Program Highlights

nuggets 

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

Graphene

Graphene-Enhanced Ferroelectric Tunnel Junctions

Haidong Lu, Alexey Lipatov, Evgeny Tsymbal, Alexander Sinitskii, and Alexei Gruverman
Nebraska MRSEC

Sangwoo Ryu and Chang-Beom Eom
University of Wisconsin-Madison

Ferroelectric tunnel junctions exploit an ultrathin ferroelectric layer, 100,000 times thinner than a sheet of paper, so that electrons can "tunnel" through it. This layer resides between two metal electrodes that can reverse the direction of its polarization by applying electric voltage to it. A junction polarity determines its resistance to tunneling current, with one direction allowing current to flow and the other strongly reducing it, known as “on” and “off” states. This feature makes ferroelectric tunnel junctions promising for the next generation of high-speed, high-capacity random access memories.
Researchers at the Nebraska MRSEC in collaboration with their colleagues from University of Wisconsin-Madison have employed graphene, a carbon material only one atom thick, to enhance the performance of ferroelectric tunnel junctions. By transferring graphene in different solvents on the surface of the ultrathin ferroelectric BaTiO3 film, the researchers found that an interfacial ammonia layer leads to strong polarization retention and increases the resistance disparity between these "on" and "off" states by almost three orders of magnitude.
Using graphene as an electrode in FTJs reveals a new potential for graphene as a functional material. The easiness with which graphene can be transferred from a solution to a ferroelectric surface opens a possibility of using a wide variety of molecular substances for interface engineering to the further improvement of ferroelectric tunnel junctions.

This work was published in the December-2014 issue of Nature Communications.

Figure illustrating a re-orientation of the ammonia molecules in the interfacial layer upon polarization reversal in barium titanate. This behavior of molecules ensures a strong retention of polarization and, thus, resistive states of the BaTiO3-based ferroelectric tunnel junction.

synchroton

Nebraska MRSEC Partnership with the Hiroshima Synchrotron Radiation Center

Takashi Komesu, Keisuke Fukutani, and Peter A. Dowben
Nebraska MRSEC

Kenya Shimada and Hideaki Iwasawa
Hiroshima Synchrotron Radiation Center

Angle-resolved photoemission spectroscopy (ARPES) is a powerful method for the exploration of the intricate details of the electronic structure of materials, details responsible for the material’s functional properties. The method is based on collecting and analyzing electrons of different wave vectors and energy emitted from a material as a result of exposure to a photon flux. High resolution light polarization dependent ARPES requires the use of synchrotrons – powerful sources of the photon radiation not available at a university setting.
The Nebraska MRSEC has established a long standing partnership with the Hiroshima Synchrotron Radiation Center to explore a wide variety of two-dimensional semiconducting materials including the transition metal chalcogenides, such as molybdenum disulfide, MoS2. These materials are striking due to their potential to serve as efficient conducting channels in the field effects transistors – electronic devices lying in the heart of the semiconductor industry.
ARPES makes possible to visualize changes in the position of electronic bands when alkali metal atoms are absorbed by MoS2 (see Figure). The adsorption on sodium, Na, leads to a charge transfer to the MoS2 surface causing an effect similar to electron doping of a semiconductor. The MoS2 occupied valence band shifts rigidly to greater binding energy with a little change in the occupied state dispersion, but causing a narrowing of the MoS2 bandgap and increased metallicity. This alkali metal adsorption on the surface, in fact, leads to band bending in the region of the surface, and thus dopes the surface region of MoS2.

Six MRSEC papers have resulted from this international collaboration in the last 4 years.

Figure: ARPES spectrum showing a shift of the occupied electronic bands when the alkali metal, sodium, is adsorbed on MoS2. EF denotes the Fermi energy.

IRG1 nugget

Laves Phase Ferromagnetism and Core-Shell Nanoparticles

Mark Koten, Balasubramanian Balamurugan, Ralph Skomski, David Sellmyer, and Jeffrey Shield
Nebraska MRSEC

Advanced processing tools such as inert gas condensation cluster deposition systems can be used to fabricate alloys on the nanoscale with novel crystal structures and complex morphologies such as core-shell and onion-like chemical partitioning. Different nanostructures can be designed through precise composition control and fine-tuning of gas-condensation parameters. Phases that are a challenge to form using bulk, top-down routes due to, for example, large differences in melting temperature between component elements can be synthesized. This is the case for the Fe-W Laves phase, which demands an exact composition with minimal error for its production. This phase has the formula Fe2W, and is predicted by theoretical calculations to be ferromagnetic. Recent synthesis of this phase via inert gas condensation demonstrated a saturation magnetization as high as 60 emu/g at low temperatures, consistent with the calculated value and is reasonably high for a rare-earth-free material. Also within this system, but at a more Fe-rich composition, core-shell nanoparticles were designed by taking advantage of the disparity between Fe and W surface energies and the immiscibility present within the system. Particles with this chemical segregation pattern often demonstrate interesting physical, chemical, or electrical properties as well as improved stability.

Picture: Transmission electron microscopy (TEM) image of an Fe2W nanoparticle (a) and a map of the component atoms of the W-Fe nanoparticle with a W core (b).

seed project

A Nickelate Channel for a Ferroelectric Field Effect Transistor

Vijay Raj Singh and Xia Hong
Nebraska MRSEC

A field-effect transistor (FET) is a basic building block of modern electronic technology. It consists of a thin-film conducting channel whose in-plane conductivity depends strongly on a voltage applied across an insulating layer adjacent to it, known as the gate. Transistors are used to amplify and switch electronic signals, making them valuable for performing memory and logic operations in modern computers.
An important functional characteristic of a FET is the ON/OFF ratio, defined as the conductivity change between the ON and OFF states controlled by the voltage applied to the gate. Choice of the material for a conducting channel is critical for enhancing the ON/OFF ratio.
One of the possibilities is to exploit properties of the so-called strongly correlated materials where resistance changes dramatically with external stimulus, exhibiting a metal-insulator transition. Among these materials are nickelates, RNiO3 (where R is a rare earth ion), one of the canonical families of pseudo-cubic perovskites. These materials undergo a metal-insulator transition and may serve as efficient conducting channels in ferroelectric FETs where the switchable polarization of a ferroelectric gate layer modulates the conductivity of the nickelate channel in a non-volatile manner.
Researchers at Nebraska MRSEC employ an epitaxial oxide composite structure consisting of a ferroelectric gate, such as Pb(Zr,Ti)O3, and a correlated oxide conducting channel, such as (Sm,Nd)NiO3, as a model system to explore the coupling between the interface electric polarization and conductivity of the nickelate. By applying voltage pulses to the Pb(Zr,Ti)O3 gate with different polarities, they found induced reversible switching in a 4 nm (Sm,Nd)NiO3 channel between the low and high resistance states, which can be used to represent the ON and OFF states. This novel field effect device can be used to build high speed, low power electronic devices, such as nonvolatile memories and logic devices.

Figure: Nonvolatile, reversible resistance switching in a 4 nm Sm0.5Nd0.5NiO3 thin film through ferroelectric field effect.

wophys14

Nebraska MSREC WoPhyS14 Conference

Axel Enders
Nebraska MRSEC

The University of Nebraska- Lincoln (UNL), under Nebraska MRSEC’s leadership, held its sixth Conference for Undergraduate Women in Physical Sciences, WoPhyS, on November 6-8, 2014.
WoPhyS is the centerpiece of Nebraska MRSEC’s efforts to make working in physics more enjoyable for young women, to create a climate that offers equal chances for women and men, ultimately encouraging US undergraduate students of both genders to pursue careers in science. The primary goal of this conference is thus to provide participants with a great opportunity to present their scientific work to their peers and to share their experience with other undergraduate students.
Under this year’s scientific theme “Nano Rising” the conference highlighted current progress in materials and nanoscience through a series of plenary and invited talks. Conference participants had the opportunity to visit the labs and materials research centers at UNL, to learn about the graduate programs, and to meet with faculty. Hallmarks of the meeting are a career session where role model scientists discuss their experiences and give career advice, a poster session, dinner with faculty from UNL, and social events. Students have the opportunity to present invited talks at this meeting upon nomination by their faculty, which gives them exposure to the community and helps them improve their academic record on their resume.
WoPhyS 2014 “Nano Rising” has been attended by 84 registered participants from universities across the U.S., plus UNL students. Several of the conference events have been publicly advertised, such as the lecture by NASA scientist Dr. Kamlesh Lulla and the plenary talk by Prof. Amber Boehnlein (see picture), so that audiences of over 150 students and faculty were reached.
WoPhyS has grown considerably in reputation over the past 6 years, as seen from the numbers of students and the stature of plenary speakers we are able to attract. It has a direct impact on minority student recruitment into UNL’s graduate program and advertises of UNL’s research capabilities to other students in the USA.

nat lab

Structural and Magnetic Evolution of Bimetallic MnAu Clusters

Xiao Cheng Zeng, Jeffrey Shield, and David Sellmyer
Nebraska MRSEC

He Kai
Brookhaven National Laboratory

Matthew J. Kramer
Ames National Laboratory

Highly-symmetrized MnAu nanoalloys may possess high magnetic moments for potential application. The magnetic properties of MnAu nanoclusters exhibit strong dependence on the cluster sizes and morphologies. Determining the most stable morphologies as well as their spin-polarization patterns is important for their further application. Researchers at University of Nebraska MRSEC in collaboration with researchers at Brookhaven and Ames Laboratory performed a joint theoretical/experimental study to investigate structural and magnetic evolution of MnAu clusters (Nano Letters 14, 1362 (2014)). They found the MnAu clusters order into the L10 structure, and monotonic size-dependences develop for the composition and lattice parameters, which are well reproduced by the density functional theory calculations by the theoretical group. Simultaneously, Mn diffusion forms 5 Å nanoshells on larger clusters inducing significant magnetization in an otherwise antiferromagnetic system. The differing atomic mobilities yield new cluster nanostructures that can be employed generally to create novel physical properties. This study shows the first case of utilizing the different mobilities of atoms in a solid matrix as a driving force for nanoscale nanoparticle engineering. By adding small/meta-stable clusters of desirable compositions to the initial cluster ensemble, nanoparticles with complex heterostructures can be built up layer by layer. These techniques create potentially powerful strategies to fabricate unique nanoscale heterostructures by modifying preformed nanoparticles.

Figure: Top: Transmission electron microscopy (TEM) images of as-produced (left) and annealed (right) MnAu clusters. The insets show the size distributions. Bottom: Representative TEM images for annealed MnAu clusters with various sizes.

 

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