17 research outputs found
Development of quantitative in situ transmission electron microscopy for nanoindentation and cold-field emission
Cette thĂšse porte sur le dĂ©veloppement d'analyse quantitative d'expĂ©rience in situ de microscopie Ă©lectronique en transmission (MET). Nous avons utilisĂ© un porte objet spĂ©cial, qui combine les fonctions de polarisation Ă©lectrique locale et tests de micro-mĂ©canique. La mĂ©thode des Ă©lĂ©ments finis (MEF) a Ă©tĂ© mise en Ćuvre afin de comparer les rĂ©sultats issus de la modĂ©lisation avec les rĂ©sultats expĂ©rimentaux. En plus des techniques d'imagerie classique, l'holographie Ă©lectronique a Ă©tĂ© employĂ©e pour mesurer des champs Ă©lectriques et de dĂ©formation. La premiĂšre partie traite de l'Ă©mission de champ d'une nanopointe faite d'un cĂŽne de carbone (CCnT). Ce nouveau type de matĂ©riaux pourrait remplacer les pointes de tungstĂšne qui sont utilisĂ©s dans les canons d'Ă©lectrons les plus avancĂ©s. Quand un champ Ă©lectrique suffisamment fort est appliquĂ© au CCnT, les Ă©lectrons peuvent passer Ă travers la barriĂšre d'Ă©nergie avec le vide par effet tunnel, ce qui correspond au phĂ©nomĂšne d'Ă©mission de champ. En combinant holographie Ă©lectronique avec les simulations MEF, une valeur quantitative du champ Ă©lectrique local a Ă©tĂ© obtenue pour l'Ă©mission (2,5 V/nm). En faisant appel aux Ă©quations de Fowler-Nordheim, une valeur de la fonction de travail de sortie du CCnT est dĂ©terminĂ©e (4,8±0,3 eV). Nous avons Ă©galement mesurĂ© les charges sur le CCnT, avant et aprĂšs le dĂ©but de l'Ă©mission de champ. La deuxiĂšme partie porte sur la dĂ©formation plastique d'un film mince d'Al pour tester les interactions des dislocation - interface. Une dislocation Ă proximitĂ© d'une interface avec un matĂ©riau plus rigide doit ĂȘtre repoussĂ©e par celle-ci. Ici, nous constatons que les dislocations qui vont vers l'interface oxydĂ©e sont absorbĂ©es par cette interface rigide, mĂȘme Ă tempĂ©rature ambiante. La contrainte locale est dĂ©terminĂ©e par une combinaison de mesures de forces par le capteur et de calculs MEF. Enfin, des rĂ©sultats prĂ©liminaires de combiner indentation in situ et holographie Ă©lectronique en champ sombre sont prĂ©sentĂ©s.This thesis has focused on the development of quantitative in situ transmission electron microscopy (TEM) techniques. We have used a special nano-probe sample holder, which allows local electrical biasing and micro-mechanical testing. The finite element method (FEM) was used to compare models with the experimental results. In addition to conventional imaging techniques, electron holography has been used to measure electric fields and strains. The first part addresses cold-field emission from a carbon cone nanotip (CCnT). This novel type of carbon structure may present an alternative to W-based cold-field emission sources, which are used in the most advanced electron guns today. When a sufficiently strong electric field is applied to the CCnT, electrons can tunnel through the energy barrier with the vacuum, which corresponds to the phenomenon of cold-field emission. Using electron holography and FEM, a quantified value of the local electric field at the onset of field emission was found (2.5 V/nm). Combining this with one of the Fowler-Nordheim equations, the exit work function of the CCnT was determined to be 4.8±0.3 eV. The number of charges on the CCnT before and after the onset of field emission was also measured. The second part focuses on the plastic deformation of Al thin films to test dislocation-interface interactions. A dislocation close to an interface with a stiffer material should be repelled by it. Here, we find to the contrary that dislocations moving towards the oxidized interface are absorbed, even at room temperature. The stress was derived from a combination of load-cell measurements and FEM calculations. Finally, preliminary experiments to combine in situ indentation and dark-field electron holography are reported
An STM â SEM setup for characterizing photon and electron induced effects in single photovoltaic nanowires
Vertical arrays of semiconductor nanowires show great potential for material-efficient and high-performance solar cells. The characterization and correlation between material structure and properties of the individual nanowires are crucial for the continued performance improvement of such devices. In this work, we developed a method with a scanning tunneling microscope (STM) probe inside a scanning electron microscope (SEM) to enable the studies of single photovoltaic nanowires. The STM probe is used to contact individual nanowires in ensembles. We combine the STM-SEM with an in situ light emitting diode (LED) illumination source to study both the electrical and photovoltaic properties of vertical GaAs nanowires with radial p-i-n junctions. We also illustrate that the local charge separation ability within the nanowires can be studied by electron beam induced current (EBIC) measurements. The in situ SEM setup allows the correlation between properties and nanowire structure. The data show that the quality of the electrical contact to the semiconductor nanowire is crucial to be able to investigate the inherent properties of the nanowires. We have established a procedure to make high-quality ohmic contacts to the nanowires with the STM probe. We also show that the effect of mechanical strain on the electrical properties can be investigated by the STM-SEM setup
Localized resistance measurements of wrinkled reduced graphene oxide using in-situ transmission electron microscopy
The tunable electrical properties of reduced graphene oxide (rGO) make it an ideal candidate for many applications including energy storage. However, in order to utilize the material for applications it is essential to understand the behavior of the material on the nanoscale, especially how naturally occurring phenomena like wrinkling affect the electronic transport. Here, we use a transmission electron microscope (TEM) with electrical probe in-situ holder to perform localized electrical measurements on wrinkled, supported rGO flakes. The TEM allows for observation of the local wrinkled structure of the rGO and simultaneously an electrical probe is used to perform localized resistance measurements. For these measurements, there is no correlation between the electrode distance and the measured resistance indicating that contact resistance varies and dominates the measurements. There is, however, a correlation between increasing number of wrinkles underneath the probe and decreasing resistance, indicating that the wrinkles can provide surface area for contact with the probe and thus lower the resistance. The overall resistance is on the order of single kOhm, if the contact between the probe and the rGO is optimized. These measurements give evidence that rGO with wrinkling can compete as a leading type of graphene for certain applications
DĂ©veloppement de la microscopie Ă©lectronique en transmission in situ quantitative pour la nanoindentation et l'Ă©mission en champ froid
This thesis has focused on in situ transmission electron microscopy (TEM) techniques and especially quantitative in situ TEM. We have used a special TEM nano-probing sample holder, which combines local electrical biasing and micromechanical testing. Finite element method (FEM) modeling was used to compare with the experimental results. Different electron holography techniques have been used to measure electric fields and strains.The first part of this thesis addresses cold-field emission of a carbon cone nanotip (CCnT). This novel type of carbon structure may be used as an alternative to W-based cold-field emission guns (C-FEG), which are the most advanced electron guns used in TEMs today. When a sufficiently strong electric field is applied to the CCnT, electrons can tunnel through the energy barrier with the vacuum, which corresponds to the phenomenon of cold-field emission. The important parameters are the local electric field around the tip and the exit work function of the material.The experiment was realized by applying, inside the TEM holder, a potential to an anode facing the CCnT. By approaching the CCnT to the anode and increasing the bias, the electric field increased until field emission began. The electrons in the imaging beam of the TEM, arriving perpendicular to the electrons emitted from the CCnT, acquire a phase shift when traveling through the strong electric field. A map of the relative phase shift was obtained using off-axis electron holography. Combining the results with FEM, a quantitative value of the critical local electric field around the tip was obtained for the CCnT emission (2.5 V/nm). Finally, using this information together with one of the Fowler-Nordheim equations, which describes the field emission process, a value of the exit work function of the CCnT is determined (4.8±0.3 eV). We have also measured the charges on the CCnT, before and after the onset of field emission.The second part of the thesis focuses on the plastic deformation of an Al thin film deposited on an oxidized substrate to test dislocation-interface interactions. Here, we used a diamond-equipped microelectromechanical system (MEMS) sensor, to measure the force transmitted to a cross-sectional type sample. This configuration allows the simultaneous observation of moving dislocations processes in the sample and a measure of the applied force. Nanoindentation of a thin sample causes it to bend, and impede a stable image formation. Here, focused ion beam (FIB) was used to sculpture electron transparent sample windows in an H-bar configuration, which provides support for the sample. FEM was used to find the optimum window size that is a good balance between the stiffness provided by the H-bar shape and the side effects generated from the bulk part of the sample.According to dislocation theory, a dislocation close to an interface with a stiffer material should be repelled by it. The force being inversely proportional to the distance, a dislocation under an applied stress should be stationary at a certain distance from the interface. Here, we find that dislocations moving towards the oxidized interface are absorbed by this stiffer interface at room temperature. The stress at which this absorption occurs is derived from a combination of load-cell measurements and FEM calculations, and compared with supposed image force. This extends the findings of dislocation absorption at Al/SiO2 interfaces made at higher temperatures. Finally, a first try to combine in situ indentation and dark-field electron holography is reported. The goal there is to acquire a strain map of the indented sample directly from phase analysis.In addition of being a unique tool to see mechanisms unraveling in materials, in situ TEM techniques can nowadays provide quantitative information. This is achieved both by the development of sensor equipped TEM holders and by expanding previously static imaging techniques, modeling and analysis.Cette thĂšse porte sur la microscopie Ă©lectronique Ă transmission (MET) in situ et surtout Ă lâaspect quantitatif de cette technique. Nous avons utilisĂ© un porte objet MET spĂ©cial Ă pointe, qui combine polarisation Ă©lectrique locale et tests de micromĂ©canique. La cartographie par modĂ©lisation aux Ă©lĂ©ments finis (MEF) a Ă©tĂ© utilisĂ©e pour comparer les diffĂ©rents rĂ©sultats expĂ©rimentaux. Lâholographie Ă©lectronique a aussi Ă©tĂ© utilisĂ©e pour mesurer des champs Ă©lectriques et de dĂ©formation.La premiĂšre partie de cette thĂšse traite de lâĂ©mission de champ froid dâune nanopointe faite dâun cĂŽne de carbone (CCnT). Ce nouveau type de structure carbone peut ĂȘtre utilisĂ© comme une solution de rechange aux canons Ă cathode froide (C-FEG) Ă pointe de tungstĂšne qui sont les sources dâĂ©lectrons les plus avancĂ©es dans les MET modernes. Quand un champ Ă©lectrique suffisamment fort est appliquĂ© au CCnT, les Ă©lectrons peuvent passer par effet tunnel Ă travers la barriĂšre dâĂ©nergie avec le vide, ce qui correspond au phĂ©nomĂšne dâĂ©mission de champ froid. Les paramĂštres importants sont le champ Ă©lectrique local autour de la pointe et le travail de sortie du matĂ©riau. LâexpĂ©rience a Ă©tĂ© rĂ©alisĂ©e en appliquant, Ă lâintĂ©rieur du porte-objet, un potentiel Ă une anode faisant face au CCnT. En sâapprochant le CCnT de lâanode et en augmentant la polarisation, le champ Ă©lectrique augmente jusquâĂ ce que lâĂ©mission de champ se produise. La pointe est observĂ©e en mĂȘme temps avec le faisceau dâĂ©lectrons rapide du MET. Lâholographie Ă©lectronique consiste Ă faire interfĂ©rer des Ă©lectrons qui ont passĂ© prĂšs de la pointe avec les Ă©lectrons qui ont traversĂ© une rĂ©gion de faible champ, ce qui permet dâobtenir une carte du dĂ©phasage relatif. En combinant les rĂ©sultats avec les simulations MEF, une valeur quantitative du champ Ă©lectrique local critique autour de la pointe CCnT a Ă©tĂ© obtenue pour lâĂ©mission (2,5 V/nm). Enfin, en utilisant ces informations en mĂȘme temps que lâĂ©quation de Fowler-Nordheim, qui dĂ©crit le processus dâĂ©mission de champ, une valeur de la fonction de travail de sortie du CCnT est dĂ©terminĂ©e (4,8±0.3 eV). Nous avons Ă©galement Ă©tudiĂ© les charges sur le CCnT, avant et aprĂšs le dĂ©but de lâĂ©mission de champ.La deuxiĂšme partie de la thĂšse porte sur la dĂ©formation plastique dâun film mince dâAl dĂ©posĂ© sur un substrat oxydĂ© pour tester les interactions des dislocation â interface. Ici, nous avons utilisĂ© une pointe diamant montĂ©e sur un capteur de force micro-Ă©lectro-mĂ©canique (MEMS) pour mesurer la force transmise Ă un Ă©chantillon en section transverse. Cette configuration permet lâobservation simultanĂ©e des processus de dislocations dans lâĂ©chantillon et une mesure de la force appliquĂ©e. La nanoindentation dâun film mince impose une flexion du film, ce qui perturbe lâacquisition dâimage. Ici, un microscope ionique Ă sonde focalisĂ©e (FIB) a Ă©tĂ© utilisĂ© pour sculpter des fenĂȘtres transparentes aux Ă©lectrons dans une configuration dite "Hâbar", qui offre un maintien mĂ©canique Ă lâĂ©chantillon. Les simulations MEF ont Ă©tĂ© utilisĂ©es pour trouver la taille optimale de la fenĂȘtre, câest Ă dire le bon Ă©quilibre entre la rigiditĂ© grĂące Ă la forme en H et les effets de bord gĂ©nĂ©rĂ©s par la partie massive de lâĂ©chantillon. Selon la thĂ©orie des dislocations, une dislocation Ă proximitĂ© dâune interface avec un matĂ©riau plus rigide doit ĂȘtre repoussĂ©e par celle-ci. La force Ă©tant inversement proportionnelle Ă la distance, une dislocation sous contrainte appliquĂ©e doit sâarrĂȘter Ă une certaine distance de lâinterface. Ici, nous constatons que les dislocations qui vont vers lâinterface oxydĂ©e sont absorbĂ©es par cette interface rigide Ă tempĂ©rature ambiante. La contrainte Ă laquelle cette absorption se produit est dĂ©rivĂ©e dâune combinaison de mesures de forces par le capteur et de calculs MEF. Ils sont comparĂ©s Ă la force image supposĂ©e de la dislocation. Cela Ă©tend la validitĂ© des rĂ©sultats dâabsorption de dislocations aux interfaces Al/SiO2 faites Ă des tempĂ©ratures plus Ă©levĂ©es. Enfin, un premier essai pour combiner indentation in situ et holographie Ă©lectronique en champ sombre est rapportĂ©. Lâobjectif est dâacquĂ©rir une carte des contraintes de lâĂ©chantillon indentĂ© directement Ă partir de lâanalyse de phase.En plus dâĂȘtre un outil unique pour voir les mĂ©canismes actifs Ă lâĂ©chelle du nanomĂštre dans les matĂ©riaux, les techniques de MET in situ peuvent aujourdâhui fournir des informations quantitatives. Ceci est dĂ» Ă la fois au dĂ©veloppement des porte-objets MET Ă©quipĂ© de capteurs et Ă lâĂ©largissement des techniques dâimagerie statiques
Absorption of crystal/amorphous interfacial dislocations during in situ TEM nanoindentation of an Al thin film on Si
cited By 6International audienceIndentation of an Al film cross-section in situ in a transmission electron microscope at room temperature provides a direct observation of the absorption of threading dislocations by the metal/amorphous interface. The process occurs at a stress of about 100 MPa, a value that has been both calculated using finite-element modelling and directly measured from the dislocation radii. These results indicate that the strength of metallic thin films on oxidized substrates may therefore be weakly dependent on threading and interfacial dislocations
Three Dimensional Visualization of Electromagnetic Fields from One Dimensional Nanostructures
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In Situ TEM Observation of the Behavior of an Individual Fullerene-Like MoS2 Nanoparticle in a Dynamic Contact
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Determining the work function of a carbon-cone cold-field emitter by in situ electron holography
International audienceCold-field emission properties of carbon cone nanotips (CCnTs) have been studied in situ in the transmission electron microscope (TEM). The current as a function of voltage, i(V), was measured and analyzed using the FowlerâNordheim (FâN) equation. Off-axis electron holography was employed to map the electric field around the tip at the nanometer scale, and combined with finite element modeling, a quantitative value of the electric field has been obtained. For a tip-anode separation distance of 680 nm (measured with TEM) and a field emission onset voltage of 80 V, the local electric field was 2.55 V/nm. With this knowledge together with recorded i(V) curves, a work function of 4.8 ± 0.3 eV for the CCnT was extracted using the FâN equation
Low-noise cold-field emission current obtained between two opposed carbon cone nanotips during in situ transmission electron microscope biasing
International audienceA dedicated transmission electron microscope sample holder has been used to study in situ the cold-field emission process of carbon cone nanotips (CCnTs). We show that when using a CCnT instead of a Au plate-anode, the standard deviation of the emission current noise can be decreased from the 10ânA range to the 1ânA range under vacuum conditions of 10â5âPa. This shows the strong influence of the anode on the cold-field emission current noise
Electric-field-controlled reversible order-disorder switching of a metal tip surface
While it is well established that elevated temperatures can induce surface roughening of metal surfaces, the effect of a high electric field on the atomic structure at ambient temperature has not been investigated in detail. Here we show with atomic resolution using in situ transmission electron microscopy how intense electric fields induce reversible switching between perfect crystalline and disordered phases of gold surfaces at room temperature. Ab initio molecular dynamics simulations reveal that the mechanism behind the structural change can be attributed to a vanishing energy cost in forming surface defects in high electric fields. Our results demonstrate how surface processes can be directly controlled at the atomic scale by an externally applied electric field, which promotes an effective decoupling of the topmost surface layers from the underlying bulk. This opens up opportunities for development of active nanodevices in, e.g., nanophotonics and field-effect transistor technology as well as fundamental research in materials characterization and of yet unexplored dynamically controlled low-dimensional phases of matter.peerReviewe