The emission of light from metal-oxide-metal tunnel junctions has been studied as a probe of the interaction of the tunneling current with the surface plasmon polaritons of the tunnel junction structure. The slow mode, with large fields in the oxide layer, is most directly coupled to the tunneling current. In this experiment, diffraction gratings with periods between 70 and 100 nm provide the momentum for the slow mode to radiate. We report the unambiguous measurement of the direct emission from the slow mode showing that it does contribute to the light emission.
Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.
The ability to acquire high time resolution movies, dubbed Movie Mode Dynamic Transmission Electron Microscope (DTEM), expands the DTEM's science capabilities in single-shot mode by providing detailed histories of unique material events on the nanometer and nanosecond scale. Prior DTEM hardware only allowed single-pump/single-probe operation, building up a process's typical time history by repeating an experiment with varying time delays at different sample locations [1,2]. The Movie Mode DTEM upgrade enables single-pump/multi-probe operation [2]. It provides the ability to track the creation, motion, and interaction of individual defects, phase fronts, and chemical reaction fronts, providing invaluable information of the chemical, microstructural and atomic level features that influence the dynamics and kinetics of rapid material processes. For example, the potency of a nucleation site is governed by many factors related to defects, local chemistry, etc. While a single pump-probe snapshot provides statistical data about these factors, a multi-frame movie of a unique event allows all of the factors to be identified and the progress of nucleation and growth processes can be explored in detail. It provides unprecedented insight into the physics of rapid material processes from their early stages (e.g. nucleation) to completion, giving direct, unambiguous information regarding the dynamics of complex processes.The two core components of the Movie Mode technology (Figure 1) are the arbitrary waveform generator (AWG) cathode laser system and a high-speed electrostatic deflector array. The AWG cathode drive laser enables continuously variable and controlled electron pulse durations from 250 µs down to 5 ns in which a series of laser pulses is produced with user-defined pulse durations and delays that stimulates a defined photoemitted electron pulse train for a single sample drive event. Each pulse captures an image of the sample at a specific time. A fast-switching electrostatic deflector located below the sample directs each image to a separate patch on a large, high-resolution CCD camera. At the end of the experiment, the entire CCD image is read-out and segmented into a time-ordered series of images, i.e. a movie. The current technology produces 9 frame movies but near-term modification to the system should enable up to 25-frame movies with interframe times as low as 25 ns. This frame rate is six orders of magnitude faster than modern video-rate in situ TEM. Future versions of movie mode may also include fast-framing CCD technology, which can capture hundreds of frames within a few µs. The operating principle of these devices is that the photoelectron CCD data from multiple frames is stored in on-chip buffers that are read out at the end of the acquisition. This presentation will discuss the Movie Mode DTEM technology in the context of materials science examples in which salient, irreversible events of rapid phase transformations are revealed through multiple frame movies.References:[1] T.
Free-standing, polycrystalline Al/Ni multilayer thin films, or reactive multilayer foils (RMLF), react to form intermetallic compounds in a rapid, self-propagating reaction. The exothermic reaction front heats up to temperatures above 1120 K [1] and travels at a high velocity of ~10m/s in air. Since RMLFs produce extremely high heat over a small surface area, they are used in application to fuse dissimilar materials such as metal and ceramic, exposing only contacting surfaces to destabilizing heat.In this study, we perform a comparative study of transient intermetallic phase formation within the reaction front region using both optical and electron microscopy. Five bilayers of Ni and Al are repeated for an electron transparent 125 nm thick sample. STEM and TEM micrographs in Fig. 1 show the specimen geometry which appears to have insignificant interphase premixing from the deposition process. The formation reaction is initiated by a Nd:YLF ultraviolet laser inside the vacuum of the microscope. Optical time-resolved imaging is done to observe reaction front velocities and the evolution of a steady-state propagation of intermetallic formation. The specimen generates light during this exothermic formation and is detected every 2 µs for direct observation of this dynamic process. In Fig. 2, the reaction initiates at the position of the laser spot and gradually radiates outward until the ends of the jagged-shaped sample have been illuminated. The intensity can be related to temperature of the foil and the passing of the reaction front. It appears that the reaction front travels at a greater velocity than that mentioned above due to the vacuum pressure and sample dimensions. Analysis of the detected light levels indicates a non-steady state propagation of the reaction front. The reaction products are investigated with postmortem STEM and TEM studies to characterize any changes in the reaction front behavior.The metastable phase formation is also studied with the in-situ methods of the DTEM. The DTEM maintains the high resolution of conventional TEM but is also capable of fast (ns) time dependence studies. The instrument at Lawrence Livermore National Laboratory is modeled after the DTEM made by . It uses high-speed ultraviolet laser pulses to induce electron photoemission, creating a fast, pulsed incident electron beam. Because the dynamic evolution of phases are contained within the RMLF reaction front region alone, a single timeresolved image or diffraction pattern of this significant area gives significant information on transient states, rate of formation and also fundamental mesoscale kinetics. Such data will develop a complete picture of intermetallic phase formation in RMLFs down to the atomic and nanosecond scale. When this is captured at high time and spatial resolution, the self propagating reaction fronts can be related to the foil parameters, improving modeling [3] currently based upon diffusion and energy.
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