Tunneling Calculation in the Field Ion Microscope

In this work we describe calculations of tunneling rate constants for the Field Ion Microscope (FIM) using one-dimensional model potential that simulates the ionization process in a FIM. We obtain expressions for the ionization rate constant (ionization probability per unit of time) of inert gas atoms as a function of their position above the surface. In order to calculate the probability of barrier penetration we have used the semiclassical (JWKB) approximation. We have also calculated ionization zone widths as the distance between points where ionization rate is a maximum and half of this value. An application to helium as the imaging gas is presented to highlight the power of the method.

My Life With Erwin: The Beginning of an Atom-Probe Legacy

The atom-probe field ion microscope was introduced in 1967 at the 14th Field Emission Symposium held at the National Bureau of Standards (now, NIST) in Gaithersburg, Maryland. The atom-probe field ion microscope was, and remains, the only instrument capable of determining “the nature of one single atom seen on a metal surface and selected from neighboring atoms at the discretion of the observer”. The development of the atom-probe is a story of an instrument that one National Science Foundation (NSF) reviewer called “impossible because single atoms could not be detected”. It is also a story of my life with Erwin Wilhelm Müller as his graduate student in the Field Emission Laboratory at the Pennsylvania State University in the late 1960s and his strong and volatile personality, perhaps fostered by his pedigree as Gustav Hertz’s student in the Berlin of the 1930s. It is the story that has defined by scientific career.

Characterization and modeling of nanotips fabricated in the field ion microscope

Nanotips are considered significant elements in some of nanotechnology instruments. They are used in scanning probe microscopes and electron microscopes to characterize materials at the nano and atomic scales. Therefore, the size and profile of the nanotip determines the performance of these microscopes. The advancement of nanotip fabrication techniques has enabled the fabrication of ultra-sharp tips and even single-atom tips. However, the characterization of nanotips with an apex of a few nanometers is still premature, while the conventional characterization methods of the tip size, such as the ring counting method, have shown some limitation at this nano scale. In this paper, we review the various nanotip fabrication methods with a focus on the most recent one, which is called the local electron bombardment method. We demonstrate an approach for estimating the nanotip radius with good approximation using ball crystal models. We also model the overall nanotip profile using finite element simulation tools based on the hyperboloidal geometry. The modeling and radius estimation approach is applied on tips fabricated by the local electron bombardment method, which will be explained in detail as well.