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The Swirling Spins of Hedgehogs

January 24, 2023|

Though microscopes have been in use for centuries, there is still much that we cannot see at the smallest length scales. Current microscopies range from the simple optical microscopes used in high school science classes, to x-ray microscopes that can image through visibly-opaque objects, to electron microscopes that use electrons instead of light to capture images of vaccines and viruses. However, there is a great need to see beyond the static structure of an object—to be able capture how a nano- or biosystem functions in real time, or to visualize magnetic fields on nanometer scales. A team of researchers from the STROBE Center have been working together to overcome these challenges. STROBE is an NSF Science and Technology Center that is building the microscopes of tomorrow. A large multidisciplinary team from the Miao and Osher groups from UCLA, the Kapteyn-Murnane group at CU Boulder, Ezio Iacocca from CU Colorado Springs, David Shapiro and collaborators at Lawrence Berkley National Laboratory, and the Badding and Crespi groups from Pennsylvania State University. They developed and implemented a new method to use x-ray beams to capture the 3D magnetic texture in a material with very high 10-nanometer spatial resolution for the first time (published in Nature Nanotechnology, see reference below).

Hedgehogs and Anti-Hedgehogs

The team investigated a nanostructured magnetic sample, consisting of tiny spheres of nickel, only ~30nm across, connected together by slender few-nm “necks” of nickel, that together form a structure called a magnetic metalattice. This complex nanostructured magnet is expected to produce swirling magnetic fields with topological spin textures that are far more complex than in a uniform magnet. These are called 3D topological magnetic monopoles – or hedgehogs, due to their spiny shape in magnetic rotation – if the magnetic field points outward. Conversely, they can be thought of anti-hedgehogs if the magnetic field points inward.  However, until recently, there was no experimental method to measure the 3D spin texture at the deep nanoscale. Using advanced algorithms to recover the image, and a microscope at the x-ray synchrotron light source at the Lawrence Berkley National Laboratory, the researchers overcame these challenges.

Imaging spin textures is extremely important, as it can help physicists to better understand magnetism at a fundamental level, and to design more energy-efficient data storage, memory, and nanodevices.  Using electron microscopy, one can capture beautiful 2D images of a static spin-texture, but it is challenging to capture a full 3D image. In the past, other scientists were able to capture a 3D image at a spatial resolution of about 100 nanometers, but they had to make assumptions about the sample to extract the 3D image. With this new technique, researchers do not have to make any assumptions.

Armed with this new visualization technique, the team of researchers is excited to study spin textures further. STROBE is developing tabletop setups and helping with national facilities that can capture the static and dynamic spin texture in materials. All algorithms developed for this data analysis will be open-sourced soon. In this experiment, as with others, they found that collaboration is key for moving scientific progress forward.

A. RanaC. LiaoE. IacoccaJ. ZouM. PhamX. LuE. SubramanianY. LoS. A. RyanC. S. BevisR. M. KarlA. J. GlaidJ. RableP. MahaleJ. HirstT. OstlerW. LiuC. M. O’LearyY. YuK. BustilloH. OhldagD. A. ShapiroS. YazdiT. E. MalloukS. J. OsherH. C. KapteynV. H. CrespiJ. V. BaddingY. TserkovnyakM. M. MurnaneJ. Miao, ""Three-dimensional topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice," Nature Nanotechnology(2023)DOI: 10.1038/s41565-022-01311-0

Assessing student engagement with teamwork in an online, large-enrollment course-based undergraduate research experience in physics

October 25, 2022|

Over the last decade, course-based undergraduate research experiences (CUREs) have been recognized as a way to improve undergraduate science, technology, engineering, and mathematics education by engaging students in authentic research practices. One of these authentic practices is participating in teamwork and collaboration, which is increasingly considered to be an important component of undergraduate research experiences and laboratory classes. For example, the American Association of Physics Teachers Recommendations for the Undergraduate Physics Laboratory Curriculum suggest that one of the goals for students in physics labs should be to develop “interpersonal communication skills” through “teamwork and collaboration.” Teamwork can have tremendous benefits for students, including increased motivation, creativity, and reflection; however, it can also pose an array of new social and environmental challenges, such as differing styles of communication, levels of commitment, and understanding of concepts. It can also be difficult for lab course instructors to evaluate and assess. In this work, we study student teamwork in a large-enrollment physics CURE. The CURE was specifically designed to emphasize teamwork as a scientific practice. We use the two sources of data, the adaptive instrument for regulation of emotions questionnaire and students’ written memos to future researchers, to measure the students’ teamwork goals, challenges, self, co-, and socially shared regulations, and perceived goal attainment. We find that students overwhelmingly achieved their teamwork goals by overcoming obstacles using primarily socially shared regulatory strategies, and that the vast majority of students felt teamwork was an essential part of their research experience. We discuss implications for the design of future CUREs and lab courses and for lab instructors desiring to assess teamwork in their own courses.

A. Werth, et al., "Assessing student engagement with teamwork in an online, large-enrollment course-based undergraduate research experience in physics," Physical Review Physics Education Research18020128(2022). DOI: 10.1103/physrevphyseducres.18.020128

Structural and Elastic Properties of Nanostructured Materials Extracted Via Nondestructive Coherent Extreme UV Scatterometry and Electron Tomography

September 2, 2022|

Semiconductor metalattices consisting of a linked network of 3D nanostructures with periodicities on length scales <100nm can enable tailored functional properties due to their complex nanostructuring. For example, by controlling both the porosity and pore size, thermal transport in these phononic metalattices can be tuned—making them promising candidates for efficient thermoelectrics or thermal rectifiers. Thus, the ability to characterize the porosity, and other physical properties, of metalattices is critical but challenging, due to their nanoscale structure and thickness. To date, only metalattices with high porosities, close to the close-packing fraction of hard spheres, have been studied experimentally. Recently, a STROBE team characterized the porosity, thickness, and elastic properties of a low-porosity, empty-pore silicon metalattices for the first time. Laser-driven nanoscale surface acoustic waves were probed by EUV scatterometry to nondestructively measure the acoustic dispersion in these thin silicon metalattice layers. The Young’s modulus, porosity and metalattice layer thickness were then extracted. These advanced characterization techniques are critical for informed and iterative fabrication of energy-efficient devices based on nanostructured metamaterials.

Knobloch et al., “Structural and elastic properties of empty-pore metalattices extracted via nondestructive coherent extreme UV scatterometry and electron tomography, ACS Applied Materials and Interfaces 14, 41316 (2022). Abad et al., “Nondestructive measurements of the mechanical and structural properties of nanostructured metalattices,” Nano Letters 20, 3306 (2020). DOI: 10.1021/acs.nanolett.0c00167

Unveiling the spontaneous blistering of graphene

March 22, 2022|

The outstanding electrical and optical properties of graphene are intricately linked to its extraordinary mechanical behaviors. We report that for monolayer and few-layer graphene on common silicon and glass substrates, acidic solutions induce fast, spontaneous generation of solution-enclosing blisters/bubbles. Using interference reflection microscopy (IRM), a method we developed to visualize graphene structure and defects with outstanding contrast, we monitor the blister-generating process in situ, and show that at pH<~2, nanoscale to micrometer-sized graphene blisters, up to ~100 nm in height, are universally generated with high surface coverages on hydrophilic, but not hydrophobic, surfaces. The spontaneously generated blisters are highly dynamic, with growth, merging, and reconfiguration occurring at second-to-minute time scales. Moreover, we show that in this dynamic system, graphene behaves as a semipermeable membrane that allows the relatively free passing of water, impeded passing of the NaCl solute, and no passing of large dye molecules. Consequently, the blister volumes can be fast and reversibly modulated by the solution osmotic pressure.

Li, B. et al., "Dynamic, spontaneous blistering of substrate-supported graphene in acidic solutions," ACS Nano, 16, 6145-6152, 2022. DOI: 10.1021/acsnano.1c11616

Nano-imaging functional materials at their elementary scale

February 28, 2022|

Any realistic operation of quantum technologies will require counteracting the effects of dissipation and dephasing. In particular, the wide range of photovoltaic, molecular electronic, or novel semiconductors are subject to many coupled internal degrees of freedom, structural heterogeneities, and coupling to the environment leading to the premature loss from conduction carriers to quantum information. Understanding and ultimately controlling the coupled quantum dynamics requires imaging the elementary excitations on their natural time and length scales. To achieve this goal, STROBE developed a new scanning probe microscope with ultrafast and shaped laser pulse excitation for multiscale spatial, spectral, and temporal optical nano-imaging. In this work the researchers developed heterodyne visible-pump IR-probe nano-imaging with far from equilibrium excitation to selectively probe excited state dynamics related to material function. In corresponding ultrafast movies, the fundamental quantum dynamics down to the few-femtosecond regime with nanometer spatial resolution can be resolved. This allowed to visualize in space and time competing electron and phononic processes in the application to solar-cell relevant polaron dynamics in perovskites as well as the insulator-to-metal transition in a correlated quantum material. Resolving these elementary processes related to the performance of these and other functional materials provides a perspective for the future targeted design of optimized and novel photonic and electronic material systems.

Nishida et al., “Ultrafast infrared nano-imaging of cooperative carrier and vibrational dynamics,” Nature Commun. 13, 6582 (2022). DOI: 10.1038/s41467-022-28224-9

Imaging the electron wind force

December 5, 2021|

Electromigration is one of the most critical problems limiting the future scaling of integrated circuits (ICs).  As more transistors are packed into ever smaller volumes, the total length of interconnecting wire is increasing as the wire simultaneously becomes narrower and more vulnerable to failure. Surprisingly, while chip fabs are presently churning out microprocessors at the “5-nm process node” with correspondingly tiny interconnects, before recent work by a STROBE team no measurements of electromigration pressures had been reported at length scales smaller than 10 μm.

Using electron-beam lithography, the  team fabricates tiny aluminum nanowires on electron-transparent silicon nitride membranes. They then apply an electrical current density of 108 A/cm2,  which is enough to drive electromigration, and map the nanowire’s density with parts-per-thousand precision using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).  Two separable effects appear.  First, thermal expansion decreases the wire’s density as Joule heating raises its temperature. Using aluminum’s known coefficient of thermal expansion, one can convert the density map into a temperature map with the same spatial resolution.  Second, via the electron “wind force” the electrical current simultaneously compresses one end of the wire while it tensions the other. Using aluminum’s known bulk modulus,  one can convert the density map into a pressure map.  The temperature effect is independent of the direction of the current, while the pressure effect changes sign with the current’s direction, which allows the two effects to be separated.  Thus the thermodynamic state variables T and P can be mapped with high spatial resolution everywhere inside the wire. The ability to simultaneously see the prompt thermal and electron-wind-force effects in nanoscale interconnects opens entirely new avenues for studying electromigration, a key technological problem in the microprocessors powering modern computing devices.

M. Mecklenburg, et al., "Visualizing the Electron Wind Force in the Elastic Regime," Nano Letters2110172-10177(2021). DOI: 10.1021/acs.nanolett.1c02641

Simultaneous Successive Twinning Captured by Atomic Electron Tomography

November 16, 2021|

Shape-controlled synthesis of multiply twinned nanostructures is an important area of study in nanoscience, motivated by
the desire to control the size, shape, and terminating facets of metal
nanoparticles for applications in catalysis and other technologies. Controlling both the size
and shape of solution-grown nanoparticles relies on an understanding
of how synthetic parameters alter nanoparticle structures during
synthesis. However, while nanoparticle populations at the end of synthesis can be studied with standard electron microscopy methods, transient
structures that appear during some synthetic routes are difficult to observe. This is because these structures are often polycrystalline, with complicated overlapping crystal grains when that are difficult to interpret from a two-dimensional image.

A STROBE team from UC Berkeley and LBNL collaborated to study the
prevalence of transient structures during growth of multiply twinned
particles while also employing atomic electron tomography to reveal the atomic-scale three-dimensional structure of a Pd nanoparticle undergoing a shape transition, from decahedron to icosahedron. By identifying over 20,000 atoms within the structure, then classifying them according to their local crystallographic environment, we observe a multiply twinned structure consistent with a simultaneous successive twinning from a decahedral to icosahedral structure.

P. M. Pelz, et al., "Simultaneous Successive Twinning Captured by Atomic Electron Tomography," ACS Nano16588-596(2021). DOI: 10.1021/acsnano.1c07772

Three-dimensional atomic packing in amorphous solids with liquid-like structure

October 18, 2021|

Liquids and solids are two fundamental states of matter. Although the structure of crystalline solids has long been solved by crystallography, our understanding of the 3D atomic structure of liquids and amorphous materials remained speculative due to the lack of direct experimental determination. Now, a collaborative team from UCLA, Lawrence Berkeley National Lab and Brown University has advanced atomic electron tomography to determine for the first time the 3D atomic positions in monatomic amorphous materials, including a Ta thin film and two Pd nanoparticles. Despite different chemical composition and synthesis methods, they observed that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Contrary to traditional understanding, most pentagonal bipyramids do not assemble icosahedra, but are closely connected to form networks extending to medium-range scales. Molecular dynamics simulations further revealed that pentagonal bipyramid networks are prevalent in monatomic metallic liquids, which rapidly grow in size and form more icosahedra during the quench from the liquid to the glass state. These results expand our fundamental understanding of the atomic structure of amorphous solids and will encourage future studies on amorphous-crystalline phase and glass transitions in non-crystalline materials with three-dimensional atomic resolution.

Y. Yuan, D.S. Kim, J. Zhou, D.J. Chang, F. Zhu, Y. Nagaoka, Y. Yang, M. Pham, S. J. Osher, O. Chen, P. Ercius, A. K. Schmid and J. Miao, “Three-dimensional atomic packing in amorphous solids with liquid-like structure”, Nature Materials, 21, 95–102 (2021). DOI: 10.1038/s41563-021-01114-z

Cool it: Nano-scale discovery could help prevent overheating in electronics

September 27, 2021|

A team of physicists and engineers at CU Boulder have solved the mystery behind a perplexing phenomenon in the nano realm: why some ultra-small heat sources cool down faster if you pack them closer together. The research began with an unexplained observation: a team led by Margaret Murnane and Henry Kapteyn at JILA were experimenting with metallic nanolines on a silicon substrate that when heated with a laser, something strange occurred. Nanoscale heat sources do not usually dissipate heat efficiently. But if you pack them close together, they cool down much more quickly.

Now, the researchers know why it happens. The team joined forces with a group of theorists led by Mahmoud Hussein in Aerospace Engineering Sciences to use computer-based simulations to track the passage of heat from their nano-sized bars. The simulations were so detailed that they could follow the behavior of each and every atom in the model—millions of them in all. They discovered that when they placed the heat sources close together, the vibrations of energy (called phonons) they produced bounced off each other more efficiently when other heat sources were nearby, scattering heat away and cooling the bars down.

The group’s results highlight a major challenge in designing the next generation of tiny devices, such as microprocessors or quantum computer chips. When you shrink down to very small scales, heat does not always behave the way you think it should.

H. HonarvarJ. L. KnoblochT. D. FrazerB. AbadB. McBennettM. I. HusseinH. C. KapteynM. M. MurnaneJ. N. Hernandez-Charpak, "Directional thermal channeling: A phenomenon triggered by tight packing of heat sources," Proceedings of the National Academy of Sciences118e2109056118(2021). DOI: 10.1073/pnas.2109056118

Capturing 3D atomic defects and phonon localization at the 2D heterostructure interface

September 15, 2021|

2D lateral and vertical heterostructures have been actively studied for fundamental interest and practical applications. Although aberration-corrected electron microscopy and scanning probe microscopy have been used to characterize a wide range of 2D heterostructures, the 3D local atomic structure and crystal defects at the heterostructure interface have thus far defied any direct experimental determination. Now, a collaborative team from UCLA, Harvard University, MIT and UC Irvine demonstrates a correlative experimental and first principles method to determine the 3D atomic positions and crystal defects in a MoS2-WSe2 heterojunction with picometer prevision and capture the localized vibrational properties at the epitaxial interface. They observe various crystal defects, including vacancies, substitutional defects, bond distortion and atomic-scale ripples, and quantitatively characterize the 3D atomic displacements and full strain tensor across the heterointerface. The experimentally measured 3D atomic coordinates, representing a metastable state of the  heterojunction, are used as direct input to first principles calculations to reveal new phonon modes localized at the heterointerface, which are corroborated by spatially resolved electron energy-loss spectroscopy. In contrast, the phonon dispersion derived from the minimum energy state of the  heterojunction is absent of the local interface phonon modes, indicating the importance of using experimental 3D atomic coordinates as direct input to better predict the properties of heterointerfaces. Looking forward, it is expected that the ability to couple the 3D atomic structures and crystal defects with the properties of heterostructure interfaces will transform materials design and engineering across different disciplines.

X. TianX. YanG. VarnavidesY. YuanD. S. KimC. J. CiccarinoP. AnikeevaM. LiL. LiP. NarangX. PanJ. Miao, "Capturing 3D atomic defects and phonon localization at the 2D heterostructure interface," Science Advances7eabi6699(2021). DOI: 10.1126/sciadv.abi6699
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