Program 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


Nanoscience Vlog – a New Way to Communicate P-SPINS Research

Axel Enders, Krista Adams, and Jocelyn Bosley
Nebraska MRSEC

During the summer of 2015, Research Experiences for Teachers (RET) participant Courtney Matulka of Millard Public Schools together with Seed Project leader Krista Adams and Professor-Student Pairs participant Sharmin Sikich of Doane College developed a video blog, or “vlog,” to highlight the cutting-edge research happening in the nanosciences at the University of Nebraska-Lincoln (UNL). Working with Nebraska MRSEC Education/Outreach Director Axel Enders, Matulka interviewed and filmed Nebraska MRSEC faculty and Professor-Student Pairs collaborators, creating a series of short videos which aims to educate, excite, and engage secondary students (grades 6-12) in the nanosciences. Each one of the two- to three-minute vlog entries features the work of a Nebraska MRSEC research group, discussing the scientific background, instruments used, material outcomes, and implications of the research for future technological advancements.
A UNL P-SPINS YouTube channel has been created to share this nanoscience vlog with teachers and students, as well as with the interested public: The vlog is designed to function as a standalone resource for teachers, but teachers are also encouraged to use specific vlog entries in conjunction with related nanoscience kits created by the National Informal Science Education (NISE) Network.  
Following its summer 2015 launch, the vlog project will be maintained and expanded by Nebraska MRSEC Education/Outreach Coordinator Jocelyn Bosley. It is expected that the UNL P-SPINS vlog will serve as a useful model for other MRSECs nationally, and for other departments on the University of Nebraska campus, to communicate their research to a broad audience in a way that is dynamic, compelling, and contemporary.

Picture: Courtney Matulka of Millard Public Schools (left) filming Sharmin Sikich (right) working at the scanning electron microscope.


Nebraska MRSEC Partnership with Universities of Strasbourg and Bordeaux

Axel Enders, Sumit Beniwal, Xin Zhang, and Peter Dowben 
Nebraska MRSEC

Bernard Doudin, Lucie Routaboul, and Pierre Braunstein  
University of Strasbourg

Guillaume Chastanet, Nathalie Daro, Patrick Rosa, and Jean-François Letard  
University of Bordeaux

Molecules with switchable magnetic moment could become of considerable importance for the emerging field of organic spintronics, where the control of spin degrees of freedom may be performed electrically on the molecular scale.  Spin crossover complexes based on magnetic iron (Fe) ions are interesting in this regard because they exhibit a reversible transition between a high-spin and low-spin magnetic state, which can be induced by external stimuli such as temperature, pressure, electric field, and by light. The switching of the spin state is strongly dependent of the local environment of the Fe ion, which is susceptible to crystal packing or the presence of an extraneous matrix, and also to long-range effects such as cooperativity between adjacent molecules.
Nebraska MRSEC researchers are involved in a long-standing partnership with French scientists from Universities of Strasbourg and Bordeaux. Recently they have shown that the self-assembly and the structural conformation of the spin crossover molecules Fe((H2B)pz2)2(bipy) and Fe(phen)2(NCS)2 (see figure) are influenced by a supporting gold substrate, with dramatic consequences for spin state and spin state transitions. This demonstration of how the spin state of spin crossover molecules can be manipulated through interface interactions, but also through molecular dipole fields, is new and exciting.
Eleven MRSEC papers have resulted from this international collaboration in the last 5 years

Figure: Scanning tunneling microscopy images showing clear differences in how spin crossover molecules Fe((H2B)pz2)2(bipy) (left) and Fe(phen)2(NCS)2 (right) are aligned on the Au(111) surface to form weakly bound ordered (left) and strongly bound and site-selectively adsorbed disordered (right) layers.

IRG2 nugget

Search for Ferroelectricity in Two-Dimensional Molecular Crystals

Paolo Costa, Jacob Teeter, Alexander Sinitskii and Axel Enders
Nebraska MRSEC

Molecular ferroelectrics are a special class of organic materials that exhibit a bistable electric polarization that can be reversed by applying an external electric field. Molecular ferroelectrics can be useful in ways most inorganic materials cannot. They are inherently more flexible and they do not require epitaxy or other constraining structures for fabrication, and therefore can be made on nearly any surface – including flexible sheets and fabrics. Imidazole-based crystals are a promising class of molecular ferroelectrics. These organic materials are based on molecular chains with hydrogen bonds of the NH...N type that can be bistable in electric polarity and electrically switchable through field-induced proton migration along the hydrogen bond and tautomerization. 
This MRSEC team investigated the formation of two-dimensional assemblies of two-types of imidazole-based molecules: benzimidazole and 2-trifluoromethyl benzimidazole, deposited on gold single crystal substrates. Scanning tunneling microscopy images reveal that in both cases the two-dimensional assemblies of molecules consist of hydrogen-bonded molecular chains. This observation is consistent with the model established for bulk imidazole-based crystals, so that ferroelectricity could potentially exist in those chains along the chain axes. This first direct visualization of NH...N-bonded assemblies of the two-dimensional molecular ferroelectrics of this class opens new opportunities to engineer new low-dimensional structures of molecular ferroelectrics.

Picture: Scanning tunneling microscopy images of molecular chains of benzimidazole (left) and 2-trifluoromethyl benzimidazole (right) molecules on Au(111). Bottom: structure model of benzimidazole chains, highlighting the tautomerization that is at the origin of ferroelectricity in benzimidazole crystals.

seed project

Magnetic Textures Stabilized by Strong Spin-Orbit Interactions

Utkan Güngördü, Rabindra Nepal, Kirill Belashchenko, and Alexey Kovalev  
 Nebraska MRSEC

Oleg Tretiakov  
Institute for Materials Research, Tohoku University, Japan

Recent progress in material science has resulted in the realization of new magnetic materials in which the magnetization vector follows complex patterns, including magnetic spirals and skyrmion lattices. Magnetic skyrmion is a vortex like configuration of magnetic moments where they reverse their direction in the core by forming a whirling twist (see figure). Because skyrmions are stable at finite temperatures and can be manipulated by electric currents and fields, realization of magnetic skyrmions may open new possibilities for applications in magnetic memories and logic devices.
An underlying requirement for the formation and stability of these complex magnetic patterns is a strong spin-orbit coupling, a relativistic effect that refers to the coupling of electron spin with the orbital angular momentum. The strong spin-orbit coupling may be achieved, for example, in layered magnetic/non-magnetic thin films, layered oxide heterostructures, and in thin ferromagnetic layers deposited on topological insulators – emerging materials which have non-trivial electronic properties.
Nebraska MRSEC researchers in collaboration with a researcher from Tohoku University are performing a theoretical modeling of complex magnetic patterns, which may occur in these systems. Their results demonstrate a rich phase diagram of stable magnetic configurations as functions of crystalline anisotropies, spin-orbit interaction and magnetic field, revealing regions of the phase diagram that have not previously been analyzed. These phases include spirals and skyrmionic lattices of differing symmetries (see figure). These results provide a new insight into complex magnetic configurations and their control through materials properties.

Figure: Left: Direction of the magnetization vector in a skyrmion. The magnetization in the core of the skyrmion is opposite to the magnetization on the outskirts. Right: direction of the magnetization vector, forming a skyrmion lattice, with 4-fold (top) and 6-fold symmetry (bottom).


Strain on the Fly

Uday Singh and Shireen Adenwalla
Nebraska MRSEC

Strain refers to the expansion or contraction of a material and can have dramatic effects on the properties of materials, even at very small strain levels of well below 1%.  This sensitivity has been exploited to alter material behavior.  Thin-film materials can be strained by growing them on a substrate with a slight mismatch in the atomic lattice. The slightly altered atomic spacings in the strained thin films can result in dramatically different behavior from bulk crystals, changing the magnetic or superconducting ordering temperatures, for example. Usually, strain sensitivity is explored by growing a series of samples on different substrates, with slight variations in the atomic spacing, but this method results in strains that are fixed by the choice of the substrate and do not allow for any variation on the fly. 
Nebraska MRSEC researchers are exploring methods to strain thin films using intense, high frequency sound waves, which alternately squeeze and stretch a thin film. This allows variation of the strain on the fly, as well as investigating what happens to a material when the strain is varied very quickly. The researchers produce these high frequency strains by patterning a series of curved electrodes on a piezoelectric material – a material that can convert electricity to strain. The curved electrodes focus all the sound wave energy into a central spot. The figure shows light reflecting off a surface excited by a high frequency standing strain wave produced by such an arrangement, with the strain varying from compressive to tensile at close to one hundred millions times a second. The strain distorts the surface, and the reflected light beam wavers in synchrony with the distorted surface, producing a map of high strain and low strain regions, with the highest strain occurring between the two darkest red areas.
The pattern is reminiscent of a stone was dropped into still water.  In this case, however, the wave moves inward to the center rather than outward from the center. Moreover, because the piezoelectric material is not isotropic, the inward moving wave forms elliptical rather than circular rings.

The figure shows optical reflectivity measurements of a standing strain waves produced on a piezoelectric surface at high frequencies (88MHz).  The dark red regions are regions of high strain gradient and the blue regions represent low strain gradients. The highest strain is produced in the regions between the two darkest red areas and may be as high as 1%.

shared facilities

Shared MRSEC Facility Strengthening Collaboration

Axel Enders, Sumit Beniwal, Haidong Lu, Peter Dowben, Alexei Gruverman, and Chang-Beom Eom
Nebraska MRSEC

The MRSEC Thin Film Growth and Characterization Facility combines state-of-the-art tools for the fabrication and in-situ characterization of thin films, nano- and hybrid structures under ultrahigh vacuum conditions. The facility stimulates collaboration between MRSEC investigators to explore new materials and to discover novel phenomena at the nanoscale, by providing a unique suite of synthesis tools for a broad range of materials classes, and a comprehensive set of ultra-sensitive, high-resolution equipment to investigate their properties.
Recently, the Nebraska MRSEC researchers used the Facility to investigate the stability of the ferroelectric polarization of barium titanate films as thin as only a few nanometers. Shown below are piezoresponse force microscopy phase images, taken on a 48-unit-cell-thick (about 19 nanometers) ferroelectric film of barium titanate on a strontium titanate substrate in ultrahigh vacuum. The films have been polarized perpendicular to the surface in square-in-a-square geometry, and the polarization has then been probed as a function of time. The right image shows that the polarization pointing into the plane (the inner square) is unstable and disappears over the time of 5 minutes, due to a loss of stabilizing surface charges in vacuum. Dipolar adsorbates, such as quinonoid zwitterion molecules, can be added to the surface to study how such adsorbates would affect long-term stability and surface polarization of the ferroelectric layers. Experiments like these aide to study the role of surface charges in ferroelectric thin films, and the surface pyroelectric effect. The shared facility is key in enabling this collaboration, by bringing together PIs with expertise in oxide film growth, local probe microscopy, and interface physics to jointly work on this pressing scientific question of stability and scalability of ferroelectrics.

Picture: In-situ piezoresponse force microscopy phase images of a polarization pattern electrically written in a thin BaTiO3 film on the SrTiO3 substrate. The image on the right was taken minutes after the image on the left was written and shows degradation of the remanent polarization of the inner square where the sample is polarized into the film plane, whereas outward polarization in the outer square appears more stable.

nat lab

The P-SPINS Bridge Program – 
A Pipeline for Minority Students to UNL’s Graduate Program

Axel Enders and Jocelyn Bosley
Nebraska MRSEC

Central to Nebraska MRSEC’s Diversity Strategic Plan is a new Bridge Program, which builds on strong strategic partnerships with minority-serving undergraduate institutions to create a pipeline for underrepresented students into the UNL’s graduate program in materials science research. Partnering institutions in the Bridge program include North Carolina A&T State University (NCAT), which is a historically black college, as well as the University of Puerto Rico (UPR) and the California State University at San Bernardino (CSUSB), which both are primarily Hispanic-serving institutions.
Through the Bridge Program, teams of faculty and undergraduate students from the partner institutions are supported to do summer research at UNL. During this time, those teams are involved in P-SPINS research and receive training in the use of UNL’s shared facilities. It is expected that these collaborations continue throughout the following academic year and result in multiple publications.
A second component of the Bridge Program is the organization of educational activities at the partner institutions, involving faculty and students from both P-SPINS and the partner institution. For example, a materials science workshop for undergraduate students, to be held at NCAT during the Fall break of 2015 for the first time and annually thereafter, is currently being organized by Dhananjay Kumar, who is NCAT faculty and a P-SPINS PI, and UNL P-SPINS PI Eva Schubert. Another example is the development of a low-cost scanning tunneling microscope for use in classrooms at CSUSB, through collaboration between Axel Enders and CSUSB faculty Paul Dixon. These activities aim to draw students from partner institutions into P-SPINS activities at multiple levels, ultimately encouraging and enabling them to join UNL’s graduate program.

Picture: 2015 Bridge Program participants. From left: Fernand Torres (UPR), Daniel Vegerano (CSUSB), and Rina Mudanyi (NCAT).


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