T. LaGrange

The evolution of ultrafast electron microscope instrumentation.

Extrapolating from a brief survey of the literature, we outline a vision for the future development of time-resolved electron probe instruments that could offer levels of performance and flexibility that push the limits of physical possibility. This includes a discussion of the electron beam parameters (brightness and emittance) that limit performance, the identification of a dimensionless invariant figure of merit for pulsed electron guns (the number of electrons per lateral coherence area, per pulse), and calculations of how this figure of merit determines the trade-off of spatial against temporal resolution for different imaging modes. Modern photonics ability to control its fundamental particles at the quantum level, while enjoying extreme flexibility and a very large variety of operating modes, is held up as an example and a goal. We argue that this goal may be approached by combining ideas already in the literature, suggesting the need for large-scale collaborative development of next-generation time-resolved instruments.

Solving the accelerator-condenser coupling problem in a nanosecond dynamic transmission electron microscope

We describe a modification to a transmission electron microscope (TEM) that allows it to briefly (using a pulsed-laser-driven photocathode) operate at currents in excess of 10 mA while keeping the effects of condenser lens aberrations to a minimum. This modification allows real-space imaging of material microstructure with a resolution of order 10 nm over regions several microm across with an exposure time of 15 ns. This is more than six orders of magnitude faster than typical video-rate TEM imaging. The key is the addition of a weak magnetic lens to couple the large-diameter high-current beam exiting the accelerator into the acceptance aperture of a conventional TEM condenser lens system. We show that the performance of the system is essentially consistent with models derived from ray tracing and finite element simulations. The instrument can also be operated as a conventional TEM by using the electron gun in a thermionic mode. The modification enables very high electron current densities in microm-sized areas and could also be used in a nonpulsed system for high-throughput imaging and analytical TEM.

Imaging and controlling plasmonic interference fields at buried interfaces.

Capturing and controlling plasmons at buried interfaces with nanometre and femtosecond resolution has yet to be achieved and is critical for next generation plasmonic devices. Here we use light to excite plasmonic interference patterns at a buried metal–dielectric interface in a nanostructured thin film. Plasmons are launched from a photoexcited array of nanocavities and their propagation is followed via photon-induced near-field electron microscopy (PINEM). The resulting movie directly captures the plasmon dynamics, allowing quantification of their group velocity at ∼0.3 times the speed of light, consistent with our theoretical predictions. Furthermore, we show that the light polarization and nanocavity design can be tailored to shape transient plasmonic gratings at the nanoscale. This work, demonstrating dynamical imaging with PINEM, paves the way for the femtosecond and nanometre visualization and control of plasmonic fields in advanced heterostructures based on novel two-dimensional materials such as graphene, MoS2, and ultrathin metal films.

Laser-Induced Skyrmion Writing and Erasing in an Ultrafast Cryo-Lorentz Transmission Electron Microscope

We demonstrate that light-induced heat pulses of different duration and energy can write Skyrmions in a broad range of temperatures and magnetic field in FeGe. Using a combination of camera-rate and pump-probe cryo-Lorentz transmission electron microscopy, we directly resolve the spatiotemporal evolution of the magnetization ensuing optical excitation. The Skyrmion lattice was found to maintain its structural properties during the laser-induced demagnetization, and its recovery to the initial state happened in the sub-μs to μs range, depending on the cooling rate of the system.

Quantitative Phase Analysis of Rapid Solidification Products in Al-Cu Alloys by Automated Crystal Orientation Mapping in the TEM

There has been significant interest in laser-based-processing methods for the manufacturing of complex components (e.g. additive manufacturing) or post-processing/repair work (e.g. laser welding). Since laser processing tends to result in the formation of rapidly solidified microstructures, it becomes increasingly important to understand the microstructural development under such non-equilibrium conditions. Pulsed-laser-based-processing methods have been foci of investigations on rapidly solidified microstructures. This method has been shown to produce unique microstructures and micro-constituents in metallic thin films at the nanometer-scale [1].

Multifunctional molecular modulators for perovskite solar cells with over 20% efficiency and high operational stability

Perovskite solar cells present one of the most prominent photovoltaic technologies, yet their stability, scalability, and engineering at the molecular level remain challenging. We demonstrate a concept of multifunctional molecular modulation of scalable and operationally stable perovskite solar cells that exhibit exceptional solar-to-electric power conversion efficiencies. The judiciously designed bifunctional molecular modulator SN links the mercapto-tetrazolium (S) and phenylammonium (N) moieties, which passivate the surface defects, while displaying a structure-directing function through interaction with the perovskite that induces the formation of large grain crystals of high electronic quality of the most thermally stable formamidinium cesium mixed lead iodide perovskite formulation. As a result, we achieve greatly enhanced solar cell performance with efficiencies exceeding 20% for active device areas above 1u2009cm2 without the use of antisolvents, accompanied by outstanding operational stability under ambient conditions.Engineering hybrid perovskites at the molecular level to solve the stability problem remains a challenge. Here Grätzel et al. design a multifunctional molecular modulator that interacts with the perovskite via modes elucidated by solid state NMR spectroscopy and show high efficiency and operational stability.

Imaging Unsteady Propagation of Reaction Fronts in Reactive Multilayer Foils with Multi-Frame Dynamic TEM

Reactive multilayer foils (RMLFs) are composites formed of thin alternating layers of metals that undergo exothermic mixing reactions to form intermetallic compounds. The exothermic reaction provides adequate energy to cause self-propagation of the reaction beyond the local area where the reaction is initiated by an external energy source. RMLFs are used as local heat sources for brazing and soldering as well as for other commercial and defense applications.

Phase Transitions in Nanomaterials using Movie Mode Dynamic Transmission Electron Microscopy

2D layered nanomaterials including graphene, Bi2Se3, MoO3, and many others, have attracted enormous interest for both their countless industrial uses and their seemingly boundless ability to produce fascinating and unexpected physical behavior. This broad class of materials exhibits extraordinary electronic properties including Dirac-cone dispersion relations and topological insulating behavior while also being ideal for such literally down-to-earth applications as solid lubricants in fossil fuel drilling. 2D layered nanomaterials have also proven capable of reversibly absorbing surprisingly large amounts of foreign material within the van der Waals gaps separating the layers—which is to say, they are nearly ideal hosts for chemical intercalation.

Principles and Implementation of an Ultrafast Transmission Electron Microscope

In this communication we report on the performance of a fs-resolved transmission electron microscope, installed at the École Polytechnique Fédérale de Lausanne (EPFL), in the Laboratory for Ultrafast Microscopy and Electron Scattering (LUMES). The microscope, constructed by Integrated Dynamic Electron Solutions (IDES), is a variant of the Dynamic Transmission Electron Microscope developed at Lawrence Livermore National Laboratory capable of probing photoinitiated processes in materials on femtosecond timescales with2 Ångström resolution. Ultrafast temporal resolution is achieved by generating a photoelectron probe using 266nm laser pulses that are optically delayed relative to infrared pump pulses hitting the sample. To meet the energy, duration and repetition rate requirements we use a prototype laser system from KMLabs that delivers 80fs pulses a ta tunable repetition rate, 200kHz to 2MHz,with an energy of 1.55eV per photon and an average power of 3 W. The microscope is a modified JEOL JEM-2100TEM equipped with IDES constructed laser port and C0 lens sections that enable two pulsed laser beams to enter the column; an ultraviolet beam to illuminate the LaB6 cathode, generating electron pulses, and an infrared beam to stimulate excitations in the sample figure 1). In stroboscopic operation, to avoid space-charge effects and achieve femtosecond time resolution, every bunch of electrons should contain as few as 1 electron. This poses limitations in terms of integration time however this is overcome by the high repetition rate of the laser system and the improved coupling into the condenser system provided by the C0 lens. It is of capital importance to control the amount and spatial distribution of the charge photoemitted from the LaB6 tip. To do this, the optical set-up is designed in order to match the UV spot size to the flat surface of the cathode (50 μm); this limits the emission area of the tip, without the relying on high Wehnelt bias settings. The emitted electrons are then coupled in the column via an additional electromagnetic lens (C0lens) located just below the electron gun. This solution allows optimal coupling of the photoemitted electrons to the condenser system. High resolution images on a gold nanoparticle test sample have been collected to demonstrate that the modifications to the electron-optical system did not limit the spatial resolution of the microscope ( gure 2). The energy spread of the electron beam has been also characterized via the post column Gatan imaging filter. We notice here that the addition of the C0 lens allows us to efficiently couple electrons emitted from the lament and conserve the source brightness. Because the C0 lens allows much higher throughput, all the electrons emitted from the filament can be coupled to the standard TEM optics, producing currents as high as few microamps on the sample and detector. Thus lower lament heating currents can be used while maintaining reasonable signals on the detector, reducing the energy spread of the source and increasing spectroscopic energy resolution. Figure3 shows the width of the zero loss peak as measured using the GIF Quantum EELS spectrometer for di erent expected, using a lower in a narrower zero-loss peak. In addition, since the C0 lens improves coupling and allows high throughput, low Wehnelt bias voltages can be used to minimize the gun crossover and limit adverse Boersch e ects that increase energy spread. We envision that with the C0 lens modi low loss energy resolutionof0.4eV can be achieved with currents >100 nA on the detector, a vast improvement over standard thermionic TEM technology. The C0 lens 600 doi:10.1017/S1431927612004850 Microsc. Microanal. 18 (Suppl 2), 2012

In Situ Transmission Electron Microscopy of Rapidly Solidifying Aluminum

How microstructures evolve during solidification is of scientific and technological importance. Here we studied the morphological and structural changes in metallic thin films induced by rapid liquidsolid transformations with nanoscale spatial and 15 nanosecond temporal resolution using the dynamic transmission electron microscope (DTEM) at Lawrence Livermore National Laboratory (Figure 1a) [1]. This unprecedented time resolution allows the detailed study of phenomena associated with slow as well as ultrafast solidification processes. We discuss the rapid solidification of laser melted zones in nanocrystalline 80 nm thick Al films deposited on 100 nm thick amorphous Si3N4 membranes. The DTEM experiments were performed by, first, melting the Al films with a single 12ns, 1064nm laser pulse (Figure 1b). After a pre-selected time delay, as short as 15 ns, an electron pulse then illuminates this previously laser-pulse molten area to acquire image or diffraction data (Figure 1b). Acquiring series of pulsed electron exposures with different time delays permits direct observation of morphological and structural changes during the entire solidification process.