CRISP Materials Research: Unlocking the Potential of Complex Metal Oxides

Andrew Moir
By Andrew Moir November 22, 2008 21:29

Few undergrads may know this, but Yale is the home of one of the nation’s premier materials science research centers. The Center for Research on Interface Structures and Phenomena, or CRISP, is currently conducting pioneering research in the field of complex metal oxides, a subset of materials with tantalizing potential applications.

CRISP also provides quality educational opportunities not only to the Yale community, but to the Greater New Haven area. First and foremost, CRISP represents Yale’s commitment to materials science, an interdisciplinary field which, although not as established as some of the other physical sciences, promises to yield many new developments in the years to come.

The story of CRISP began in 2005, when the National Science Foundation (NSF) announced that it was going to hold an open competition for universities to join its Materials Research Science & Engineering Centers (MRSEC) program.

Through the MRSEC program, the NSF partners with major research institutions across the country by providing six years of extensive research and development funding to “support interdisciplinary and multidisciplinary materials research and education of the highest quality.”

Yale, in collaboration with nearby Southern Connecticut State University (SCSU) and the Brookhaven National Laboratories in Long Island, submitted an application to the MRSEC program. After a rigorous evaluation process, Yale, along with the University of Washington in Seattle, was chosen as a new site for a MRSEC program.

The first year of the MRSEC program, during which CRISP received a stipend of nearly $2 million, consisted mainly of laying the groundwork for the research to come.

This large capital source allowed the scientists involved with CRISP, such as CRISP Director John Tully, to secure research facilities for the program and to invest in all-new, large scale equipment – the type of machinery that, according to Tully, “individual professors would never be able to afford on their own.”

With the facilities and equipment in place, the scientists of CRISP were now ready to get to work doing what won them the NSF’s support in the first place: cutting edge materials science research.

Why Complex Metal Oxides?

The research CRISP focuses on the interfaces of complex metal oxides, both solid-solid interfaces (complex metal oxides layered with other complex metal oxides) and solid-gas interfaces (complex metal oxides exposed to air or other gaseous media).

Complex metal oxides are, at a basic level, quite simple: they are solid compounds that consist of lattices of oxygen molecules and a variety of metals, such as iron and cobalt, usually with more than one metal present in a given compound.

A graphical representation of the interface between Fe3O4 (bottom) and NiO (top).

First and foremost, CRISP’s goal is to gain a fundamental understanding of the processes that occur at solid-solid and gas-solid interfaces on the atomic level. However, since materials science is such an interdisciplinary and multifaceted field, this requires extensive knowledge in a variety of different areas.

Gaining a full understanding of the behavior of an interface is a manifold task, requiring a theoretical or computational model of the problem to provide a hypothetical explanation for what occurs at the surface, engineering expertise to be able to actually synthesize the material containing the interface, and the analytical capabilities necessary to con firm the purity and identity of the synthesized product.

Consequently, the Center’s associated faculty includes researchers from the Chemistry, Chemical Engineering, Electrical Engineering, Mechanical Engineering, and Applied Physics departments, with research interests ranging from the theoretical, employing computational models to simulate interface processes, to the experimental, using advanced equipment to actually synthesize the interfaces studied by the theorists in the group.

Bringing together such a diverse group of researchers allows the interface problem to be attacked from all sides, making a problem otherwise overwhelming in scope much more tractable.

CRISP also has some state-of-the-art equipment in its toolbox to help it achieve this goal. According to Professor Eric Altman, Yale’s molecular beam epitaxy (MBE) machine enables the construction of these solid oxide interfaces. Part of the difficulty in creating these interfaces is that the layers of material themselves must be extremely smooth, with no cracks or defects even on the scale of a few atoms.

A front view of Yale’s Molecular Beam Epitaxy (MBE) machine, assembled by Myrtle-Rose Padmore, DC ’08. On top of being used to conduct research, this machine will also be utilized as a teaching tool in undergraduate lab classes.

MBE achieves this uniformity by heating the substances to be deposited in a separate chamber until they begin to sublime, at which point they effuse through a tiny hole in the chamber into an ultra high vacuum chamber containing the deposition wafer.

The deposition on the wafer occurs slowly enough that the material deposits epitaxially, one atomic layer at a time, forming an extremely uniform crystal structure on the wafer. This process can be repeated multiple times to form the desired layered substances.

Once the material is deposited, its purity, uniformity, and identity must be examined, a task that requires atomic resolution to ensure that the substance is pure. While scanning tunneling microscopy, a widely-used form of electron microscopy, can achieve atomic resolution, it cannot be used with many complex metal oxides since it requires that the examined material be a conductor of electricity.

Thus, Professor Udo Schwarz developed a type of microscopy called Non-Contact Atomic Force Microscopy (NC-AFM) to visualize these nonconducting surfaces with the desired resolution. Yale’s NCAFM machine, the only one of its kind in the U.S., consists of a sharp probe, whose width is on the order of 1 nm, that scans across the surface of the complex oxide and “feels” the interactions that the tip experiences with the surface.

Unlike typical AFM approaches, which push the probe directly onto the surface and hence see tens or hundreds of atoms at a time (a typical atom is usually about 0.2 nm wide), Schwarz’s NC-AFM backs the probe off the surface so that it does not disturb the surface.

If the tip is vibrated at a high frequency with a very small amplitude of oscillation, its distance to the surface will change, and hence the force it feels due to its interaction with the surface will change. By monitoring changes in this force as a function of tip position, NC-AFM can achieve visualization with atomic resolution.

Although the precision of these results is usually limited by transient vibrations in the experimental setup, Yale has alleviated this problem by housing its NC-AFM apparatus in the vibrationless ground floor of the Malone Engineering Center. These technological innovations are allowing the researchers at CRISP to achieve an unparalleled understanding of these complex metal oxide interfaces.

What Can Complex Metal Oxides Do for You?

Although the interactions of complex metal oxides at their interfaces is an interesting and rich physical and chemical problem in its own right, the researchers at CRISP are also interested in complex metal oxides’ myriad applications, many of which are in the field of electronics.

As computers and other electronics become simultaneously more powerful and more compact, more and more transistors must be packed onto smaller and smaller chips. The most popular transistor design, known as MOSFET, turns the transistor polarity “on” and “off ” by changing the conductivity of the silicon through the continuous application of an electric field via a gate electrode.

However, in order to change the polarity without inducing a current through the transistor, a very powerful insulator must be placed in the transistor to prevent the flow of current. As transistors are getting smaller and smaller, the insulators commonly used today are simply not strong enough to insulate the transistor, drastically reducing their efficiency.

Complex metal oxides may offer the solution to this dilemma. Professors T.P. Ma and Charles Ahn, the leaders of the CRISP complex oxide research effort, have revolutionized transistor technology by replacing the typical gate electrode with a ferroelectric complex metal oxide.

Analogous to a ferromagnetic substance, which has a permanent magnetic dipole, a ferroelectric substance has a permanent electric dipole, or separation of positive and negative charges within the substance. Thus, the polarity of the transistor is now represented by the polarity of the electric dipole within the ferroelectric, which can be easily switched via a pulsed electric field.

A schematic of a ferroelectric transistor. Unlike a traditional MOSFET transistor, which requires continuous application of an electric field to maintain polarization, a ferroelectric transistor retains its polarization after application of the field, requiring less energy to operate and reducing its scale.

This approach replaces the insulator in the transistor and also drastically reduces its power consumption, since once the ferroelectric’s polarity is switched, it will remain in this state until switched back with another electric pulse. This transistor technology is only one of the many applications of complex metal oxides.

Metal oxides are used to achieve “colossal magnetoresistance” (an improvement on the magnetic hard drive technology used in desktops and laptops), and they are also the materials used to make high temperature superconductors.

Complex metal oxides also have fantastic potential applications in the field of alternative energy, serving as conducting substrates in solar cells, an application that Tully says CRISP hopes to explore more in the near future.

CRISP’s Commitment to Education

CRISP is not just about top-notch materials science research; part of its role as a MRSEC is providing many educational opportunities not only to the Yale community, but to the people of Greater New Haven as well. In addition to traditional outreach programs such as curriculum enhancement and demonstrations in local schools, CRISP tries to make the most of its resources by focusing on helping to educate New Haven’s future science teachers.

As Tully put it, “The best way to leverage a small amount of resources into K- 12 education is to influence the teachers, since they will influence the K-12 students in turn.” CRISP achieves this goal through a summer program in collaboration with SCSU’s Master’s Program in Education, as well as current teachers participating in SCSU’s Continuing Education programs.

Coordinated by SCSU Physics Professor Christine Broadbridge, CRISP places these individuals in MRSEC Initiative for Multidisciplinary Education and Research (MIMER) teams, which assign the participants into small groups that include 1-2 Master’s students or current teachers, a CRISP faculty member, graduate students in that faculty member’s lab, and undergraduate students visiting Yale as REU (Research Experience for Undergraduate) students.

Under the supervision of the CRISP faculty member, the teachers and undergraduates will collaborate to create new materials using the MBE machine, to analyze their products using NC-AFM and other methods, and to explore potential applications of this new material. In this way, local teachers will gain hands-on experience doing cutting-edge research, which will benefit them as teachers.

This MIMER program would not be possible, however, without equipment sophisticated enough to do high-quality research, yet accessible to those with limited knowledge in the field. To meet this need, Chemical Engineering major Myrtle-Rose Padmore DC ’08 began building an MBE machine that would be accessible to both undergraduates and local teachers.

Collaborating with Dr. Fred Walker, a research scientist at CRISP, Myrtle-Rose began her project in the spring of 2007, putting together the machine and writing up documentation that would allow future undergraduate and graduate students to use the MBE in their laboratory classes.

Yale’s unique MBE machine features several innovations which make it more accessible for general use. Since MBE machines operate under ultra high vacuum (UHV) conditions, most of the time spent in using an MBE machine consists of waiting for the machine to pump down to high vacuum after inserting the sample, which can often take hours.

This problem was circumvented by creating a smaller load lock chamber completely separated from the main chamber of the machine. Thus, when a student wants to put a sample into the machine, he or she can simply open the load lock chamber, place the sample inside, and then pump down only the smaller load lock chamber, which takes much less time.

The main chamber remains at UHV conditions, eliminating the time constraints usually present in MBE experiments and making the apparatus suitable for use in undergraduate courses. Myrtle’s experience with the MBE machine has been such a success that she is currently utilizing the machine to conduct her senior project, in which she is examining a particular gas-solid phase transition of strontium deposited on silicon.

Using her data, she will be able to determine the heat of condensation of this transition, and compare this value to a theoretical value which has been calculated by CRISP professor Sorab Ismail-Beigi. Thus, CRISP’s commitment to education is already paying dividends with the opportunities that it allows undergraduates and local teachers to do high-quality materials science research.

With CRISP, Yale is poised as a major player in the burgeoning field of advanced materials science research. With its multidisciplinary emphasis, cutting edge research equipment, and commitment to education, CRISP serves as a shining example of what the NSF is hoping to accomplish with its MRSEC program. Though not the most high profile group on campus, CRISP is quietly conducting innovative research accessible to all levels.

ABOUT THE AUTHOR
ANDREW MOIR is a junior Chemistry major in Berkeley College. Aside from school, he enjoys playing rugby, hiking, and the San Diego Chargers.

ACKNOWLEDGEMENTS
Thanks to Professors John Tully, Charles Ahn, and undergrad Myrtle-Rose Padmore for their consultation on this article.

FURTHER READING
CRISP website: http://www.crisp.yale.edu
C.H. Ahn et al., Science 284, 1152 (1999)
U.D. Schwarz et al., Appl. Surf. Sci. 188, 245-251 (2002)

Andrew Moir
By Andrew Moir November 22, 2008 21:29