Um estudo de espectroscopias eletrônica e ótica em um microscópio de varredura de tunelamento e em um microscópio eletrônico de varredura por transmissão

Orientador: Luiz Fernando ZagonelDissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Física Gleb WataghinResumo: O desenvolvimento desse trabalho foi convenientemente dividido em três partes. A primeira inicia com o Microscópio de Varredura de Tunelamento (STM), no qual medidas padrão da reconstrução 7x7 do Silício (111) foram realizadas e discutidas. O modelo Dimer-Adatom-Stacking Fault foi usado para comparar nossas medidas, que incluíam a densidade local de estados esperada e a distância entre os adatoms. Nós observamos imagens atomicamente resolvidas e espectros de espectroscopia de tunelamento de elétrons (STS) consistentes com a combinação de sinais de adatoms e de rest atoms. Os primeiros foram utilizados para determinar a resolução espacial da ponta e o último para lhe caracterizar a densidade de estados. Na segunda parte desse trabalho, nós desenvolvemos um sistema óptico de coleção para o mesmo STM usado na primeira parte. Esse projeto teve mais enfoque em instrumentação e o objetivo era desenvolver um sistema de alinhamento robusto que tivesse uma precisão aproximadamente 50 nm para poder usar um espelho de alta coleção e detectar emissão de fótons da junção túnel. O pré-alinhamento foi feito visualmente usando o conceito de magnificação infinita e o alinhamento fino foi feito com o sinal de luz emitido pela junção túnel. Nós coletamos luz emitida da junção via decaimento de plásmons de superfície excitados pela ponta no qual os picos de luminescência estavam centrados em aproximadamente ~!LSP = 1.75 eV e com fatores de qualidade de Q = 4:25. A eficiência quântica, definida como a razão entre os fótons emitidos por unidade de elétrons tunelados, foi estimada em 10????7-10????8 fótons/elétrons, dependendo das condições de tunelamento e de detecção. Na terceira parte desse trabalho, nós usamos um micróscopio eletrônico de varredura por transmissão (STEM) para também estudar modos de plásmons de superfície com resolução espacial nanométrica. Nós realizamos majoritariamente espectroscopia de ganho de energia de elétrons (sEEGS), no qual o trabalho focou em melhorar a densidade de potência ao mudar o acoplamento da iluminação externa por fibra ótica para o espaço livre. Estruturas invertidas e nanocubos de prata foram estudados e, para esse último, a probabilidade de acoplamento entre o elétron e o fóton melhorou 200xAbstract: The development of this work was conveniently divided in three parts. The first opened the subject of Scanning Tunneling Microscope, in which standard measurements on Silicon (111) 7x7 reconstruction were performed and discussed. Dimer-Adatom-Stacking Fault model was used to compare our measurements, which included the expected local density of states and the distance between atoms. We have observed atomically resolved images and Scanning Tunneling Spectroscopy (STS) spectra consistent with a combination of signals from adatoms and rest atoms. The former was used to determine tip spatial resolution and the later to characterize the tip density of states (DOS). In the second part of this work, we have developed an optical collection system for the same STM used in the first part. This project was axed on instrumentation and the goal was to develop a robust alignment system, with a precision of approximately 50 nm, in order to use a high collective mirror to detect photon emission from the tunnel junction. The pre-alignment of the system was done visually using the concept of infinite magnification and the fine alignment was done using the actual light signal. We were able to collect light emitted from the junction by tip induced surface plasmon in which we saw peaks at approximately ~!LSP = 1.75 eV and quality factors of Q = 4:25. The quantum yield, defined as the ratio of emitted photons per unit of tunneling electrons, was determined to be around 10????7-10????8 photons/electrons, depending on the tunneling and detection conditions. In the third part of this work, we used a Scanning Transmission Electron Microscope (STEM) to also study surface plasmon modes with nanometric spatial resolution. We have performed mostly Stimulated Electron Energy Gain Spectroscopy (sEEGS), in which our work was focused on improving the system power density changing from guided space light coupling to a free space coupling. Inverted structures and silver nanocubes were studied and, for the later, coupling probability between the fast electron and the photon improved by a factor of 200xMestradoFisica AplicadaMestre em Física2017/00259-5  FAPES

Time calibration studies for the Timepix3 hybrid pixel detector in electron microscopy

Direct electron detection is currently revolutionizing many fields of electron microscopy due to its lower noise, its reduced point-spread function, and its increased quantum efficiency. More specifically to this work, Timepix3 is a hybrid-pixel direct electron detector capable of outputting temporal information of individual hits in its pixel array. Its architecture results in a data-driven detector, also called event-based, in which individual hits trigger the data off the chip for readout as fast as possible. The presence of a pixel threshold value results in an almost readout-noise-free detector while also defining the hit time of arrival and the time the signal stays over the pixel threshold. In this work, we have performed various experiments to calibrate and correct the Timepix3 temporal information, specifically in the context of electron microscopy. These include the energy calibration, and the time-walk and pixel delay corrections, reaching an average temporal resolution throughout the entire pixel matrix of $1.37 \pm 0.04$ ns. Additionally, we have also studied cosmic rays tracks to characterize the charge dynamics along the volume of the sensor layer, allowing us to estimate the limits of the detector's temporal response depending on different bias voltages, sensor thickness, and the electron beam ionization volume. We have estimated the uncertainty due to the ionization volume ranging from about 0.8 ns for 60 keV electrons to 8.8 ns for 300 keV electrons

Design and implementation of a device based on an off-axis parabolic mirror to perform luminescence experiments in a scanning tunneling microscope

We present the design, implementation, and illustrative results of a light collection/injection strategy based on an off-axis parabolic mirror collector for a low-temperature Scanning Tunneling Microscope (STM). This device allows us to perform STM induced Light Emission (STM-LE) and Cathodoluminescence (STM-CL) experiments and in situ Photoluminescence (PL) and Raman spectroscopy as complementary techniques. Considering the \'Etendue conservation and using an off-axis parabolic mirror, it is possible to design a light collection and injection system that displays 72% of collection efficiency (considering the hemisphere above the sample surface) while maintaining high spectral resolution and minimizing signal loss. The performance of the STM is tested by atomically resolved images and scanning tunneling spectroscopy results on standard sample surfaces. The capabilities of our system are demonstrated by performing STM-LE on metallic surfaces and two-dimensional semiconducting samples, observing both plasmonic and excitonic emissions. In addition, we carried out in situ PL measurements on semiconducting monolayers and quantum dots and in situ Raman on graphite and hexagonal boron nitride (h-BN) samples. Additionally, STM-CL and PL were obtained on monolayer h-BN gathering luminescence spectra that are typically associated with intragap states related to carbon defects. The results show that the flexible and efficient light injection and collection device based on an off-axis parabolic mirror is a powerful tool to study several types of nanostructures with multiple spectroscopic techniques in correlation with their morphology at the atomic scale and electronic structure.Comment: 19 pages, 14 Figure

High efficiency coupling of free electrons to sub-λ3\lambda^3 modal volume, high-Q photonic cavities

We report on the design, realization and experimental investigation by spatially resolved monochromated electron energy loss spectroscopy (EELS) of high quality factor cavities with modal volumes smaller than $\lambda^3$, with $\lambda$ the free-space wavelength of light. The cavities are based on a slot defect in a 2D photonic crystal slab made up of silicon. They are optimized for high coupling of electrons accelerated to 100 kV, to quasi-Transverse Electrical modes polarized along the slot direction. We studied the cavities in two geometries. The first geometry, for which the cavities have been designed, corresponds to an electron beam travelling along the slot direction. The second consists in the electron beam travelling perpendicular to the slab. In both cases, a large series of modes is identified. The dielectric slot modes energies are measured to be in the 0.8- 0.85 eV range, as per design, and surrounded by two bands of dielectric and air modes of the photonic structure. The dielectric even slot modes, to which the cavity mode belongs, are highly coupled to the electrons with up to 3.2$\%$ probability of creating a slot photon per incident electron. Although the experimental spectral resolution (around 30 meV) alone does not allow to disentangle cavity photons from other slot photons, the remarkable agreement between the experiments and FDTD simulations permits us to deduce that amongst the photons created in the slot, around 30$\%$ are stored in the cavity mode. A systematic study of the energy and coupling strength as a function of the photonic band gap parameters permits to foresee increase of coupling strength by fine-tuning phase matching. Our work demonstrates free electron coupling to high quality factor cavities with low mode density, sub-$\lambda^3$ modal volume, making it an excellent candidate for applications such as quantum nano-optics with free electrons

Excitation's lifetime extracted from electron-photon (EELS-CL) nanosecond-scale temporal coincidences

Electron-photon temporal correlations in electron energy loss (EELS) and cathodoluminescence (CL) spectroscopies have recently been used to measure the relative quantum efficiency of materials. This combined spectroscopy, named Cathodoluminescence excitation spectroscopy (CLE), allows the identification of excitation and decay channels which are hidden in average measurements. Here, we demonstrate that CLE can also be used to measure excitation's decay time. In addition, the decay time as a function of the excitation energy is accessed, as the energy for each electron-photon pair is probed. We used two well-known insulating materials to characterize this technique, nanodiamonds with \textit{NV$^0$} defect emission and h-BN with a \textit{4.1 eV} defect emission. Both also exhibit marked transition radiations, whose extremely short decay times can be used to characterize the instrumental response function. It is found to be typically 2 ns, in agreement with the expected limit of the EELS detector temporal resolution. The measured lifetimes of \textit{NV$^0$} centers in diamond nanoparticles (20 to 40 ns) and \textit{4.1 eV} defect in h-BN flakes ($<$ 2 ns) matches those reported for those materials previously

Nouvelles spectroscopies nanosecondes et millielectronvolts au microscope électronique et leurs applications à la nano-optique

In this thesis, a myriad of electron/matter/light interactions have been explored in a scanning transmission electron microscope (STEM) combining traditional electron spectroscopies, such as electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) with novel techniques, such as electron energy-gain spectroscopy (EEGS) and cathodoluminescence excitation spectroscopy (CLE), in which this work has contributed to developing. In particular, the narrow-band resonances of an optical whispering gallery mode resonator (WGMR) have motivated the development of EEGS, a promising technique that could combine the spectral resolution of laser sources with the spatial resolution of modern electron microscopes. Besides, the time-scale of the desired resonances, in the nanosecond range, has triggered instrumentation developments of time-resolved fast deflectors and detectors, such as Timepix3 (TPX3). WGMRs were studied either bare or coupled to a metallic nanoparticle (MNP) by using all the three main techniques aforementioned, i.e. EELS, CL, and EEGS. More than 80 gallery modes could be simultaneously identified in the bare resonator, while weak coupling was observed in the coupled system in the lower-order plasmonic modes of the MNP. Additionally, the event-based nature of TPX3 allowed the development of a hyperspectral image reconstruction that can be performed limited by the scanning unit instead of the long-lasting limitation of the readout time of frame-based detectors, such as in charged-coupled devices (CCDs). With this, the decomposition of calcite (CaCO3) was used to explore the time-resolved potentials of such detectors. Finally, the presence of time-to-digital converters (TDCs) in the TPX3 also triggered the development of CLE because electrons and photon events can be timestamped with the same reference clock and thus relate to the electron associated with a photon emission. CLE was used to study phase-locked electron-photon interactions, such as in transition radiation and localized surface plasmons, and non-phase-locked interactions, such as near-band edge and bulk plasmon absorption. For such, a silica-shelled gold nanosphere and a h-BN flake were used as samples.Dans cette thèse, un ensemble d’interactions électron/matière/lumière ont été explorées dans un microscope électronique en transmission à balayage (STEM, scanning transmission electron microscope) combinant les spectroscopies électroniques traditionnelles, telles que la spectroscopie de perte d’énergie des électrons (EELS, electron energy-loss spectroscopy) et la cathodoluminescence (CL) avec de nouvelles techniques, telles que la spectroscopie de gain d’énergie des électrons (EEGS, electron energy-gain spectroscopy) et la spectroscopie d’excitation de cathodoluminescence (CLE, cathodoluminescence excitation spectroscopy), au développement desquelles ce travail a contribué. En particulier, la mesure des résonances à bande étroite d’un micro-résonateur à modes de galerie (WGMR, whispering-gallery mode resonators) optique ont motivé le développement de l’EEGS, une technique prometteuse qui pourrait combiner la résolution spectrale des sources laser avec la résolution spatiale des microscopes électroniques modernes. En outre, l’échelle de temps des résonances souhaitées, de l’ordre de la nanoseconde, a déclenché le développement de l’instrumentation de déflecteurs et de détecteurs rapides à résolution temporelle, tels que la Timepix3 (TPX3). Les WGMRs ont été étudiés seuls ou couplés à une nanoparticule métallique (MNP, metallic nanoparticle) en utilisant les trois principales techniques mentionnées ci-dessus, c’est-à-dire l’EELS, la CL et l’EEGS. Plus de 80 modes de galerie ont pu être identifiés simultanément dans le résonateur seul, tandis qu’un faible couplage a été observé dans le système couplé dans les modes plasmoniques d’ordre inférieur du MNP. En plus, la nature « event-based » du TPX3 a permis le développement d’une reconstruction d’image hyperspectrale qui peut être exécutée en étant uniquement limitée par l’unité de balayage au lieu de la limitation du temps de lecture des détecteurs à frame, comme c’est le cas pour les CCDs (charge-coupled devices). Ainsi, la décomposition de la calcite (CaCO3) a été utilisée pour explorer les potentiels de résolution temporelle de ces détecteurs. Enfin, la présence de time-to-digital converters (TDCs) dans la TPX3 a également déclenché le développement de la CLE car les électrons et les événements photons peuvent être codés avec la même horloge de reference et ainsi associé l’électron à une émission photon. La CLE a été utilisé pour étudier les interactions électrons-photons en phase, telles que le rayonnement de transition et les plasmons de surface localisés, et les interactions sans relation de phase, telles que l’absorption proche de le band gap et d’un plasmon de volume. Une nanosphère d’or à coquille de silice et un flocon de h–BN ont été utilisés comme échantillons

Time-Resolved Cathodoluminescence in a Transmission Electron Microscope

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Cathodoluminescence excitation spectroscopy: nanoscale imaging of excitation pathways

Following the lifespan of optical excitations from their creation to decay into photons is crucial in understanding materials optical properties. Macroscopically, techniques such as the photoluminescence excitation spectroscopy provide unique information on the photophysics of materials with applications as diverse as quantum optics or photovoltaics. Materials excitation and emission pathways are affected by nanometer scale variations directly impacting devices performances. However, they cannot be directly accessed, despite techniques, such as optical spectroscopies with free electrons, having the relevant spatial, spectral or time resolution. Here, we explore optical excitation creation and decay in two representative optical devices: plasmonic nanoparticles and luminescent 2D layers. The analysis of the energy lost by an exciting electron that is coincident in time with a visible-UV photon unveils the decay pathways from excitation towards light emission. This is demonstrated for phase-locked interactions, such as in localized surface plasmons, and non-phase-locked ones, such as the light emission by individual point defects. This newly developed cathodoluminescence excitation spectroscopy images energy transfer pathways at the nanometer scale. It widens the toolset available to explore nanoscale materials

Dataset to manuscript Excitation's lifetime extracted from electron-photon (EELS-CL) nanosecond-scale temporal coincidences

&lt;p&gt;The datasets provided are raw data after the initial electron-photon temporal lists have been analyzed to determine the electron-photon pairs and their temporal delay. These lists are too long and not very meaningful to be provided in full.&lt;/p&gt;&lt;p&gt;The datasets were analyzed with the following Python libraries: Numpy 1.23.5, Matplotlib 3.6.2, Scipy 1.10.0, Hyperspy 1.7.3.&lt;/p&gt;&lt;p&gt;The provided jupyter notebooks were used to generate the figures in the manuscript.&lt;/p&gt

μeV electron spectromicroscopy using free-space light

Abstract The synergy between free electrons and light has recently been leveraged to reach an impressive degree of simultaneous spatial and spectral resolution, enabling applications in microscopy and quantum optics. However, the required combination of electron optics and light injection into the spectrally narrow modes of arbitrary specimens remains a challenge. Here, we demonstrate microelectronvolt spectral resolution with a sub-nanometer probe of photonic modes with quality factors as high as 104. We rely on mode matching of a tightly focused laser beam to whispering gallery modes to achieve a 108-fold increase in light-electron coupling efficiency. By adapting the shape and size of free-space optical beams to address specific physical questions, our approach allows us to interrogate any type of photonic structure with unprecedented spectral and spatial detail