Gian Carlo Gazzadi

Highly efficient electron vortex beams generated by nanofabricated phase holograms

We propose an improved type of holographic-plate suitable for the shaping of electron beams. The plate is fabricated by a focused ion beam on a silicon nitride membrane and introduces a controllable phase shift to the electron wavefunction. We adopted the optimal blazed-profile design for the phase hologram, which results in the generation of highly efficient (25%) electron vortex beams. This approach paves the route towards applications in nano-scale imaging and materials science.

Nanofabrication and the realization of Feynman’s two-slit experiment

Two nanosized slits are opened by focused ion beam milling in a membrane to observe, with a transmission electron microscope, electron interference fringes. Then, on the same sample, one of the slits is closed by focused ion beam induced deposition and the corresponding transmitted intensity is recorded. The comparison between the two measurements provides an impressive experimental evidence of the probability amplitude of quantum mechanics following step by step the original idea proposed by Feynman [The Feynman Lectures on Physics (Addison-Wesley, Reading, MA, 1966), Vol. 3, Chap. 1].

Electrical characterization and Auger depth profiling of nanogap electrodes fabricated by I2-assisted focused ion beam

Iodine (I2)-assisted, 30keV Ga+ focused ion beam (FIB) has been employed to fabricate nanogap Au electrodes and has been compared to conventional FIB milling. Electrical characterization shows that I2 assistance improves insulation resistance from 300–400GΩto20–50TΩ. Auger depth profiling reveals that the Ga concentration profile in FIB-milled samples has a peak value of 25at.% at 7nm and extends, with a decreasing Gaussian tail, down to 40nm, whereas in I2-processed samples Ga concentration is reduced below 5at.%. I2 assistance is found to increase minimum gap size from 8to16nm and to markedly roughen Au surface morphology.

Holographic Generation of Highly Twisted Electron Beams

Free electrons can possess an intrinsic orbital angular momentum, similar to those in an electron cloud, upon free-space propagation. The wave front corresponding to the electrons wave function forms a helical structure with a number of twists given by the angular speed. Beams with a high number of twists are of particular interest because they carry a high magnetic moment about the propagation axis. Among several different techniques, electron holography seems to be a promising approach to shape a conventional electron beam into a helical form with large values of angular momentum. Here, we propose and manufacture a nanofabricated phase hologram for generating a beam of this kind with an orbital angular momentum up to 200ℏ. Based on a novel technique the value of orbital angular momentum of the generated beam is measured and then compared with simulations. Our work, apart from the technological achievements, may lead to a way of generating electron beams with a high quanta of magnetic moment along the propagation direction and, thus, may be used in the study of the magnetic properties of materials and for manipulating nanoparticles.

Young’s double-slit interference experiment with electrons

In this short Note we report a method for producing samples containing two nano-sized slits suitable for demonstrating to undergraduate and graduate students the double-slit electron interference experiment in a conventional transmission electron microscope.

Fabrication of 5nm gap pillar electrodes by electron-beam Pt deposition

Using a focused ion beam (FIB)-scanning electron microscope (SEM) workstation, free-standing nanoelectrodes were grown by SEM-assisted Pt deposition between FIB-patterned Au pads. Two pillar electrodes were first grown with opposite-tilted geometries up to a spacing of 120nm. By SEM scanning over the pillar tips, under a precursor gas flow, gap reduction down to 5nm was monitored in live imaging mode. As shown by transmission electron microscopy (TEM) analysis, the deposit consisted of Pt crystallites embedded in amorphous C. Local annealing by high-current TEM irradiation increased the size of the Pt grains, which produced clear diffraction rings. The annealing procedure did not affect the overall shape of the tips, indicating good mechanical stability of the pillars. We show how this FIB-SEM approach is suitable to fabricate multielectrode nanostructures by depositing a third pillar electrode below the gap of the tilted electrodes.

Two and three slit electron interference and diffraction experiments

Current nanotechnology techniques make possible the preparation of slits in the submicrometer range so that electron interference and diffraction experiments can be done even with a conventional electron microscope. If the instrument is also equipped with a field emission source, it is possible to follow almost in real time the transition from the image of the slits to their Fraunhofer pattern through the intermediate Fresnel diffraction images. We discuss our results for the two-slit experiment and illustrate them for the three-slit case.

Build-up of interference patterns with single electrons

A conventional transmission electron microscope, equipped with a fast recording system able to measure the electron arrival time and the position of single electrons, is used to show the build-up of interference patterns. Two experiments are presented. The first is the electron version of the Grimaldi and Young experiments performed with light, where single electrons strike on an opaque thin wire. Interference fringes are observed in the geometrical shadow of the wire and diffraction effects are clearly displayed at the wire edges. The second, original experiment reports the build-up of two-slit interference patterns with single electrons.

Angular anisotropy of electron-excited secondary electron emission

Abstract We studied the influence of the crystalline structure of III–V compound semiconductors on the intensity of the energy distribution of secondary electrons. Modulation of the amplitude of the primary electron waves within the crystal by electron-atom scattering results in a dependence of secondary emission intensity on the incidence angle. We measured the intensity anisotropy of the electron energy distribution in a wide energy range on either GaAs(110) and InP(110) surfaces, for different values of the primary beam energy between 0.6 and 5 keV. We focused our attention on the elastic and inelastic Auger emission and on the background intensity. The Monte Carlo method was used to simulate the sequences of events experienced by the primary and secondary electrons in the material. Anisotropy was assumed to arise in electron elastic focusing on atomic sites. The calculated anisotropy dependence on the electron kinetic energy is consistent with experimental results. Differences in anisotropy of electrons with different probing depth is related to the different relative importance of surface and bulk contributions to the overall emission intensity. Defocusing along forward scattering directions does not occur over a depth of several tens of A, indicating that the defocusing length is longer than had been usually assumed for close-packed directions.

Realization of electron vortices with large orbital angular momentum using miniature holograms fabricated by electron beam lithography

Free electron beams that carry high values of orbital angular momentum (OAM) possess large magnetic moments along the propagation direction. This makes them an ideal probe for measuring the electronic and magnetic properties of materials, and for fundamental experiments in magnetism. However, their generation requires the use of complex diffractive elements, which usually take the form of nano-fabricated holograms. Here, we show how the limitations of focused ion beam milling in the fabrication of such holograms can be overcome by using electron beam lithography. We demonstrate experimentally the realization of an electron vortex beam with the largest OAM value that has yet been reported (L = 1000h\bar), paving the way for even more demanding demonstrations and applications of electron beam shaping.