We present the first demonstration of ultrafast laser-induced field emission and measurement of the energy distribution of electrons from a nanotip based on a carbon nanotube (CNT). Our experimental ...setup extends the studies performed on conventional tungsten or gold tips by using this new innovative tip. The carbon tip consists of concentric carbon layers in the shape of a cone, and has been previously studied as a very good candidate for cold field emission. The first laser-induced field emission from a CNT-based nanotip has been observed and we measured the energy spectrum as well as the polarization dependance of the emission. We also characterize the damage threshold of the tip, when illuminated by a high repetition rate femtosecond laser. These first results are encouraging further studies of electron emission from CNT-based carbon nanotips.
•First demonstration of ultrafast-laser induced emission from a CNT based nanotip.•Nanotip consists of concentric carbon layers in the shape of a cone.•Measurements of the energy spectrum and polarization dependence of emission.•Characterization of tip damage threshold.
A newly developed carbon cone nanotip (CCnT) has been used as field emission cathode both in low voltage SEM (30kV) electron source and high voltage TEM (200kV) electron source. The results clearly ...show, for both technologies, an unprecedented stability of the emission and the probe current with almost no decay during 1h, as well as a very small noise (rms less than 0.5%) compared to standard sources which use tungsten tips as emitting cathode. In addition, quantitative electric field mapping around the FE tip have been performed using in situ electron holography experiments during the emission of the new tip. These results show the advantage of the very high aspect ratio of the new CCnT which induces a strong enhancement of the electric field at the apex of the tip, leading to very small extraction voltage (some hundred of volts) for which the field emission will start. The combination of these experiments with emission current measurements has also allowed to extract an exit work function value of 4.8eV.
•We develop a new field emission cathode based on carbon material.•We determine the exit work function of this new cathode using a combination of in situ electron holography and finite element modeling.•We show that the stability of cold-field emitted current can be improved with no decay during one hour of emission with a lower emission noise (less than 0.5%).•We used this cathode both for 200kV TEM and 30kV SEM cold field emission source.•As a TEM source, we also observe an increase of the spatial coherence using Fresnel fringes contrast.
The metal-oxide-semiconductor (MOS) capacitor is one of the fundamental electrical components used in integrated circuits. While much effort is currently being made to integrate new dielectric or ...ferroelectric materials, capacitors of silicon dioxide on silicon remain the most prevalent. It is perhaps surprising therefore that the electric field within a such capacitor has never been measured, or mapped out, at the nanoscale. Here we present results from operando electron holography experiments showing the electric potential across a working MOS nanocapacitor with unprecedented sensitivity and reveal unexpected charging of the dielectric material bordering the electrodes.
The fabrication of multi-gigabit magnetic random access memory (MRAM) chips requires the patterning of magnetic tunnel junctions at very small dimensions (sub-30 nm) and a very dense pitch. This ...remains a challenge due to the difficulty in etching magnetic tunnel junction stacks. We previously proposed a strategy to circumvent this problem by depositing the magnetic tunnel junction material on prepatterned metallic pillars, resulting in the junction being naturally shaped during deposition. Upon electrical contact, the deposit on top of the pillars constitutes the magnetic storage element of the memory cell. However, in this process, the magnetic material is also deposited in the trenches between the pillars that might affect the memory cell behaviour. Here we study the magnetic interactions between the deposit on top of the pillars and in the trenches by electron holography, at room temperature and up to 325 °C. Supported by models, we show that the additional material in the trenches is not perturbing the working principle of the memory chip and can even play the role of a flux absorber which reduces the crosstalk between neighboring dots. Besides, in the studied sample, the magnetization of the 1.4 nm thick storage layer of the dots is found to switch from out-of-plane to an in-plane configuration above 125 °C, but gradually decreases with temperature. Electron holography is shown to constitute a very efficient tool for characterizing the micromagnetic configuration of the storage layer in MRAM cells.
Electron holography investigation of the magnetic induction field of perpendicular magnetic tunnel junctions for the fabrication of magnetic random access memories (MRAM), and temperature behavior.
We present an experiment studying the interaction of a strongly focused 25 fs laser pulse with a tungsten nanotip, investigating the different regimes of laser-induced electron emission. We study the ...dependence of the electron yield with respect to the static electric field applied to the tip. Photoelectron spectra are recorded using a retarding field spectrometer and peaks separated by the photon energy are observed with a 45% contrast. They are a clear signature of above threshold photoemission (ATP), and are confirmed by extensive spectrally resolved studies of the laser power dependence. Understanding these mechanisms opens the route to control experiment in the strong-field regime on nanoscale objects.
One-dimensional (1D) nanostructures have been regarded as the most promising building blocks for nanoelectronics and nanocomposite material systems as well as for alternative energy applications. ...Although they result in confinement of a material, their properties and interactions with other nanostructures are still very much three-dimensional (3D) in nature. In this work, we present a novel method for quantitative determination of the 3D electromagnetic fields in and around 1D nanostructures using a single electron wave phase image, thereby eliminating the cumbersome acquisition of tomographic data. Using symmetry arguments, we have reconstructed the 3D magnetic field of a nickel nanowire as well as the 3D electric field around a carbon nanotube field emitter, from one single projection. The accuracy of quantitative values determined here is shown to be a better fit to the physics at play than the value obtained by conventional analysis. Moreover the 3D reconstructions can then directly be visualized and used in the design of functional 3D architectures built using 1D nanostructures.
•Novel method for tomography of 3D electromagnetic fields from a single image is presented.•The method relies upon using cylindrical symmetry and is applied to 1D nanostructures.•The 3D magnetic field of a Nickel nanowire is reconstructed.•The 3D electric field from a biased carbon cone nanotip is reconstructed.•Our method improves the quantitative measurement of the 3D electromagnetic fields.
We probe magnetic domain walls in cylindrical soft magnetic nanowires using electron holography. We detail the modelling of expected contrast for both transverse and Bloch point domain walls and ...provide comparison with experimental observations performed on NiCo nanowires, involving also both magnetic and electrostatic contribution to the electron holography map. This allows the fast determination of the domain wall type without the need for uneasy and time-consuming experimental removal of the electrostatic contribution. Finally, we describe and implement a new efficient algorithm for calculating the magnetic contrast.
Topological insulators (TIs) hold great promise for new spin-related phenomena and applications thanks to the spin texture of their surface states. However, a versatile platform allowing for the ...exploitation of these assets is still lacking due to the difficult integration of these materials with the mainstream Si-based technology. Here, we exploit germanium as a substrate for the growth of Bi2Se3, a prototypical TI. We probe the spin properties of the Bi2Se3/Ge pristine interface by investigating the spin-to-charge conversion taking place in the interface states by means of a nonlocal detection method. The spin population is generated by optical orientation in Ge and diffuses toward the Bi2Se3, which acts as a spin detector. We compare the spin-to-charge conversion in Bi2Se3/Ge with the one taking place in Pt in the same experimental conditions. Notably, the sign of the spin-to-charge conversion given by the TI detector is reversed compared to the Pt one, while the efficiency is comparable. By exploiting first-principles calculations, we ascribe the sign reversal to the hybridization of the topological surface states of Bi2Se3 with the Ge bands. These results pave the way for the implementation of highly efficient spin detection in TI-based architectures compatible with semiconductor-based platforms.
We develop the self-assembly of epitaxial submicrometer-sized face-centered-cubic (fcc) Co(111) dots using pulsed laser deposition. The dots display atomically flat facets, from which the ratios of ...surface and interface energies for fcc Co are deduced. Zero-field magnetic structures are investigated with magnetic force and Lorentz microscopies, revealing vortex-based flux-closure patterns. A good agreement is found with micromagnetic simulations.
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