Determining the work function of a carbon-cone cold-field emitter by in situ electron holography

Measuring Electrical Resistivity at the Nanoscale in Phase-Change Materials

Electrical resistivity is the key parameter in the active regions of many current nanoscale devices, from memristors to resistive random-access memory and phase-change memories. The local resistivity of the materials is engineered on the nanoscale to fit the performance requirements. Phase-change memories, for example, rely on materials whose electrical resistance increases dramatically with a change from a crystalline to an amorphous phase. Electrical characterization methods have been developed to measure the response of individual devices, but they cannot map the local resistance across the active area. Here, we propose a method based on operando electron holography to determine the local resistance within working devices. Upon switching the device, we show that electrical resistance is inhomogeneous on the scale of only a few nanometers.

Extended Charge Layers in Metal-Oxide-Semiconductor Nanocapacitors Revealed by Operando Electron Holography

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 such a 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.

Single Electron Precision in the Measurement of Charge Distributions on Electrically Biased Graphene Nanotips Using Electron Holography

We use off-axis electron holography to measure the electrostatic charge density distributions on graphene-based nanogap devices that have thicknesses of between 1 and 10 monolayers and separations of between 8 and 58 nm with a precision of better than a single unit charge. Our experimental measurements, which are compared with finite element simulations, show that wider graphene tips, which have thicknesses of a single monolayer at their ends, exhibit charge accumulation along their edges. The results are relevant for both fundamental research on graphene electrostatics and applications of graphene nanogaps to single nucleotide detection in DNA sequencing, single molecule electronics, plasmonic antennae, and cold field emission sources.

Optimization of off-axis electron holography performed with femtosecond electron pulses

We report on electron holography experiments performed with femtosecond electron pulses in an ultrafast coherent Transmission Electron Microscope based on a laser-driven cold field emission gun. We first discuss the experimental requirements related to the long acquisition times imposed by the low emission/probe current available in these instruments. The experimental parameters are first optimized and electron holograms are then acquired in vacuum and on a nano-object showing that useful physical properties can nevertheless be extracted from the hologram phase in pulsed condition. Finally, we show that the acquisition of short exposure time holograms assembled in a stack, combined with a computer-assisted shift compensation of usual instabilities encountered in holography, such as beam and biprism wire instabilities, can yield electron holograms acquired with a much better contrast paving the way to ultrafast time-resolved electron holography.

Quantitative measurement of nanoscale electrostatic potentials and charges using off-axis electron holography: Developments and opportunities

Off-axis electron holography has evolved into a powerful electron-microscopy-based technique for characterizing electromagnetic fields with nanometer-scale resolution. In this paper, we present a review of the application of off-axis electron holography to the quantitative measurement of electrostatic potentials and charge density distributions. We begin with a short overview of the theoretical and experimental basis of the technique. Practical aspects of phase imaging, sample preparation and microscope operation are outlined briefly. Applications of off-axis electron holography to a wide range of materials are then described in more detail. Finally, challenges and future opportunities for electron holography investigations of electrostatic fields and charge density distributions are presented.

200 keV cold field emission source using carbon cone nanotip: Application to scanning transmission electron microscopy

We report the use of a pyrolytic carbon cone nanotip as field emission cathode inside a modern 200 kV dedicated scanning transmission electron microscope. We show an unprecedented improvement in the probe current stability while maintaining all the fundamental properties of a cold field emission source such as a small angular current density together with a high brightness. We have also studied the influence of the low extraction voltage, as enabled by the nanosized apex of the cones, on the electron optics properties of the source that prevent the formation of a virtual beam cross-over of the gun. We have addressed this resolution-limiting issue by coming up with a new electron optical source design.

The dissipation of field emitting carbon nanotubes in an oxygen environment as revealed by in situ transmission electron microscopy

In this work, we report the first direct experimental observations of carbon nanotubes (CNT) field emitting in an oxygen environment, using aberration-corrected environmental transmission electron microscopy in combination with an electrical biasing specimen holder under low-dose, field-free imaging conditions. Our studies show that while the CNTs remain stable during high vacuum field emission, they experience abrupt decreases in length, also termed "burn-back", when field-emitting in an oxygen environment at around 30 Pa pressure. Furthermore, we perform correlative field-free and aberration-corrected, high-resolution transmission electron microscopy imaging to understand how the structure of the CNTs - particularly the opening of the nanotube caps - is influenced by its gas environment during field emission. This work provides significant insight into the mechanism of carbon nanotube behavior under non-ideal field emission conditions.

Quantitative 3D electromagnetic field determination of 1D nanostructures from single projection

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.