Program Highlights

 

Highlights:

Research and education highlights are brief and digestible summaries of recent significant research results or education activities, chosen for their potential interest to a broad audience.

 

Resent Research and Education Highlights

IRG1 highlight 2018

Ferroelectrically-Controlled Magnetic Anisotropy

Anil Rajapitamahuni, Lingling Tao, Jingfeng Song, Evgeny Tsymbal, and Xia Hong
Nebraska MRSEC

Using electric fields or voltage to manipulate magnetically ordered states is a promising approach to develop novel energy-efficient electronic devices. Nebraska MRSEC researchers have explored a new path to the electrically controlled magnetism. They exploited an electric polarization of lead zirconate titanate (PZT) to control magnetic anisotropy of a nm-thick film of lanthanum strontium manganite (LSMO). PZT is a ferroelectric material which allows reversible switching of its polarization by voltage. The researchers found that magnetic anisotropy of the LSMO film changes by about 20% when the ferroelectric polarization of PZT is reversed. Magnetic anisotropy determines magnetization orientation, and thus this work has demonstrated a route to achieve non-volatile voltage control of the magnetic state via switching the ferroelectric polarization, which is critical for developing low-power device applications.

Picture: Schematic of a device where magnetization of  lanthanum strontium manganite (LSMO)  is affected by ferroelectric polarization (indicated by dipoles) of lead zirconate titanate (PZT) via a gate voltage (Vg).

IRG1 2018

Direct Observation of Ferrimagnetism in a Multiferroic Hexagonal Ferrite

Xiaoshan Xu, Peter Dowben, and Evgeny Tsymbal
Nebraska MRSEC

Multiferroics is a class of materials that exhibits a coexistence of electric and magnetic polarizations. Coupling of these polarizations is potentially useful for energy-efficient information storage and processing. Hexagonal rare-earth ferrites (h-RFeO3, where R is rare-earth element and Fe is iron) are new family of multiferroic materials. Magnetic interactions between rare-earth and iron ions in h-RFeO3, may amplify the weak ferromagnetic moment of iron, making these materials more appealing as multiferroics. Using element-specific magnetic characterization techniques, Nebraska MRSEC researchers have elucidated the nature of the interaction between the rare-earth ion, ytterbium (Yb), and the iron ion, in multiferroic h-YbFeO3. Their results directly reveal anti-alignment of the magnetic moments of ytterbium and iron at low temperature, known as a ferrimagnetic order. These findings suggest an important role which is played by the rare-earth ions in tuning the multiferroic properties of hexagonal ferrites (S. Cao et al, Phys. Rev. B 95, 224428 (2017)).

Picture: Magnetic structure of hexagonal ytterbium ferrite (h-YbFeO3) in which iron (Fe) magnetic moments (indicated by blue arrows) are anti-aligned to ytterbium (Yb) magnetic moments (indicated by grey arrows), thus revealing ferrimagnetism of multiferroic hexagonal ferrite h-YbFeO3.

IRG2 2018

Optical Control of Polarization in Hybrid 2D-Ferroelectric Structures

Alexei Gruverman, Alexander Sinitskii, and Chang-Beom Eom
Nebraska MRSEC

Switchable electric polarization of ferroelectric materials can serve as a state variable in advanced electronic systems, such as non-volatile memories and logic. Control of ferroelectric polarization by external stimuli is the key component for these systems. Nebraska MRSEC researchers have discovered an optical control of the hybrid structures comprising a two-dimensional (2D) semiconducting material, molybdenum disulfide (MoS2), and ultrathin ferroelectric barium titanate (BaTiO3). They showed that optical excitation of MoS2 by ultraviolet light leads to polarization reversal in the BaTiO3 film accompanied by a significant change in electric resistance of the hybrid structure. These findings reveal a great potential of the 2D semiconductor-ferroelectric structures for future nanoelectronic devices with optically controlled functionalities.

Picture: Optically-induced polarization reversal in hybrid MoS2/BaTiO3 (BTO) structures: geometry of experiment (left panel) and polarization state of BaTiO3 (right panels). The BaTiO3 surface is partially covered with MoS2. Under ultraviolet (UV) illumination polarization of the BaTiO3 film underneath the MoS2 flake is reversed as indicated by color.

seed project

Solar Cell Enhancement by Ionic Defect Passivation

Jinsong Huang and Xiao Cheng Zeng
Nebraska MRSEC

Organic–inorganic halide perovskites are promising materials for efficient conversion of solar energy to electricity. However, the presence of ionic defects at the surfaces and grain boundaries in there materials are detrimental to both the efficiency and stability of perovskite solar cells. Nebraska MRSEC researchers have shown that quaternary ammonium halides can effectively passivate ionic defects in several different types of hybrid perovskites with negative- and positive-charged components. The defect passivation reduces the charge trap density and elongates the carrier recombination lifetime, boosting the efficiency to a certified value of above 20%. Moreover, the defect healing significantly enhances the stability of films in ambient conditions. These findings provide an avenue to further improve both the efficiency and stability of solar cells. (X. Zheng et al., Nature Energy 2, 17102 (2017) doi:10.1038/nenergy.2017.102).

Picture: Quaternary ammonium halides can passivate both cationic and anionic defects, which enhances solar cells efficiency and stability.

seed

NanoThermoMechanical Thermal Computing

Sidy Ndao
Nebraska MRSEC

Limited performance and reliability of electronic devices at extreme temperatures, intensive radiation found in space exploration missions and earth-based applications requires the development of alternative computing technologies. Nebraska MRSEC researchers have designed and prototyped the world’s first high-temperature thermal diode. They have demonstrated the use of near-field thermal radiation from smooth and metamaterial surfaces to achieve thermal rectification at high temperatures. They named the technology NanoThermoMechanical thermal computing. Unlike in electronics, a NanoThermoMechanical device uses heat instead of electricity to record, store, and process data. Furthermore, the researchers were able to design thermal logic gates based on NanoThermoMechanical diodes and transistors. Thermal computing has a potential to unlock the mysteries of outer space, explore and harvest our own planet’s deep-beneath-the-surface geology, and harness waste heat for more efficient-energy utilization.

Figure: Scanning Electron Microscope Image of a High-Temperature NanoThermoMechanical Diode.

highlight 2018

UNL-NCAT Joint Workshop in Materials Science and Engineering

Rebecca Lai, Jocelyn Bosley, Eva Schubert, and Dhananjay Kumar
Nebraska MRSEC

In Fall 2017, the University of Nebraska-Lincoln (UNL) offered its second joint workshop in materials science and engineering at North Carolina A&T State University (NCAT), an HBCU and partner institution in the Nebraska MRSEC Bridge Program. Seminars introduced 33 undergraduate students and 10 graduate students to fundamental concepts and techniques in materials science, connecting these with current research at NCAT and UNL. Faculty from both institutions offered seminars on photovoltaic materials, advances in thin-film research; soft materials, and characterization methods. The workshop included a poster session for NCAT students, and hands-on tours of NCAT facilities for pulsed laser deposition, sputtering, X-ray diffraction, scanning electron microscopy, and atomic force tomography. Workshop participants cited exposure to cutting-edge research, learning about novel applications, and the opportunity to move beyond the curriculum as highlights of the experience.

Picture: Dr. Stephen Morin of Nebraska MRSEC presents a seminar on the chemistry and mechanics of soft materials to NCAT workshop participants.

highlight 2018

Nebraska MRSEC Puts a “Spark” in Summer Learning

Rebecca Lai, Jocelyn Bosley, and Krista Adam
Nebraska MRSEC

In Summer 2017, Nebraska MRSEC partnered with the Foundation for Lincoln Public Schools to offer a new, STEAM-based summer learning program. Spark Summer Learning provides opportunities for students in grades K-5 to explore science, technology, engineering, art, and math in an immersive setting, engaging students in problem-based learning through hands-on “maker” projects. For the week-long experience, Nebraska MRSEC Assistant Director of Education and Outreach Jocelyn Bosley adapted and extended lesson plans developed by preservice teachers in the science education classes of MRSEC PI and Assistant Prof. of Teaching, Learning, and Teacher Education Krista Adams. Inspired by Nebraska MRSEC research, these lessons introduced students to properties of magnets, nanomedicine, and applications of magnetism to nanomedicine. The workshop was led by MRSEC undergraduate researchers Peter Kosch and Spencer Prockish.

Picture: Nebraska MRSEC undergraduates Spencer Prockish (left) and Peter Kosch help elementary students design their own experiments to determine how a magnet’s size and shape affect its strength.

highlight 2018

Characterizing Complex Nanostructured Materials with Atomic-Scale Resolution

Jeffrey Shield
Nebraska MRSEC

The ability to precisely engineer well-defined nanostructures in ever more complex systems, including tailored, multielemental geometries with unique atomic configurations, enhances functionalities of nanoscale devices. For example, core/shell nanoparticles (Co@ZnO) consisting of a cobalt (Co) core and a zinc oxide (ZnO) shell offer unique opportunities for multifunctional behavior by combining two different phases in a single nanostructure. To understand and design these materials, it is important to characterize the structure and chemistry at high spatial resolution. Scanning transmission electron microscopy (STEM) allows scientists to visualize the structure on the atomic scale and, through chemical mapping, determine elemental distributions within the nanoparticle. Nebraska MRSEC researchers exploit central facilities available at the University of Nebraska, such as high-resolution STEM, to perform interdisciplinary research at the frontiers of nanoscience.

Picture: Top: High-resolution transmission electron micrograph of the Co@ZnO nanoparticles. Bottom: Chemical maps obtained by x-ray energy dispersive spectroscopy in STEM mode, revealing a Co core (green) and ZnO shell (blue and red).

highlight 2018

Nebraska MRSEC Facility: Synthesis and Characterization of Graphene-Like Boron-Carbon-Nitrogen Monolayers

Axel Enders, Peter Dowben, and Alexander Sinitskii
Nebraska MRSEC

The emergence of two-dimensional (2D) materials, which are only one atom or one structural unit cell thick, has stimulated an enormous range of research effort. The well-known example is graphene – a zero band gap semiconductor, which exhibits outstanding charge carrier mobility. However, the absence of a band gap is a major hindrance in implementing graphene in 2D electronics. The question arises whether other graphenic systems of mono-atomic thickness, with useful electronic properties, can be realized. Nebraska MRSEC researchers have demonstrated that a ternary network of boron (B), carbon (C), and nitrogen (N) atoms that occupy the graphenic sites in a 2D sheet can produce the required electronic properties. They synthesized a BCN monolayer on the iridium (Ir) surface, modelled its atomic structure, and characterized its electronic properties. Thus, long predicted, the BCN material has been finally realized showing a new route to implementing 2D electronics. (S. Beniwal et al., ACS Nano 11, 2486 (2017).

Picture: Modelled corrugation of a BCN monoatomic layer on iridium (Ir) surface, where red (blue) colors indicate high (low) BCN elevations. “h”, “H”, and “T” sites refer to the position of the B and N atoms with respect to the Ir substrate atoms. The top left panel shows the scanning tunneling microscopy image of the simulation cell.

 

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