Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode

Bücker, Kerstin; Picher, Matthieu; Crégut, O.; LaGrange, Thomas; Reed, Bryan W.; Park, S.T.; Masiel, Daniel J.; Banhart, Florian

doi:10.1016/j.ultramic.2016.08.014

Cited by 65 publications

(55 citation statements)

References 21 publications

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“…Excitation, lifetime and decay pathways of excited states of the most common oxygen vacancies in alumina have been studied by photoluminescence [12] and cathodoluminescence [13]. The development of femtosecond electron sources [14,15] makes it now possible to study ultrafast electron dynamics by a novel technique called Ultrafast Electron Microscopy [16][17][18], which combines the spatial resolution of an Electron Microscope (EM) and the temporal resolution typical of an ultrafast optical pump-probe configuration: the sample is excited by two ultrashort pulses, one optical and one electronic, and the effect on typical electron microscope probes, such as SE, is measured as a function of the delay between the two pulses. The nanometer escape depth of the SE probe [19,20] gives the potential to address dynamics at surfaces and interfaces of today's nano-scale devices, where many applications rely on the interplay between semiconductors and insulators.…”

Section: Introductionmentioning

confidence: 99%

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

et al. 2018

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Section: Introductionmentioning

confidence: 99%

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

et al. 2018

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“…The shank-emitted electrons are subject to large aberrations, including substantial path-length differences that greatly worsen temporal resolution. It was recently demonstrated experimentally that shank-emitted photoelectrons and photoelectrons emitted from the flat truncated surface of a conventional cathode geometry arrive at the sample with a relative time delay when operating an UEM with low bias on the Wehnelt electrode 11 . Thus, in order to perform imaging and diffraction experiments at high temporal resolution, the UEM must be operated with a sufficiently high Wehnelt bias voltage to reject all shank-emitted electrons from entering the limiting aperture of the column.…”

Section: Introductionmentioning

confidence: 99%

“…A potential solution is the use of disc cathodes. In particular, tantalum disc cathodes with a diameter exceeding that of the photo-exciting laser beam have shown promising results 11,12 . However, during conventional thermionic operation, a large cathode is detrimental to the performance of the microscope.…”

Section: Introductionmentioning

confidence: 99%

Influence of cathode geometry on electron dynamics in an ultrafast electron microscope

Piazza

Cao

et al. 2017

Structural Dynamics

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Efforts to understand matter at ever-increasing spatial and temporal resolutions have led to the development of instruments such as the ultrafast transmission electron microscope (UEM) that can capture transient processes with combined nanometer and picosecond resolutions. However, analysis by UEM is often associated with extended acquisition times, mainly due to the limitations of the electron gun. Improvements are hampered by tradeoffs in realizing combinations of the conflicting objectives for source size, emittance, and energy and temporal dispersion. Fundamentally, the performance of the gun is a function of the cathode material, the gun and cathode geometry, and the local fields. Especially shank emission from a truncated tip cathode results in severe broadening effects and therefore such electrons must be filtered by applying a Wehnelt bias. Here we study the influence of the cathode geometry and the Wehnelt bias on the performance of a photoelectron gun in a thermionic configuration. We combine experimental analysis with finite element simulations tracing the paths of individual photoelectrons in the relevant 3D geometry. Specifically, we compare the performance of guard ring cathodes with no shank emission to conventional truncated tip geometries. We find that a guard ring cathode allows operation at minimum Wehnelt bias and improve the temporal resolution under realistic operation conditions in an UEM. At low bias, the Wehnelt exhibits stronger focus for guard ring than truncated tip cathodes. The increase in temporal spread with bias is mainly a result from a decrease in the accelerating field near the cathode surface. Furthermore, simulations reveal that the temporal dispersion is also influenced by the intrinsic angular distribution in the photoemission process and the initial energy spread. However, a smaller emission spot on the cathode is not a dominant driver for enhancing time resolution. Space charge induced temporal broadening shows a close to linear relation with the number of electrons up to at least 10 000 electrons per pulse. The Wehnelt bias will affect the energy distribution by changing the Rayleigh length, and thus the interaction time, at the crossover.

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“…In previous reports from other groups, the 0.34 nm lattice fringes of graphitized carbon were resolved when pulsed photoelectron packets were generated at the repetition rate of 25 MHz 2 . Recently, the image of gold crystalline nanoparticles with the lattice fringes of 0.23 nm was successfully taken at 2 MHz 30 . This high repetition, however, narrows the choice of a specimen because the specimen upon photoexcitation must fully revert to its original configuration in less than 1 microsecond ( μ s), the case of which is practically rare.…”

mentioning

confidence: 97%

“…This space-charge effect limits the formation and propagation of fs-long electron pulses considerably; ultrashort electron pulses broaden in space and lengthen in time 30,31 . Typically, the spatial resolution of pulsed electron packets generated by optical pulses of a few hundreds of femtoseconds at lower repetition rates than MHz seems to reside around several nanometers at best 32 .…”

mentioning

confidence: 99%

Ultrafast electron microscopy integrated with a direct electron detection camera

Lee

Kim

et al. 2017

Structural Dynamics

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In the past decade, we have witnessed the rapid growth of the field of ultrafast electron microscopy (UEM), which provides intuitive means to watch atomic and molecular motions of matter. Yet, because of the limited current of the pulsed electron beam resulting from space-charge effects, observations have been mainly made to periodic motions of the crystalline structure of hundreds of nanometers or higher by stroboscopic imaging at high repetition rates. Here, we develop an advanced UEM with robust capabilities for circumventing the present limitations by integrating a direct electron detection camera for the first time which allows for imaging at low repetition rates. This approach is expected to promote UEM to a more powerful platform to visualize molecular and collective motions and dissect fundamental physical, chemical, and materials phenomena in space and time.

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Electron beam dynamics in an ultrafast transmission electron microscope with Wehnelt electrode

Cited by 65 publications

References 21 publications

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

Charge dynamics in aluminum oxide thin film studied by ultrafast scanning electron microscopy

Influence of cathode geometry on electron dynamics in an ultrafast electron microscope

Ultrafast electron microscopy integrated with a direct electron detection camera

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