Session L1: Focus Session: Imaging and Modifying Materials at the Limits of Space and Time Resolution

In-Situ Transmission Electron Microscopy with Nanosecond Temporal Resolution

The dynamic transmission electron microscope (DTEM) can obtain both high spatial ($\sim $1nm or better) and high temporal ($\sim $1$\mu $s or faster) resolution. The high temporal resolution is achieved by using a short pulse laser to create the pulse of electrons through photo-emission. This pulse of electrons is propagated down the microscope column in the same way as in a conventional high-resolution TEM. The only difference is that the spatial resolution is limited by the electron-electron interactions in the pulse (a typical 10ns pulse contains $\sim $10$^{9}$ electrons). To synchronize this pulse of electrons with a particular dynamic event, a second laser is used to ``drive'' the sample a defined time interval prior to the arrival of the laser pulse. The important aspect of the DTEM is that one pulse of electrons is used to form the whole image, allowing irreversible transitions and cumulative phenomena such as nucleation and growth, to be studied directly in the microscope. The use of the drive laser for fast heating of the specimen presents differences and several advantages over conventional resistive heating \textit{in-situ} TEM -- such as the ability to drive the sample into non-equilibrium states. So far, the drive laser has been used for \textit{in-situ} processing of nanoscale materials, rapid and high temperature phase transformations, and controlled thermal activation of materials. In this presentation, a summary of the development of the DTEM and in-situ stages to control the environment around the specimen will be described. Particular attention will be paid to the potential for gas stages to study catalytic processes and liquid stages to study biological specimens in their live hydrated states. The future potential improvements in spatial and temporal resolution that can be expected through the implementation of upgrades to the lasers, electron optics and detectors will also be discussed.   

Design and performance of a Near Ultra High Vacuum Helium Ion Microscope

The advent of He Ion Microscopy (HIM) as a new nanoscopic technique to image materials has enabled a new look at materials that is based on the interaction of swift light ions with matter. Initial HIM instruments have demonstrated high-resolution imaging, combined with great surface sensitivity, the ability to neutralize charge very efficiently, and with enhanced materials contrast when ion induced secondary electrons are used for imaging. To achieve ultimate performance, the chamber vacuum of the existing platform may be improved. For instance, carbon deposits due to beam interaction are readily seen due to the surface sensitivity of the technique. At high current densities the sharply focused beam may very efficiently decompose residual hydrocarbons. Not only can this obscure a clear view of the sample, thereby negating the benefits of the small spot size, it also limits the available acquisition time. This has proven extremely useful for nanopatterning for sensors, and other device fabrication applications at the sub-10nm level. However, it is undesirable when the instrument is used for materials characterization. We will discuss the basic considerations that went into the design of a Near-UHV He Ion Microscope [1]. First applications that the instrument was used for will be highlighted and its impact in surface physics and other research areas that require increased imaging sensitivity will be discussed. \\[4pt] [1] R.van Gastel et al, \textit{Microscopy and Microanalysis} \textbf{17}, 928-929 (2011)   

Instrumentation Development for Dynamic Atom Probe Tomography

Atom probe tomography (APT) is a materials characterization technique widely recognized as having the highest combined spatial (sub-nm) and chemical (less than 10$^{17}$ atoms / cm$^3$) resolution. This imaging time of flight mass spectrometry technique typically utilizes laser pulsed field emission in semiconductors and dielectrics. Laser pulsed field emission is widely considered to be a thermal effect, with specimen temperatures from 100 K to greater than 1500 K observed. Following the thermal spike, specimens typically cool to their base temperature in less than 5 ns, depending upon their thermal transport and geometry. Combining the laser pulsed atom probe experiment with pulsed transmission electron diffraction will enable thermal annealing and quenching experiments at atomic resolution with nanosecond temporal resolution. The combined instrument, titled a Dynamic Atom Probe, will be used to monitor solid state processes including solute drag effects on grain boundary motion, atomic scale kinetics of crystallization in amorphous semiconductors and oxides, atomic scale nucleation and spinodal decomposition kinetics in oxides, and phase transformations in metallic alloys. This discussion will include the specifics of the instrumentation currently under development as well as proof of principle first experiments.   

Simulation of ultrashort photoelectron pulses as a guide for developing reliable ultrafast electron diffraction systems

The development of a reliable experimental ultrafast electron diffraction and imaging system requires a theoretical understanding of the underlying physical phenomena and an accurate modeling of the optical elements present in the beam column. To achieve this goal, we have developed two types of computer simulations: a mean-field Gaussian approximation, in which the linear effects of realistic optical elements are incorporated. Due to the limitations inherent in the theory, it fails to capture the intricate behavior of a real system but it is computationally very inexpensive and offers valuable information on the relevant parameter ranges. The second type of simulation considered is an explicit N particle model which uses a O(N) method for calculating the space charge effects, enabling simulations of over one million electrons in a pulse. While being computationally very expensive, it offers the advantage of incorporating realistic descriptions for the electromagnetic fields of the optical elements, along with their linear and non-linear contributions. The results of this work show the limits of the validity of the mean-field approach and offer a detailed and highly accurate physical description of the beam dynamics under different operational regimes.   

Charged nanoparticle dynamics in water induced by scanning transmission electron microscopy

Using scanning transmission electron microscopy we image $\sim 4$~nm platinum nanoparticles deposited on an insulating membrane, where the membrane is one of two electron-transparent windows separating an aqueous environment from the microscope's high vacuum. Upon receiving a relatively moderate dose of $\sim 10^4\,e^-$/nm$^2$, initially immobile nanoparticles begin to move along trajectories that are directed radially outward from the center of the field of view. As the dose rate is increased the particle motion becomes increasingly dramatic. These observations demonstrate that even under mild imaging conditions, the \emph{in situ} electron microscopy of aqueous environments can produce charging effects that dominate the dynamics of nanoparticles under observation. Such effects provide a new tool for modifying \emph{in vitro} environments such as those used for TEM studies of wet biological systems.   

Coherent X-ray microscopy of real materials at 10nm resolution

Coherent X-ray microscopy replaces image-forming optics by mathematical algorithms to ``reconstruct'' the micrograph from the distribution of scattered light. ~In case of X-rays, its popularity is largely based on its ability to alleviate limitations of resolution and efficiency of available optics. Various methodologies have been developed, and high resolving power could be demonstrated. Yet, stringent constraints on sample preparation and data quality had to be addressed before the technique could advance from proof of principle studies to application. About a decade after the first demonstration using X-rays [J.W. Miao et al., Nature 400 (1999) 342] coherent diffractive imaging (CDI) seems to have matured into a microscopy technique that proves useful in real-life scientific investigations. The major coherent diffractive imaging methods will be reviewed, i.e., its original implementation using plane waves, the so-called Fresnel CDI that exploits curvature in the illuminating wavefront, and ptychographic CDI, which is a scanning microscopy method using multiple exposures. Being fully compatible with spectroscopic contrast channels makes quantitative structural, chemical, and magnetic information accessible, and the penetration power of X-rays can be exploited by tomographic or ab initio reconstruction methods in order to yield three-dimensional reconstructions with high resolution, high specificity, and high dose efficiency. These recent advances have turned coherent X-ray microscopy into a reliable and robust method that is attractive for scientists independent on how (un-)familiar they are with X-ray microscopy. Applications from the materials and life sciences will be discussed.   

Time-Resolved Near-Edge Coherent Diffractive Imaging

Coherent diffractive imaging (CDI) with x-rays is a well-established technique that provides structural information beyond the limitations of optical microscopy. Free electron lasers provide ultrashort x-ray pulses with sufficiently high peak brightness to facilitate single-shot imaging and the extension of CDI into the time-domain. Recent progress in the generation of spatially coherent ultrashort x-ray pulses by high harmonic generation (HHG) using tabletop lasers lead to the emergence of a new field of laboratory-based CDI. While a relatively low photon flux and limited photon energies result in lower imaging resolution compared to x-ray studies at large-scale facilities, the significantly greater availability makes laboratory-based experiments well suited for developing new CDI techniques. We present a new apparatus for CDI, which provides ultrashort XUV pulses with tunable photon energies. By implementing a monochromator in a HHG-based CDI setup, the photon energy can be tuned to the inner-shell absorption edges of different elements in the sample. The wavelength-dependence of the x-ray optical constants close to the resonances facilitates to exploit the element selectivity and chemical sensitivity of x-ray transitions in time-domain CDI experiments.   

In-situ proton radiography of solidification in Sn-Bi, Al-Cu, Al-In and Bi-Ga melts

In-situ observation of the solidification phenomena in metals can lead to better understanding and control of microstructure evolution. Proton radiography offers the ability to image thick samples of high z material. Recently, in-situ proton radiography was used to directly observe dynamic processes during melting and solidification in bulk binary alloy systems. The spatial resolution was $\sim $65 microns for a 44 x 44 mm2 field of view. The time scale of each experiment was 2 to 6 hours. The data collected allowed for the determination of solidification front velocities, captured the changes in solid/liquid densities and showed evidence of convective fluid flow in the melt. Microstructural features larger than approximately 100 microns in the solid phase were observed.   

True Attosecond Imaging of Inhomogeneous Systems with Standing-Wave Inelastic X-Ray Scatterin

Inelastic X-ray scattering (IXS) has recently been used to image electron dynamics at the attosecond scale, but it has been shown that these images are spatially averaged. The problem is that the existing technique can only access the ``diagonal'' ($k_1=-k_2$) elements of the electron density response $\chi(k_1,k_2,\omega)$. It has been shown, however, that inelastic X-ray scattering at the Bragg position is sensitive to the entire response $\chi(k_1,k_2,\omega)$. With this method, a standing wave field is established in the sample by exciting a Bragg condition, allowing access to all the off-diagonal ($k_1\ne- k_2$) elements of $\chi$. In this talk I will present a simple model demonstrating that, in principle, this approach can be used to map the entire density response. In particular, I show that a one-dimensional system is experimentally impossible to probe, a two-dimensional system is experimentally accessible but typically difficult to measure in practice, and a three-dimensional system is experimentally both plausible and practicable. Finally, I propose possible experimental realizations of this technique.   

Sub-Picosecond Dynamics of Displacement Cascades

Sub-picosecond x-ray pulses produced by the Linac Coherent Light Source (LCLS) now enable real time experimental measurements of atomic displacement cascade structure and dynamics on sub-picosecond time scales. Such measurements will make possible the first direct experimental test of molecular dynamics (MD) displacement cascade simulations. Here we will discuss the potential to use single, seeded, 100-fs LCLS hard x-ray pulses focused to $\sim $100 nm diameter by diamond-based Fresnel zone plate optics to make real-time diffuse scattering measurements on 50 keV Ar-ion-induced cascades in thin single-crystal samples. We will present x-ray Bragg diffuse scattering calculations based on $\sim $ 4M atom, 25 keV primary knock-on energy MD cascade simulations demonstrating that temporally-random, 100 fs LCLS x-ray pulse measurements of diffuse scattering near low index Bragg reflections can be time-ordered from sub-picoseconds to a few picoseconds. Time ordering is made possible in this regime by the distinct nature of diffuse scattering profiles as a function of time that are produced by shock-induced pressure waves according to MD cascade simulations in Fe. The expected results and the experimental challenges anticipated to perform such measurements will be discussed.   

Nanostructure Formation on Sb$_{2}$Te$_{3}$ Thin Films Induced by Femtosecond Laser Irradiation

Sb$_{2}$Te$_{3}$ has applications in thermoelectrics, phase-change memory devices and topological insulators. In the case of thermoelectricity, nanostructure formation in this type of material has been predicted to enhance its figure of merit for thermal energy conversion. Here, we present our results on modification of the surface morphology of Sb$_{2}$Te$_{3}$ thin films after femtosecond laser irradiation. Under a narrow range of laser fluence and irradiation time, long and highly-aligned nanotracks were formed in the plane of the film, having a periodicity 10 times smaller than the irradiation laser wavelength. The laser fluence and irradiation time can result in different surface nanostructure morphologies with varying degrees of order. Finally, using an optical pump-probe technique, we find that the laser-irradiated nanostructured areas of the film have a lower thermal conductivity compared to that of the reference smooth areas not irradiated by the laser. Such Sb$_{2}$Te$_{3}$ nanostructures can be important for thermoelectric applications as well as for further studies of femtosecond laser interaction with opaque materials.