The physical mechanisms that lead to the remarkable electron emission properties of single atom tips have been investigated. The primary tool used for this purpose has been large scale computer simulation. A simulation code has been developed to study a large variety of models. The simulation results have contributed significantly to the understanding of experimental findings. In particular it has been demonstrated that most of the features of the electron emission by these nanotips can be explained, at least qualitatively, in terms of the metal vacuum barrier. A computer animation movie, which provides a vivid demonstration of the effect of the various physical mechanisms on the properties of the emitted electron, has been produced.
A fabrication technique was established for atomic size electron sources and used to characterize these sources. The following devices were constructed: an integrated field ion microscope (FIM) and field electron emission microscope (FEM) apparatus, an integrated FIM-FEM field emission electron microscope (FEES) apparatus and a video image processing unit. These devices were used to introduce a technique of fabrication of a single atom source of electrons and metallic ions based on the new concept of surface melting in the presence of a high field. This concept is the basis, not only of the electron coherent sources, but also of the atomic metallic ion emission (AMIE) sources which was therefore developed. The results obtained have led to the development of 3-dimensional calculations at the atomic scale of the electrostatic potential and field created by nanometric protusions and to the characterization of single atom nanotips.
This research studied, characterized and mapped the microelectric fields and nanoelectric fields associated with the tunnelling current emission from single atom tips. The feasibility of using nanotips and single atom tips in a transmission electron microscope (TEM) and in a scanning tunnelling microscope (STM) was explored. Studies were made of tip environment and characterization, of tip holders and of field computation and imaging by electron holography. Maps were computed of both the electric potential and the electric field for several models of tip shape. These maps were compared with experimental ones obtained by means of electron interferometry and electron holography with very satisfactory results.
Tungsten nanotips have been used in an STM working in air and comparison made with the performance of a platinum iridium alloy tip. The tungsten tips are sharper, more regular and smoother and give better performances. However, they have to be used before a surface oxide layer has formed and their quality deteriorates with time.
Initial implementation of a nanotip in the electron optical column of a scanning tunnelling electron microscope (STEM) has been done. The Fowler-Nordheim plot clearly shows the presence of nanotip tunnelling but the column could not be aligned with the nanotip in its sharpest state. However the tip was easily blunted slightly by heating its support wire. This left the tip in an ideal condition for traditional use in the STEM and the ability to regenerate tips in situ removes a significant cause of downtime on the microscope.
It has recently been reported that nanotips emit a 2 energies separated by 1 eV each with an energy spread of 0.25 eV. The chromatic aberration incurred as a result of the additional energy spread would negate most if not all of the gain in coherence. Thus 2 essential modifications are now being undertaken to enhance the performance of nanotips.
The aim of the project is to fabricate atomic-size electron sources and to characterize their emission properties by experimental and theoretical analysis.
The applicability of these sources to electron spectroscopy interferometry and holography will be assessed.
Special equipment to transfert and sharpen the tips will be developed.
Funding SchemeCSC - Cost-sharing contracts
CB2 1TN Cambridge