Nestor Zaluzec - A Transmission X-ray Detector for Nano-Particle Analysis

Abstract
In nano-materials research, one of the ubiquitous instruments we employ for characterization is the electron microscope. After electrons, the signal most often measured in these instruments is the emission of characteristic x-rays. During the last 40 years, either the solid state Si(Li) or more recently the SDD Energy Dispersive Spectrometer (EDS) have been the detector most often used for this task. On one hand these detectors are remarkably, simple and efficient devices, but on the other there nearly always remains opportunities for improvement.

One of the factors which governs the ability to measure an x-ray signal is the detector geometrical collection efficiency and is typically defined in terms of the collection solid angle . For EDS systems interfaced to a scanning electron microscope (SEM) values can range from < 0.005 to ~0.1 sR, while in transmission or scanning-transmission electron microscopes (TEM/STEMs) values of up to ~ 0.1- 0.3 sR have become routine.

To improve this situation specifically for nano-particle characterization, a prototype 42.5 mm² SDD x-ray detector which operates in a novel transmission configuration has built and has been interfaced to the column of a FEG-ESEM. The detector is enclosed in a stainless steel housing (0.75" in diameter), while the detector crystal is protected from electron irradiation by using a 12.5 µm thick Be window. In this geometry, nano-particle specimens for analysis are supported on TEM grids held in a custom built, Be shielded stage, which can be adjusted using the existing SEM stage translation mechanisms. The SDD detector is inserted beneath the specimen by means of a standard linear insertion mechanism interfaced to a side entry port of the microscope. At closest distance tested the detector solid angle just exceeds 3.14 sR. Research continues to refine the performance and solid angle of this configuration, as well as to consider alternative configurations for use in TEM geometries.

Biography
A senior scientist and principle investigator in the Electron Microscopy Center at Argonne National Laboratory, Nestor is a Fellow of both Oak Ridge National Laboratory, and the Computational Institute of the University of Chicago, a Visiting Professor of Physics at Northern Illinois University and a Fellow of the Microscopy Society of America. Nestor's research includes the development of state-of-the-art instrumentation and techniques for atomic resolution x-ray and electron spectroscopy, analytical, and scanning confocal electron microscopy. In addition to creating tools for science, he also uses these leading-edge technologies to study issues in technologically important materials. His work over the last 30 years has included studies in the areas of structural phase transformation in metals, radiation damage in alloys, ceramic oxides for geologic immobilization of nuclear waste materials, elemental segregation in semiconductors devices, magnetism, genetically engineered proteins for bio-materials nanoarrays and most recently studies of catalysis using analytical microscopy. He was one of the earliest to realize the potential impact of the Internet on science and established the first TelePresence Microscopy Collaboratory, which is serves as a model for outreach to both the scientific and education communities, providing unencumbered access to scientific resources.

Nestor received his B.S. degree in Physics at Illinois Institute of Technology in Chicago, and his PhD in 1978 from the Department of Metallurgy at the University of Illinois Urbana-Champaign.