Nanosecond time-resolved investigations using the in situ of dynamic transmission electron microscope (DTEM)

Most biological processes, chemical reactions and materials dynamics occur at rates much faster than can be captured with standard video rate acquisition methods in transmission electron microscopes (TEM). Thus, there is a need to increase the temporal resolution in order to capture and understand salient features of these rapid materials processes. This paper details the development of a high-time resolution dynamic transmission electron microscope (DTEM) that captures dynamics in materials with nanosecond time resolution. The current DTEM performance, having a spatial resolution <10nm for single-shot imaging using 15ns electron pulses, will be discussed in the context of experimental investigations in solid state reactions of NiAl reactive multilayer films, the study of martensitic transformations in nanocrystalline Ti and the catalytic growth of Si nanowires. In addition, this paper will address the technical issues involved with high current, electron pulse operation and the near-term improvements to the electron optics, which will greatly improve the signal and spatial resolutions, and to the laser system, which will allow tailored specimen and photocathode drive conditions.     

Spectrally resolved cathodoluminescence analyses in the optical near-field

By implementing a scanning near-field optical microscope into the specimen chamber of a scanning electron microscope, cathodoluminescence can be locally detected in the optical near-field. The achievable spatial resolution in this set-up is only limited by the size of the aperture in a coated fibre probe and its separation from the sample, rather than by the energy dissipation volume of the primary electrons and diffusion processes of excess carriers inside the specimen. We demonstrate how electronically active defects in polycrystalline diamond can be distinguished and localized with sub-wavelength lateral resolution by spectral filtering of the cathodoluminescence signal.     

Spectrally resolved cathodoluminescence analyses in the optical near-field

By implementing a scanning near-field optical microscope into the specimen chamber of a scanning electron microscope, cathodoluminescence can be locally detected in the optical near-field. The achievable spatial resolution in this set-up is only limited by the size of the aperture in a coated fibre probe and its separation from the sample, rather than by the energy dissipation volume of the primary electrons and diffusion processes of excess carriers inside the specimen. We demonstrate how electronically active defects in polycrystalline diamond can be distinguished and localized with sub-wavelength lateral resolution by spectral filtering of the cathodoluminescence signal.     

Modelling imaging based on core-loss spectroscopy in scanning transmission electron microscopy

Recent experimental realizations of atomic column resolution core-loss spectroscopy in the scanning transmission electron microscope have increased the importance of routinely modelling core-loss images. We discuss different approaches to wave function simulation and how they may be used in conjunction with the mixed dynamic form factor model to simulate images resulting from such inelastic scattering events. It is shown that, as resolution improves and in situations where the degree of thermal scattering is high, detailed quantitative comparisons will require the thermal scattering of electrons to be adequately modelled. Indeed, for sufficiently strong thermal scattering even qualitative interpretation may be affected: we give an example where this leads to a contrast reversal. We describe two methods suited to this purpose, the frozen lattice model and the scattering factor model, and explain how they may be combined with the mixed dynamic form factor approach.     

Resolving power of lens-less low energy electron point source microscopy

Simulations show the resolving power of lens-less low energy electron point source microscopy to be approximately 15 Å  for electron energies of between 14 and 105 eV. This resolution can be improved to 10 Å  by employing a composite hologram method. However, these values fall short of predictions of at least 3 Å  resolution for electron energies in this range because the limited divergence angle of the electron beam had not previously been taken into account. Also shown is that electron coherence from field emitting tips that terminate in a single atom will not limit the resolving power as much as the beam divergence angle. The penetration depth of electrons with energies of between 50 and 100 eV, into biological molecules, was estimated from the inelastic mean free path length to be 25 Å . This places an upper limit on the size of biological molecules for which internal structural information can be obtained using low energy electrons.     

Resolution in low voltage scanning electron microscopy

As the energy of an electron beam is reduced, the range falls and the secondary electron yield rises. A low voltage scanning electron microscope can therefore, in principle, examine without damage or charging samples such as insulators, dielectrics or beam sensitive materials. This paper investigates the way in which the choice of beam energy affects the spatial resolution of a secondary electron image. It is shown that for samples which are thin compared to the electron range, the edge resolution and contrast in the image improve with increasing beam energy. In samples that are thicker than the electron range, the resolution can be optimized at either high or low energies, but low energy operation will produce images of higher contrast. At an energy of 2 keV or less beam interaction limited resolutions of the order of 3 nm should be possible.     

Polycrystal orientation maps from TEM

Determination of topography of crystallite orientations is an important technique of investigation of polycrystalline materials. A system for creating orientation maps using transmission electron microscope (TEM) Kikuchi patterns and Convergent beam electron diffraction patterns is presented. The orientation maps are obtained using a step-by-step beam scan on a computer-controlled TEM equipped with a CCD camera. At each step, acquired diffraction patterns are indexed and orientations are determined. Although, the approach used is similar to that applied in SEM/electron back scattered diffraction (EBSD) orientation imaging setups, the TEM-based system considerably differs from its SEM counterpart. The main differences appear due to specific features of TEM and SEM diffraction patterns. Also, the resulting maps are not equivalent. On these generated by TEM, the accuracy of orientation determination can be better than 0.1 degrees. The spatial resolution is estimated to be about 10nm. The latter feature makes the TEM orientation mapping system an important tool for studies at fine scale unreachable by SEM/EBSD systems. The automatic orientation mapping is expected to be a useful complement of the conventional TEM contrast images. The new technique will be essential for characterization of fine structure materials. To illustrate that, example maps of an aluminum sample produced by severe plastic deformation are included.     

Direct electron imaging in electron microscopy with monolithic active pixel sensors

A new imaging device for dynamic electron microscopy is in great demand. The detector should provide the experimenter with images having sufficient spatial resolution at high speed. Immunity to radiation damage, accumulated during exposures, is critical. Photographic film, a traditional medium, is not adequate for studies that require large volumes of data or rapid recording and charge coupled device (CCD) cameras have limited resolution, due to phosphor screen coupling. CCD chips are not suitable for direct recording due to their extreme sensitivity to radiation damage. This paper discusses characterization of monolithic active pixel sensors (MAPS) in a scanning electron microscope (SEM) as well as in a transmission electron microscope (TEM). The tested devices were two versions of the MIMOSA V (MV) chip. This 1M pixel device features pixel size of 17 x 17 microm(2) and was designed in a 0.6 microm CMOS process. The active layer for detection is a thin (less than 20 microm) epitaxial layer, limiting the broadening of the electron beam. The first version of the detector was a standard imager with electronics, passivation and interconnection layers on top of the active region; the second one was bottom-thinned, reaching the epitaxial layer from the bottom. The electron energies used range from a few keV to 30 keV for SEM and from 40 to 400 keV for TEM. Deterioration of the image resolution due to backscattering was quantified for different energies and both detector versions.     

Fluctuation microscopy in the STEM

Fluctuation electron microscopy is a technique for studying medium-range order in disordered materials. We present an implementation of fluctuation microscopy using nanodiffraction in a scanning transmission electron microscope (STEM) at a spatial resolution varying from 0.8 to 5.0 nm. Compared to conventional TEM (CTEM), the STEM-based technique offers a denser scattering vector sampling at a reduced sample dose and easier access to variable resolution information. We have reproduced results on amorphous silicon previously obtained by CTEM-based fluctuation microscopy, and report initial variable-resolution measurements on amorphous germanium.     

Very low voltage electron microscopy.

We conclude that a 150 V scanning microscope with a resolution of 10 A is quite feasible and could have considerable value. It might consist of a field emission source, an electron gun to decelerate the electrons, a condenser lens to produce a parallel beam, a multipole corrector and a short focal length objective lens. Electrons reflected from the specimen surface would pass through a spectrometer whose principal features would be a large collecting power and low (1/200) energy resolution. Finally, we should add that such a microscope presents a considerable challenge and new opportunities for the electron optician in both lens and spectrometer design. We cannot refrain from pointing out that the Scherzer theorem does not necessarily hold for such a lens since the constraints of the theorem do not apply to this case.     

X-ray imaging of various biological samples using a phase-contrast hard X-ray microscope

In this study, we visualized the internal structures of various bio-samples and found the optimum conditions of test samples for the 7 keV hard X-ray microscope of the Pohang light source. From the captured X-ray images, we could observe the intercellular and intracellular structures of dehydrated human cells and mouse tumor tissues without using any staining materials in a spatial resolution better than 100 nm. The metastasized lung tissue, which was several tens of micrometers in thickness, was found to be very well suited to this hard X-ray microscope system, because it is nearly impossible to observe such a nontransparent and thick sample with a high spatial resolution better than 100 nm using any microscopes such as a soft X-ray microscope, an optical microscope, or an electron microscope.     

A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of sigma=18 ps and energy resolution Delta E/E=3.5%

We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5 microm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to sigma=18 ps. We observed that our electron coincidence detector has a timing resolution better than sigma=16 ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of DeltaE(FWHM)/E=3.5% for electrons with kinetic energy near 2 eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about Deltam(FWHM)/m=14150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to sigma=115 microm.     

Resolution and contrast in the electron microscope: an historical review

The development of quantitative interpretation of electron micrographs, in respect of contrast as well as resolution, has followed similar lines to those in optical microscopy, though on a faster time scale. A thorough understanding of phase contrast came relatively late in both disciplines. The definition of resolution is more complicated in the case of the electron image on account of the severe effect of lens aberrations, especially spherical aberration. Experimental measurement of the performance of an electron microscope requires an operational definition of resolving power, which must include contrast considerations as well as the limitations imposed by lack of spatial and temporal coherence of the electron source. Agreement on suitable test procedures is now being reached, at least for a very thin specimen.     

Laser pulse-induced transitions on surfaces of bulk material,traced by thermal and secondary electrons

Thermal and electron beam-released electrons were exploited to probe the dynamics of surface modifications, induced by a Q-switched frequency-doubled Nd:YAG laser within areas of 100 μm Φ on bulk silicon and metals. Changes of surface geometry and phase transitions show up as pronounced peaks and steps in the emitted electron currents. They occur within Φ 200 ns after the laser pulse and consume times from less than 5 ns up to 200 ns. Applications of pulsed radiation power for surface machining, recrystallization and vitrification or for production of microstructures on bulk substrates for integrated devices are continually extending (Bäuerle 1984). Laser pulses are most frequently used, in spite of their inhomogeneous absorption, as contrasted to electron and ion beams, because of easy handling of power and no need for vacuum in many cases. The diagnostic methods usually applied to probe the dynamics of pulse-induced transitions are still those introduced at the beginning of the “laser annealing” technique, exploiting light reflection and transmission, and to a minor extent x-ray and electron diffraction, mass spectrometry and electrical conductivity (Khaibullin 1984, Larson 1984). Naturally, each diagnostic tool has a restricted range of useful application, so further methods combining high temporal and spatial resolution and equally applicable to semiconductors and metals are highly desirable. Thermal and secondary electrons are expected to be susceptible to changes of temperature and geometry of a surface, occuring during laser pulse machining. In fact, photon-assisted thermal electron emission was used to probe thermal relaxations in laser-pulsed semiconductors (Leung and van Driel 1984), however, the thermal electrons were not used to trace geometric modifications. Furthermore, secondary electrons have not yet been tested as a probe for very fast single effects. This report describes first results, demonstrating the usability of emitted electrons as a probe for nanosecond transitions on surfaces of bulk material. An electron optical equipment was built, consisting of electron gun, condensor lens and specimen chamber, allowing synchronized laser and electron beam pulsing of the specimen. Its surface was probed by the photo-thermally emitted electrons and ions or by the secondary and backscattered electrons, which were generated by the focussed primary electron beam. The latter was pulsed in order to suppress electron radiation damage. For laser treatment a pulse of a Q-switched frequency-doubled Nd:YAG laser (FWHM 20 ns) was focussed with a lens and a dielectric mirror onto the specimen, which could be viewed with a microscope for aligning the laser and the primary electron beam. Both beams had a diameter of 100 μm (FWHM) on the specimen. The emitted charges were collected by a shielded scintillator/multiplier detector of the Everhart-Thornley type, having a rise time of = 3 ns and being protected against excessive green laser light by an edge filter. Despite the simp1e set-up, rapid changes of the surface by vaporization, melting, solidification could readily be observed within areas down to 30 μm Φ on the nanosecond time scale. Flow and disrupture of liquid layers occur after the laser pulse, delayed by several 10 ns (Figs. 1 and 2). The dispersion of metal liquids by temperature-induced gradients of the surface tension may consume times from below 5 ns (Fig. 2a) up to 200 ns (Fig. la). It is signalized by a large increase of electron emission, probably due to Schottky effect at charged transient tips within the disintegrating liquid. First order phase transitions involving latent heats are readily indicated by thermal electrons. Solidification of a melt, for instance, shows up in the emission current as a prolonged plateau with an abrupt drop within 20 … 40 ns (Fig. 3). Summarizing, secondary and thermal electrons are well suited to trace single transitions on the surface of bulk material on the nanosecond-micrometer scale. In contrast to other diagnostic probes the spatial resolution may be increased well below 1 μm by improved focussing of the primary electron beam.     

Scanning absorption nanoscopy with supercontinuum light sources based on photonic crystal fiber

We have experimentally demonstrated a scanning absorption nanoscopy system combining a near-field scanning optical microscope with an absorption spectroscope using supercontinuum radiation generated by coupling a mode-locked Ti:sapphire pulse laser to a nonlinear photonic crystal fiber as a light source. For the performance test of the system, the absorption spectrum and near-field absorption image of Rhodamine 6G were observed. As this system allows us to investigate the absorption properties and distribution of materials with high spatial resolution, it is expected to be effectively applied in various research areas.     

Angle-resolved photoemission spectroscopy with a femtosecond high harmonic light source using a two-dimensional imaging electron analyzer

An experimental setup for time- and angle-resolved photoemission spectroscopy using a femtosecond 1 kHz high harmonic light source and a two-dimensional electron analyzer for parallel energy and momentum detection is presented. A selection of the 27th harmonic (41.85 eV) from the harmonic spectrum of the light source is achieved with a multilayer MoSi double mirror monochromator. The extinction efficiency of the monochromator in selecting this harmonic is shown to be better than 7:1, while the transmitted bandwidth of the selected harmonic is capable of supporting temporal pulse widths as short as 3 fs. The recorded E(k) photoelectron spectrum from a Cu(111) surface demonstrates an angular resolution of better than 0.6 degrees (=0.03 A(-1) at E(kin,e)=36 eV). Used in a pump-probe configuration, the described experimental setup represents a powerful experimental tool for studying the femtosecond dynamics of ultrafast surface processes in real time.     

Energy loss spectroscopic profiling across linear interfaces: the example of amorphous carbon superlattices

Energy loss spectroscopic profiling is a way to acquire, in parallel, spectroscopic information across a linear feature of interest, using a Gatan imaging filter (GIF) fitted to a transmission electron microscope (TEM). This technique is capable of translating the high spatial resolution of a bright field image into a sampling of the spectral information with similar resolution. Here we evaluate the contributions of chromatic aberration and the various acquisition parameters to the spatial sampling resolution of the spectral information, and show that this can reach 0.5 nm, in a system not ordinarily capable of forming electron probes smaller than 2 nm. We use this high spatial sampling resolution to study the plasmon energy variation across amorphous carbon superlattices, in order to extract information about their structure and electronic properties. By modelling the interaction of the relativistic incident electrons with a dielectric layer sandwiched between outer layers, we show that, due to the screening of the interfaces and at increased collection angles, the plasmon energy in the sandwiched layer can still be identified for layer thicknesses down to 5 A. This allows us to measure the change in the well bandgap as a function of well width and to interpret it in terms of the changes in the sp2 -fractions due to the deposition method, as measured from the carbon K-edges, and in terms of quantum confinement of the well wavefunction by the adjacent barriers.     

Secondary electron detection in the scanning transmission electron microscope

In a dedicated scanning transmission electron microscope (STEM) secondary electron images with high spatial resolution and good contrast can be obtained. Two types of detector are described. These take into account the secondary electrons which depend on the post-specimen field strength of the objective lens. Due to the thinness of the samples and the collection geometry the images differ from those obtained in a convectional scanning microscope. Examples are given where secondary electron images augment the information obtained by the more commonly used imaging modes.