A LEAP Forward
Atom probe tomography to unveil new details for ceramics, oxides
Two in One
The $2.25 million LEAP® 4000 XHR local electrode atom probe unites time-of-flight mass spectrometry and point-projection microscopy to identify individual elements and locate them in 3-D within the bulk of a material sample.
Time-of-flight is the time an atom, ion or any object in general takes to reach or travel from one point to another. In the case of time-of-flight mass spectrometry, ions are accelerated to the same kinetic energy by an applied electrical field. The time an ion takes to reach a detector at a known distance is measured and will depend on the mass-to-charge ratio of the particle (heavy particles or lower charge state for a given mass = longer time). Using the time measurement and experimental factors, the actual mass-to-charge ratio can be deduced.
Similar to projection cameras, a point-projection microscope magnifies a point by projecting it onto a larger detector. Point-projection microscopy can be used to identify where the elements are located originally on the sample in 3-D with a magnification exceeding 1MX in LEAP.
EMSL scientists currently are testing and bringing online the LEAP® 4000 XHR local electrode atom probe, a $2.25 million atom probe tomography instrument that will enable the first-ever comprehensive and accurate 3-D chemical imaging studies of low electrical conductivity materials, such as ceramics, semiconductors and oxides. The implications could lead to advances in materials science, ceramics, metallurgy, geochemistry and biomineralization.
The instrument will be among the first of its kind available to the global scientific community through a user facility.
The LEAP will provide true 3-D images with ~0.5 nm spatial resolution and ppm elemental sensitivity with an unprecedented field-of-view, affording breakthroughs in visualization. It will augment EMSL's existing world-class suite of microscopy, surface and interfacial analysis capabilities.
EMSL's LEAP expands the capabilities of traditional atom probe field ion microscopy and atom probe tomography, which, until recently, could only characterize conductive materials. It combines time-of-flight mass spectrometry and point-projection microscopy (see Two in One). A combination of high electrical field and ultra-fast voltage (2-20 kHz) or femtosecond laser (10-200 kHz) pulse is used to remove individual atoms from sample materials through field evaporation.
"EMSL will be the first organization to focus on analysis of bulk oxides and complex mineral samples using the atom probe tomography technology, per our knowledge," said Satyanarayana Kuchibhatla, an EMSL senior research scientist. "Materials science, ceramics, metallurgy, geochemistry, biomineralization and related fields will benefit from this unique instrumentation."
LEAP's laser pulsing module means experiments can extend to low electrical conductivity materials, including ceramics, semiconductors and oxides. Oxides, minerals coming from geological or biogeochemical origins, are of particular interest to EMSL users and the Department of Energy. New techniques also will be developed in collaboration with experts in the field and the atom probe division of CAMECA (part of AMETEK's Materials Analysis Division).
"This opens up a new direction in understanding the scientific issues associated with buried interfaces, microstructures and grain boundaries," said Theva Thevuthasan, scientist and Technical Lead for the Interfacial Spectroscopy and Diffraction group at EMSL.
Man and Machine
The system will provide a wealth of data—potentially millions of data points—that include a number of details about atoms distributed in a sample. Accurate spatial reconstruction of that data poses a tangible challenge for analysts. According to Kuchibhatla, to interpret data requires researchers make fundamental assumptions founded in core science and be diligent in not introducing artifacts. This is an area with significant room for advancements, which is a challenge EMSL is eager to tackle. This essential exchange between man and machine is part of what makes the LEAP a perfect addition to EMSL.
"EMSL believes in the idea of an intelligent marriage between the experiments and theory," Kuchibhatla said. "Atom probe tomography inherently utilizes the combination of experiments and theory. So, it's a unique capability where both experimentalists and theoreticians work together to carry out better scientific research and make excellent contributions to the community."
Atom probe tomography can be effectively applied to understand structural properties of materials and microstructures within materials due to defects, elemental diffusion/segregation and defect trapping along grain boundaries and interfaces; buried interfacial analysis; dopant distribution and segregation at grain boundaries; oxide-dispersed steels; and next-generation nuclear and radiation detection materials. Explorations are currently underway to find ways to apply the method to detailed analysis of nanoparticle coatings and composition variations within individual particles and for a distribution of particles.
A New View
EMSL's LEAP features the highest voltage-pulse mass resolution available on a commercial 3-D atom probe. Its laser-based atom probe capability combined with a large field-of-view, on the order of thousands of square nanometers, make it particularly useful for materials science research with previously unachieved space increment and time-of-flight accuracy. This data will be further processed to generate 3-D sample images with resolution on the order of a fraction of a nanometer in depth and sensitivity on the order of parts per million.
"Previously, atom probes were providing a one- or two-dimensional view of a 3-D object, which means you miss some features," Kuchibhatla explained. "Current generations of the instruments are the first to provide true 3-D information and full view."
Prior to loading in the instrument, needle-like samples are prepared. Once loaded and maneuvered into position within the LEAP's ultra-high vacuum system, the laser or electrical pulse bombards the needle tip forcing positively charged atoms to "fly" from the specimen stage to the high-resolution, reflectron-based single-atom detector. The atom's mass determines how fast it can make the leap. With the help of prefabricated, microtip-based specimen holders, scientists can optimize the sample preparation step and data acquisition in the LEAP.
LEAP at Work
The technique will advance chemical imaging critical to a wide variety of scientific challenges. One of EMSL's first projects is analyzing how trace element distribution in sediments at the Hanford Site has long-term implications on understanding the fate, transport and sequestration of uranium. The insights will enrich biogeochemistry, biomineralization and subsurface science fields, major interest areas for EMSL's sponsor, DOE's Office of Science, Office of Biological and Environmental Research.
Currently, Thevuthasan and his EMSL colleagues are using the LEAP to explore metallic nanocluster and oxide matrix interfaces. They also are collaborating with scientists from Northwestern University and Carnegie Mellon University on research affecting complementary metal–oxide–semiconductor technology and indium segregation and clustering in next-generation light-emitting diode materials, respectively.
Additionally, PNNL scientists, led by Stephen Bruemmer, hope to explain the failure of high-temperature alloys by using LEAP to conduct site-specific analysis of the regions of failure. Identifying failure mechanisms via LEAP's 3-D visualization will help to design better high-temperature, radiation-tolerant materials for nuclear energy applications.
"The LEAP is driving EMSL towards comprehensive chemical imaging of a gamut of materials systems," Kuchibhatla said.
"EMSL scientists will become the first group of people who use state-of-the-art, laser-assisted atom probe tomography to answer questions related to minerals and ceramics," he concluded. "This instrument has the potential to become a marquee capability at EMSL and make the lab a 3-D chemical imaging leader. EMSL can do it."
Acquired with funding from the American Recovery and Reinvestment Act, the LEAP was manufactured for EMSL by Madison, Wis.-based Imago Scientific Instruments. In April 2010, Imago Scientific was acquired by Paoli, Pa.-based AMETEK Inc., joining CAMECA as part of AMETEK's Materials Analysis Division.