An Atom Probe Tomography (APT) Microscope is a new material analysis instrument that provides the only technique that can obtain both three-dimensional (3D) tomographic images and chemical identification at the atomic scale. It provides insight into key problems in the materials science, chemistry, physics and engineering of nanoscale structures and devices. The APT Microscope uses a rapidly pulsed electric field from a local electrode to remove atoms individually from the sample; the atom mass is determined by the time of flight to an imaging detector array. These 3D tomographic images can identify the element, and even the isotope, of individual atoms. They enable the spatial mapping of structural defects, dopants, and atomic structure and composition in nanowires, patterned materials, heterostructures and devices. The ability to completely visualize a material directly is new, and it opens a new range of investigations and applications. This proposal asks for funds to acquire a 3D APT Microscope for installation in the Center for Nanoscale Systems at Harvard University. The APT Microscope will promote collaborations between physicists, chemists, and device developers with materials scientists by allowing them to examine spatially modulated structures at the atomic scale. This information is particularly important for nanoscale devices for electronics and photonics, where atomic scale interfaces determine the behavior of the device. This instrument can be used to understand the structure of materials at the atomic level, including complex oxide semiconductors that are promising for future electronics, solar cells and fuel cells. The installation of a Atom-Probe Tomography Microscope in the Center for Nanoscale Systems at Harvard University will provide direct and open access for researchers and educators in the Northeast region of the United States. It will provide opportunities for to a diverse population of students. An innovative educational program "Nanoscale Imaging and Analysis" will use the APT Microscope to integrate research into the teaching of materials science, physics and chemistry at the undergraduate, graduate and adult levels.
Layman Summary: To make and understand atomic-scale structures, we must be able to both image the location of individual atoms in a structure and identify their chemical identity. Atom Probe Microscopy is the only technique that provides this information. A three-dimensional image displays the atoms, and different colors can be used to show which element they are. By creating images that are easy to understand, an Atom Probe Microscope will allow us to make and understand truly small atomic scale devices for computation, communication, energy production, and medical treatment. The instrument operates by using a rapidly pulsed electric field to remove an individual atom from the sample - the time of flight to the imaging detector determines the mass of each atom. The images can identify the element, and even the isotope, of each atom, as well as its location. The Atom-Probe Microscope will be installed in the Center for Nanoscale Systems at Harvard University, and provide direct and open access for researchers and educators in the Northeast region of the United States - it will enhance the education of a diverse population of students. This new instrument will allow researchers to understand the atomic structure of new electronic and optical devices, create new types of solar cells and fuel cells to ease the national energy problem, and fundamentally study the structure of materials at the atomic level.
Atom probe tomography (APT) is a state of the art technique providing 3D, sub-nm resolution chemical mapping of a specimen. APT has innumerable analytical applications ranging from surface chemistry to geology to semiconductor device analysis. Information about dopant profiles, atomic clusters, grain boundary aggregation, and superlattice structures obtained by atom probe provides details unavailable by any other technique. This information in turn can provide insights for improving materials utilized in catalysis, thermal barrier coatings, and information processing. Atom probe tomography is a mass spectroscopy based technique. The principle behind atom probe is illustrated in Figure 1. In short, a specimen is formed into a small needle with a radius of curvature of ~100 nm. This material is ablated atom by atom under the influence of either a pulsed voltage or laser source. Under the influence of an electric field the atoms travel along a flight path until they reach a position sensitive detector. The length of time each atoms take to reach the detector provides the mass over charge ratio for the individual atom while the position on the detector reveals the point of originfor the atom in the specimen. Together this data enables a full 3D reconstruction of the original specimen providing positional information for each of the collected atoms. The Atom Probe is located at the Harvard Center for Nanoscale Systems (CNS) the center is part of the NSF sponsored National Nanotechnology Infrastructure Network (NNIN). Over the past year numerous tour groups have visited our center at Harvard and we have introduced our guests to the technique of atom probe tomography. These groups have had vastly different backgrounds stimulating different approaches to teaching about the atom probe microscope. With a group of third graders from Cambridge MA we used the atom probe as a means to explain how materials are assembled from individual atoms and to stimulate their curiosity and excitement about science, see Figure 2. With a group of dignitaries from Russia, including the deputy prime minister, we explained in more detail the technological importance information about materials that is provided by atom probe, setting the stage for future scientific exchanges. To develop a process of encapsulating nanoparticles for atom probe, we developed a method where the material of interest is embedded in a glass capillary (Andrew Magyar, Adam Graham, Mor Baram and David C. Bell, Harvard University). This technique is very fast, taking only minutes to prepare. Specimens can be loaded into the capillary directly in solution phase. The nanoparticles are fully encapsulated within the microtip (Figure 3). Images of the process used to create the atom probe specimens are shown in Figure 3a. Each end of the capillary is sealed with a blow torch to encapsulate the solution and a commercial micropipette puller is used to draw the capillary to a fine point. Characterization by scanning electron microscopy showed that optimized tips have sub-200 nm radius of curvature, making them suitable for atom probe tomography, Figure 3b. A more detailed analysis reveals boron rich precipitates in the borosilicate glass tip, depicted by the isosurface shown in Figure3c and 3d. One critical area of research for Atom Probe Tomography concerns the measurement of impurities in semiconductors (Austin Akey, Amanda Youssef, and Tonio Buonassisi, MIT), recently made possible by advances in laser-assisted APT. Due to the non-metallic nature of the specimens, the evaporation and ionization conditions experienced by dissolved impurities, precipitates, and even the host material are not well understood. A variety of transition metals have been ion-implanted into silicon, then the silicon recrystallized and the impurities induced to redistribute and precipitate by a single pulse from an Nd:YAG laser (Figure 4). Superlattice semiconducting oxides exhibit very low thermal conductivity making them interesting as potential high-temperature thermoelectric (Mor Baram, Xin Liang and David R. Clarke, Harvard University). As the superlattice spacing depends directly on the composition of these modular compounds, their spacing is independent of temperature and consequently, unlike other microstructural features, does not change with coarsening at high temperatures. The thermal resistance produced by individual superlattice interfaces has recently been determined but there remains uncertainty as to their composition and structure. To quantify these, atom probe tomography has been used, providing unique new insights into these supperlattice materials, beyond the capabilities of traditional TEM and X-ray techniques, by revealing the full 3D atomic placement of the Zn, In and O ions (Figure 5). The atom probe has proved invaluable for analyzing and understanding technologically important materials that are critical to thermal barrier coatings, semiconductors for solar and computation applications, and conductive thin films. Understanding the composition and atomic arrangement of such materials at the nanoscale will lead to improved material design and consequently enable the engineering of better, more efficient or more powerful devices.
