS nanoparticles using sub-nm resolved eels spectrum-imaging, Microsc. Microanal. – Photonics 3 (7) (2016) 1157 1163. 13 (2007) 144–145. [7] R. Bourrellier, S. Meuret, A. Tararan, O. Stéphan, M. Kociak, L.H.G. Tizei, [34] F. de la Peña, P. Burdet, T. Ostasevicius, M. Sarahan, M. Nord, V.T. Fauske, J. A. Zobelli, Bright UV single photon emission at point defects in h-bn, Nano Lett. Taillon, A. Eljarrat, S. Mazzucco, G. Donval, L.F. Zagonel, M. Walls, I. Iyengar, – 16 (7) (2016) 4317 4321 (pMID: 27299915). hyperspy: Hyperspy 0.8.2, August 2015. http://dx.doi.org/10.5281/zenodo.28025. [8] L.F. Zagonel, S. Mazzucco, M. Tence, K. March, R. Bernard, B. Laslier, G. Jacopin, [35] T. Coenen, A. Polman, Polarization-sensitive cathodoluminescence fourier mi- M. Tchernycheva, L. Rigutti, F.H. Julien, R. Songmuang, M. Kociak, Nanometer croscopy, Opt. Express 20 (17) (2012) 18679–18691. http://dx.doi.org/10.1364/ scale spectral imaging of quantum emitters in nanowires and its correlation to OE.20.018679. – their atomically resolved structure, Nano Lett. 11 (2) (2011) 568 573. http:// [36] C.I. Osorio, T. Coenen, B.J.M. Brenny, A. Polman, A.F. Koenderink, Angle- dx.doi.org/10.1021/nl103549t. resolved cathodoluminescence imaging polarimetry, ACS Photonics 3 (1) (2016) [9] L.F. Zagonel, L. Rigutti, M. Tchernycheva, G. Jacopin, R. Songmuang, M. Kociak, 147–154. Visualizing highly localized luminescence in gan/aln heterostructures in nano- [37] A. Beveratos, R. Brouri, T. Gacoin, A. Villing, J.P. Poizat, P. Grangier, Single wires, Nanotechnology 23 (45) (2012) 455205. photon quantum cryptography, Phys. Rev. Lett. 89 (18) (2002) 187901. http:// [10] N. Yamamoto, K. Araya, F.J.G. de Abajo, Photon emission from silver particles dx.doi.org/10.1103/PhysRevLett.89.187901. induced by a high-energy electron beam, Phys. Rev. B 6420 (20) (2001) 205419. [38] M. Fox, Quantum Opt.: Introd. (2006). [11] A. Losquin, L.F. Zagonel, V. Myroshnychenko, B. Rodrigues, Rodríguez-González, [39] M. Kociak, L.F. Zagonel, M. Tencé, S. Mazzucco, Adjustable cathodoluminescence M. Tencé, L. Scarabelli, J. Förstner, L. Liz-Marzán, F.J. García de Abajo, O. Stéphan, detection system and microscope employing such a system, wO Patent 2,011, 148, M. Kociak, Unveiling nanometer scale extinction and scattering phenomena through 073, 2011, combined electron energy loss spectroscopy and cathodoluminescence measure- [40] B.J.M. Brenny, T. Coenen, A. Polman, Quantifying coherent and incoherent – ments, Nano Lett. 15 (2) (2015) 1229 1237 (pMID: 25603194). cathodoluminescence in semiconductors and metals, J. Appl. Phys. 115 (24) ff [12] P.M. Petro , D.V. Lang, A new spectroscopic technique for imaging the spatial (2014) 244307. http://dx.doi.org/10.1063/1.4885426. distribution of nonradiative defects in a scanning transmission electron micro- [41] N. Yamamoto, A. Toda, K. Araya, Imaging of transition radiation from thin films scope, Appl. Phys. Lett. 31 (2) (1977) 60. on a silicon substrate using a light detection system combined with tem, J. [13] S.J. Pennycock, A. Howie, Study of single-electron excitations by electron Electron Microsc. 45 (1) (1996) 64–72. microscopy. ii. cathodoluminescence image contrast from localized energy trans- [42] B.G. Mendis, A. Howkins, D. Stowe, J.D. Major, K. Durose, The role of transition – fers, Philos. Mag. A 41 (1979) 809 827. radiation in cathodoluminescence imaging and spectroscopy of thin-foils, [14] S.J. Pennycook, L.M. Brown, A.J. Craven, Observation of cathodoluminescence at Ultramicroscopy 167 (2016) 31–42. http://dx.doi.org/10.1016/j.ultra- single dislocations by stem, Philos. Mag. A 41 (1980) 589. mic.2016.05.002. [15] N. YAMAMOTO, J.C.H. SPENCE, D. FATHY, Cathodoluminescence and polar- [43] F.J.G. de Abajo, A. Rivacoba, N. Zabala, N. Yamamoto, Boundary effects in ization studies from individual dislocations in diamond, Philos. Mag. B-Phys. cherenkov radiation, Phys. Rev. B 69 (15) (2004) 155420. – Condens. Matter Stat. Mech. Electron. Opt. Magn. Prop. 49 (6) (1984) 609 629. [44] F.J.G. de Abajo, A.G. Pattantyus-Abraham, N. Zabala, A. Rivacoba, M.O. Wolf, [16] M.V. Bashevoy, F. Jonsson, K.F. MacDonald, Y. Chen, N.I. Zheludev, P.M. Echenique, Cherenkov effect as a probe of photonic nanostructures, Phys. Hyperspectral imaging of plasmonic nanostructures with nanoscale resolution, Rev. Lett. 91 (14) (2003) 143902. – Opt. Express 15 (18) (2007) 11313 11320. http://dx.doi.org/10.1364/ [45] R. Sapienza, T. Coenen, J. Renger, M. Kuttge, N.F. van Hulst, A. Polman, Deep- OE.15.011313. subwavelength imaging of the modal dispersion of light, Nat. Mater. 11 (9) (2012) [17] M. Merano, S. Sonderegger, A. Crottini, S. Collin, P. Renucci, E. Pelucchi, 781–787. http://dx.doi.org/10.1038/NMAT3402. A. Malko, M.H. Baier, E. Kapon, B. Deveaud, J.D. Ganiere, Probing carrier [46] L. Henrard, P. Lambin, Calculation of the energy loss for an electron passing near dynamics in nanostructures by picosecond cathodoluminescence, Nature 438 giant fullerenes, J. Phys. B: At. Mol. Opt. Phys. 29 (21) (1996) 5127–5141. – (7067) (2005) 479 482. http://dx.doi.org/10.1038/nature04298. [47] F.J. García de Abajo, J. Aizpurua, Numerical simulation of electron energy loss [18] E.J.R. Vesseur, R. de Waele, M. Kuttge, A. Polman, Direct observation of near inhomogeneous dielectrics, Phys. Rev. B 56 (24) (1997) 15873–15884. plasmonic modes in au nanowires using high-resolution cathodoluminescence http://dx.doi.org/10.1103/PhysRevB.56.15873. – spectroscopy, Nano Lett. 7 (9) (2007) 2843 2846. [48] F.J.G. de Abajo, A. Howie, Relativistic electron energy loss and electron-induced [19] T. Coenen, E.J.R. Vesseur, A. Polman, A.F. Koenderink, Directional emission from photon emission in lymphogenous dielectrics, Phys. Rev. Lett. 80 (23) (1998) plasmonic yagi-uda antennas probed by angle-resolved cathodoluminescence 5180–5183. – spectroscopy, Nano Lett. 11 (9) (2011) 3779 3784. http://dx.doi.org/10.1021/ [49] U. Hohenester, A. Trugler, Mnpbem - a matlab toolbox for the simulation of nl201839g. plasmonic nanoparticles, Comput. Phys. Commun. 183 (2) (2012) 370–381. [20] P.R. Edwards, R.W. Martin, Cathodoluminescence nano-characterization of http://dx.doi.org/10.1016/j.cpc.2011.09.009. semiconductors, Semicond. Sci. Technol. 26 (6) (2011) 064005. [50] N. Geuquet, L. Henrard, Eels and optical response of a noble metal nanoparticle in [21] J. Bruckbauer, P.R. Edwards, J. Bai, T. Wang, R.W. Martin, Probing light emission the frame of a discrete dipole approximation, Ultramicroscopy 110 (8) (2010) from quantum wells within a single nanorod, Nanotechnology 24 (2013) 365704. 1075–1080.

67 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69

[51] A. Arbouet, A. Mlayah, C. Girard, G.C. des Francs, Electron energy losses and [79] M. Grundmann, J. Christen, N.N. Ledentsov, J. Bohrer, D. Bimberg, cathodoluminescence from complex plasmonic nanostructures spectra, maps and S.S. Ruvimov, P. Werner, U. Richter, U. Gosele, J. Heydenreich, V.M. Ustinov, radiation patterns from a generalized field propagator, New J. Phys. 16 (2014) A.Y. Egorov, A.E. Zhukov, P.S. Kopev, Z.I. Alferov, Ultranarrow luminescence 113012. http://dx.doi.org/10.1088/1367-2630/16/11/113012. lines from single quantum dots, Phys. Rev. Lett. 74 (20) (1995) 4043–4046. [52] P. Das, T.K. Chini, Spectroscopy and imaging of plasmonic modes over a single http://dx.doi.org/10.1103/PhysRevLett.74.4043. decahedron gold nanoparticle a combined experimental and numerical study, J. [80] J.P. Garayt, J.M. Gerard, F. Enjalbert, L. Ferlazzo, S. Founta, E. Martinez- Phys. Chem. C. 116 (49) (2012) 25969–25976. http://dx.doi.org/10.1021/ Guerrero, F. Rol, D. Araujo, R. Cox, B. Daudin, B. Gayral, L.S. Dang, H. Mariette, jp3103782. Study of isolated cubic gan quantum dots by low-temperature cathodolumines- [53] F.J.G. de Abajo, M. Kociak, Probing the photonic local density of states with cence, Physica E-Low.-Dimens. Syst. Nanostruct. 26 (1–4) (2005) 203–206. electron energy loss spectroscopy, Phys. Rev. Lett. 100 (10) (2008) 106804. [81] J. Barjon, T. Tillocher, N. Habka, O. Brinza, J. Achard, R. Issaoui, F. Silva, C. Mer, [54] S.M. Collins, E. Ringe, M. Duchamp, Z. Saghi, R.E. Dunin-Borkowski, P. Bergonzo, Boron Acceptor Concentration in Diamond from Excitonic P.A. Midgley, Eigenmode tomography of surface charge oscillations of plasmonic Recombination Intensities, Phys. Rev. B 83 (7). nanoparticles by electron energy loss spectroscopy, Acs Photonics 2 (11) (2015) [82] U. Jahn, J. Ristic, E. Calleja, Cathodoluminescence spectroscopy and imaging of 1628–1635. http://dx.doi.org/10.1021/acsphotonics.5b00421. gan/(al,ga)n nanocolumns containing quantum disks, Appl. Phys. Lett. 90 (16) [55] M. Kuttge, E.J.R. Vesseur, A.F. Koenderink, H.J. Lezec, H.A. Atwater, F.J. Garcia (2007) 161117. de Abajo, A. Polman, Local density of states, spectrum, and far-field interference [83] B. Wilsch, U. Jahn, B. Jenichen, J. Lahnemann, H.T. Grahn, H. Wang, H. Yang, of surface plasmon polaritons probed by cathodoluminescence, Phys. Rev. B 79 Spatially resolved investigation of strain and composition variations in (in,ga)n/ (11) (2009) 113405. gan epilayers, Appl. Phys. Lett. 102 (5) (2013) 052109. http://dx.doi.org/ [56] R. Carminati, A. Caze, D. Cao, F. Peragut, V. Krachmalnicoff, R. Pierrat, Y. De 10.1063/1.4790591. Wilde, Electromagnetic density of states in complex plasmonic systems, Surf. Sci. [84] Y.D. Zhuang, J. Bruckbauer, P.A. Shields, P.R. Edwards, R.W. Martin, Rep. 70 (1) (2015) 1–41. http://dx.doi.org/10.1016/j.surfrep.2014.11.001. D.W.E. Allsopp, Influence of stress on optical transitions in gan nanorods [57] A. Losquin, M. Kociak, Link between cathodoluminescence and electron energy containing a single ingan/gan quantum disk, J. Appl. Phys. 116 (17) (2014) loss spectroscopy and the radiative and full electromagnetic local density of states, 174305. Acs Photonics 2 (11) (2015) 1619–1627. http://dx.doi.org/10.1021/acsphoto- [85] J. RodriguezViejo, K.F. Jensen, H. Mattoussi, J. Michel, B.O. Dabbousi, nics.5b00416. M.G. Bawendi, Cathodoluminescence and photoluminescence of highly lumines- [58] J. Zuloaga, P. Nordlander, On the energy shift between near-field and far-field peak cent cdse/zns quantum dot composites, Appl. Phys. Lett. 70 (16) (1997) intensities in localized plasmon systems, Nano Lett. 11 (3) (2011) 1280–1283. 2132–2134. http://dx.doi.org/10.1063/1.119043. [59] T. Coenen, D.T. Schoen, B.J.M. Brenny, A. Polman, M.L. Brongersma, Combined [86] L.H.G. Tizei, S. Meuret, K. March, K. Hestroffer, T. Auzelle, B. Daudin, M. Kociak, electron energy-loss and cathodoluminescence spectroscopy on individual and A polarity-driven nanometric luminescence asymmetry in aln/gan heterostruc- composite plasmonic nanostructures, Phys. Rev. B 93 (19) (2016) 195429. http:// tures, Appl. Phys. Lett. 105 (14) (2014) 143106. dx.doi.org/10.1103/PhysRevB.93.195429. [87] P. Li, R.F. Egerton, Radiation damage in coronene, rubrene and p-terphenyl, [60] N. Yamamoto, S. Ohtani, F.J.G. de Abajo, Gap and mie plasmons in individual measured for incident electrons of kinetic energy between 100 and 200 kev, silver nanospheres near a silver surface, Nano Lett. 11 (1) (2011) 91–95. http:// Ultramicroscopy 101 (2–4) (2004) 161–172. dx.doi.org/10.1021/nl102862x. [88] L.H.G. Tizei, M. Kociak, Spectrally and spatially resolved cathodoluminescence of [61] N. Yamamoto, M. Nakano, T. Suzuki, Light emission by surface plasmons on nanodiamonds local variations of the nv0 emission properties, Nanotechnology 23 nanostructures of metal surfaces induced by high-energy electron beams, Surf. (17) (2012) 175702. http://dx.doi.org/10.1088/0957-4484/23/17/175702. Interface Anal. 38 (12–13) (2006) 1725–1730. [89] Z. Mahfoud, A.T. Dijksman, C. Javaux, P. Bassoul, A.-L. Baudrion, J. Plain, [62] R. Gomez-Medina, N. Yamamoto, M. Nakano, F.J.G. Abajo, Mapping plasmons in B. Dubertret, M. Kociak, Cathodoluminescence in a scanning transmission nanoantennas via cathodoluminescence, New J. Phys. 10 (2008) 105009. http:// electron microscope a nanometer-scale counterpart of photoluminescence for the dx.doi.org/10.1088/1367-2630/10/10/105009. study of ii-vi quantum dots, J. Phys. Chem. Lett. 4 (23) (2013) 4090–4094. [63] J.J.N.Y. Takumi Sannomiya, Hikaru Saito, Coupling of Plasmonic Nanopore Pairs: [90] D.A.B. Miller, D.S. Chemla, T.C. Damen, A.C. Gossard, W. Wiegmann, T.H. Wood, Facing Dipoles Attract Each Other, Light: Science & Applications 5 (e16146). C.A. Burrus, Band-edge electroabsorption in quantum well structures - the [64] M. Kuttge, F.J.G. de Abajo, A. Polman, Ultrasmall mode volume plasmonic quantum-confined stark-effect, Phys. Rev. Lett. 53 (22) (1984) 2173–2176. nanodisk resonators, Nano Lett. 10 (5) (2010) 1537–1541. [91] J.T. Griffiths, S. Zhang, B. Rouet-Leduc, W.Y. Fu, A. Bao, D. Zhu, D.J. Wallis, [65] V. Myroshnychenko, J. Nelayah, G. Adamo, N. Geuquet, J. Rodriguez-Fernandez, A. Howkins, I. Boyd, D. Stowe, M.J. Kappers, C.J. Humphreys, R.A. Oliver, I. Pastoriza-Santos, K.F. MacDonald, L. Henrard, L.M. Liz-Marzan, N.I. Zheludev, Nanocathodoluminescence reveals mitigation of the stark shift in ingan quantum M. Kociak, F.J.G. de Abajo, Plasmon spectroscopy and imaging of individual gold wells by si doping, Nano Lett. 15 (11) (2015) 7639–7643. nanodecahedra a combined optical microscopy, cathodoluminescence, and elec- [92] S.K. Lim, M. Brewster, F. Qian, Y. Li, C.M. Lieber, S. Gradecak, Direct correlation tron energy-loss spectroscopy study, Nano Lett. 12 (8) (2012) 4172–4180. http:// between structural and optical properties of iii-v nitride nanowire heterostruc- dx.doi.org/10.1021/nl301742h. tures with nanoscale resolution, Nano Lett. 9 (11) (2009) 3940–3944. [66] P. Das, T.K. Chini, J. Pond, Probing higher order surface plasmon modes on [93] G. Tourbot, C. Bougerol, F. Glas, L.F. Zagonel, Z. Mahfoud, S. Meuret, P. Gilet, individual truncated tetrahedral gold nanoparticle using cathodoluminescence M. Kociak, B. Gayral, B. Daudin, Growth mechanism and properties of ingan imaging and spectroscopy combined with fdtd simulations, J. Phys. Chem. C. 116 insertions in gan nanowires, Nanotechnology 23 (13) (2012) 135703. http:// (29) (2012) 15610–15619. http://dx.doi.org/10.1021/jp3047533. dx.doi.org/10.1088/0957-4484/23/13/135703. [67] C. Colliex, M. Kociak, O. Stephan, Electron energy loss spectroscopy imaging of [94] G. Jacopin, A. De Luna Bugallo, P. Lavenus, L. Rigutti, F.H. Julien, L.F. Zagonel, surface plasmons at the nanometer scale, Ultramicroscopy 162 (2016) A1–A24. M. Kociak, C. Durand, D. Salomon, X.J. Chen, J. Eymery, M. Tchernycheva, http://dx.doi.org/10.1016/j.ultramic.2015.11.012. Single-wire light-emitting diodes based on gan wires containing both polar and [68] O.L. Krivanek, T.C. Lovejoy, N. Dellby, T. Aoki, R.W. Carpenter, P. Rez, E. Soignard, nonpolar ingan/gan quantum wells, Appl. Phys. Express 5 (1) (2012) 014101. J. Zhu, P.E. Batson, M.J. Lagos, R.F. Egerton, P.A. Crozier, Vibrational spectroscopy [95] A. Pierret, C. Bougerol, B. Gayral, M. Kociak, B. Daudin, Probing alloy composi- in the electron microscope, Nature 514 (7521) (2014) 209–212. tion gradient and nanometer-scale carrier localization in single algan nanowires [69] K. Takeuchi, N. Yamamoto, Visualization of surface plasmon polariton waves in by nanocathodoluminescence, Nanotechnology 24 (2013) 305703. http:// two-dimensional plasmonic crystal by cathodoluminescence, Opt. Express 19 (13) dx.doi.org/10.1088/0957-4484/24/30/305703. (2011) 12365–12374. [96] A. Pierret, C. Bougerol, M.d. Hertog, B. Gayral, M. Kociak, H. Renevier, B. Daudin, [70] M. Honda, N. Yamamoto, Size dependence of surface plasmon modes in one- Structural and optical properties of al xga 1- xn nanowires, Phys. Status Solidi dimensional plasmonic crystal cavities, Opt. Express 21 (10) (2013) RRL 7 (10) (2013) 868–873. 11973–11983. [97] R. Bourrellier, M. Amato, L.H. Galvao Tizei, C. Giorgetti, A. Gloter, M.I. Heggie, [71] H. Saito, N. Yamamoto, Size dependence of bandgaps in a two-dimensional K. March, O. Stéphan, L. Reining, M. Kociak, A. Zobelli, Nanometric resolved plasmonic crystal with a hexagonal lattice, Opt. Express 23 (3) (2015) 2524–2540. luminescence in h-bn flakes: excitons and stacking order, ACS Photonics 1 (9) [72] C.A. Klein, Radiation Ionization Energies in Semiconductors : Speculations about (2014) 857–862. the Role of Plamons, Journal of the Physics Society of Japan Supp 21. [98] X. Zhou, M.-Y. Lu, Y.-J. Lu, E.J. Jones, S. Gwo, S. GradeÄak, Nanoscale Optical [73] C.A. Klein, Bandgap dependence and related features of radiation ionization Properties of Indium Gallium Nitride/gallium Nitride Nanodisk-in-rod energies in semiconductors, J. Appl. Phys. 39 (4) (1968) 2029–2038. Heterostructures, ACS Nano 0 (0), pMID: 25661775. arXiv: 〈http://dx.doi.org/ [74] A. Rothwarf, Plasmon theory of electron-hole pair production efficiency of cathode 10.1021/nn506867b〉, http://dx.doi.org/10.1021/nn506867b. ray phosphors, J. Appl. Phys. 44 (2) (1973) 752. [99] K. Pantzas, G. Patriarche, D. Troadec, M. Kociak, N. Cherkashin, M. Hÿtch, [75] S. Meuret, L.H.G. Tizei, T. Cazimajou, R. Bourrellier, H.C. Chang, F. Treussart, J. Barjon, C. Tanguy, T. Rivera, S. Suresh, A. Ougazzaden, Role of compositional M. Kociak, Photon Bunching in Cathodoluminescence, Phys. Rev. Lett. 114 (19) fluctuations and their suppression on the strain and luminescence of ingan alloys, (2015) 197401. J. Appl. Phys. 117 (2015) 055705. http://dx.doi.org/10.1063/1.4907210. [76] D. Drouin, A.R. Couture, D. Joly, X. Tastet, V. Aimez, R. Gauvin, Casino v2. 42–a [100] A. Urban, M. Müller, C. Karbaum, G. Schmidt, P. Veit, J. Malindretos, F. Bertram, fast and easy-to-use modeling tool for scanning electron microscopy and micro- J. Christen, A. Rizzi, Optical emission of individual GaN nanocolumns analyzed analysis users, Scanning 29 (3) (2007) 92–101. with high spatial resolution, Nano Lett. 15 (8) (2015) 5105–5109. [77] M. Toth, M. Phillips, Monte carlo modeling of cathodoluminescence generation [101] M.Müller,G.Schmidt,S.Metzner,P.Veit,F.Bertram,R.A.R.Leute,D.Heinz, using electron energy loss curves, Scanning 20 (6) (1998) 425–432. J. Wang, T. Meisch, F. Scholz, J. Christen, Nanoscale cathodoluminescene [78] N.M. Haegel, Integrating electron and near-field optics dual vision for the imaging of III-nitride-based LEDs with semipolar quantum wells in a scanning nanoworld, Nanophotonics 3 (1–2) (2014) 75–89. http://dx.doi.org/10.1515/ transmission electron microscope, Phys. Stat. Sol. (b) 253 (1) (2015) nanoph-2013-0048. 112–117.

68 M. Kociak, L.F. Zagonel Ultramicroscopy 174 (2017) 50–69

[102] M. Müller, G. Schmidt, S. Metzner, P. Veit, F. Bertram, S. Krylyuk, R. Debnath, J.- [111] L.F. Zagonel, L.H.G. Tizei, G.Z. Vitiello, G. Jacopin, L. Rigutti, M. Tchernycheva, Y. Ha, B. Wen, P. Blanchard, A. Motayed, M.R. King, A.V. Davydov, J. Christen, F.H. Julien, R. Songmuang, T. Ostasevicius, F. de la Peña, C. Ducati, P.A. Midgley, Structural and optical nanoscale analysis of GaN core-shell microrod arrays M. Kociak, Nanometer-scale monitoring of quantum-confined Stark effect and fabricated by combined top-down and bottom-up process on Si(111), Jpn. J. Appl. emission efficiency droop in multiple GaN/AlN quantum disks in nanowires, Phys. Phys. 55 (5S) (2016) 05FF02. Rev. B 93 (20) (2016) 205410. [103] J.T. Griffiths, S. Zhang, J. Lhuillier, D. Zhu, W.Y. Fu, A. Howkins, I. Boyd, [112] P. Lefebvre, B. Gayral, Optical properties of gan/aln quantum dots, Comptes D. Stowe, D.J. Wallis, C.J. Humphreys, R.A. Oliver, Nano-cathodoluminescence Rendus Phys. 9 (8) (2008) 816–829. reveals the effect of electron damage on the optical properties of nitride [113] L.H.G. Tizei, M. Kociak, Spatially resolved quantum nano-optics of single photons optoelectronics and the damage threshold, J. Appl. Phys. 120 (16) (2016) 165704. using an electron microscope, Phys. Rev. Lett. 110 (15) (2013) 153604. http:// [104] X. Zhang, H. Lourenço-Martins, S. Meuret, M. Kociak, B. Haas, J.-L. Rouviere, P.- dx.doi.org/10.1103/PhysRevLett.110.153604. H. Jouneau, C. Bougerol, T. Auzelle, D. Jalabert, X. Biquard, B. Gayral, B. Daudin, [114] M.J. Holmes, K. Choi, S. Kako, M. Arita, Y. Arakawa, Room-temperature triggered InGaN nanowires with high InN molar fraction growth, structural and optical single photon emission from a iii-nitride site-controlled nanowire quantum dot, properties, Nanotechnology 27 (19) (2016) 1–10. Nano Lett. 14 (2) (2014) 982–986. http://dx.doi.org/10.1021/nl404400d. [105] L.H. Robins, L.P. Cook, E.N. Farabaugh, A. Feldman, Cathodoluminescence of [115] L. Museur, E. Feldbach, A. Kanaev, Defect-related photoluminescence of hex- defects in diamond films and particles grown by hot-filament chemical-vapor agonal boron nitride, Phys. Rev. B 78 (15) (2008) 155204. deposition, Phys. Rev. B 39 (18) (1989) 13367–13377. http://dx.doi.org/ [116] J. Barjon, J. Brault, B. Daudin, D. Jalabert, B. Sieber, Cathodoluminescence study 10.1103/PhysRevB.39.13367. of carrier diffusion in algan, J. Appl. Phys. 94 (4) (2003) 2755–2757. http:// [106] T. Kuroda, A. Tackeuchi, Influence of free carrier screening on the luminescence dx.doi.org/10.1063/1.1593797. energy shift and carrier lifetime of ingan quantum wells, J. Appl. Phys. 92 (6) [117] M. Grundmann, J. Christen, D. Bimberg, A. Fischer-Colbrie, R. Hull, Misfit (2002) 3071–3074. http://dx.doi.org/10.1063/1.1502186. dislocations in pseudomorphic in0.23ga0.77as/gaas quantum wells influence on [107] S. Kalliakos, T. Bretagnon, P. Lefebvre, T. Taliercio, B. Gil, N. Grandjean, lifetime and diffusion of excess excitons, J. Appl. Phys. 66 (5) (1989) 2214. B. Damilano, A. Dussaigne, J. Massies, Photoluminescence energy and linewidth [118] P. de Boer, J.P. Hoogenboom, B.N.G. Giepmans, Correlated light and electron in gan/aln stackings of quantum dot planes, J. Appl. Phys. 96 (1) (2004) 180–185. microscopy ultrastructure lights up!, Nat. Methods 12 (6) (2015) 503–513. http://dx.doi.org/10.1063/1.1753085. [119] T. Furukawa, H. Niioka, M. Ichimiya, T. Nagata, M. Ashida, T. Araki, [108] A. Bell, J. Christen, F. Bertram, F.A. Ponce, H. Marui, S. Tanaka, Localization M. Hashimoto, High-resolution microscopy for biological specimens via catho- versus field effects in single ingan quantum wells, Appl. Phys. Lett. 84 (1) (2004) doluminescence of Eu- and Zn-doped Y23O nanophosphors, Opt. Express 21 (22) 58–60. http://dx.doi.org/10.1063/1.1638880. (2013) 25655. [109] J. Iveland, L. Martinelli, J. Peretti, J.S. Speck, C. Weisbuch, Direct measurement [120] S. Nagarajan, C. Pioche-Durieu, L.H.G. Tizei, C.-Y. Fang, J.-R. Bertrand, E. Le of auger electrons emitted from a semiconductor light-emitting diode under Cam, H.-C. Chang, F. Treussart, M. Kociak, Simultaneous cathodoluminescence electrical injection identification of the dominant mechanism for efficiency droop, and electron microscopy cytometry of cellular vesicles labeled with fluorescent Phys. Rev. Lett. 110 (17) (2013) 177406. http://dx.doi.org/10.1103/ nanodiamonds, Nanoscale 8 (2016) 11588–11594. PhysRevLett.110.177406. [121] A. Alhaddad, M.-P. Adam, J. Botsoa, G. Dantelle, S. Perruchas, T. Gacoin, [110] J. Piprek, Efficiency droop in nitride-based light-emitting diodes, Physica Status C. Mansuy, S. Lavielle, C. Malvy, F. Treussart, J.-R. Bertrand, Nanodiamond as a Solidi A-Appl. Mater. Sci. 207 (10) (2010) 2217–2225. http://dx.doi.org/ vector for siRNA delivery to ewing sarcoma cells, Small 7 (21) (2011) 3087–3095. 10.1002/pssa.201026149.

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