Research Highlights – Page 2

Deep-Learning Electron Diffractive Imaging

January 6, 2023|

Coherent diffractive imaging (CDI) is revolutionizing the physical and biological science fields by first measuring the diffraction patterns of nano-crystals or non-crystalline samples and then inverting them to high-resolution images. The well-known phase problem is solved by the combination of coherent illumination and iterative computational algorithms. In particular, ptychography – a powerful scanning CDI method – has found wide applications with synchrotron radiation, high harmonic generation, electron and optical microscopy. However, iterative algorithms are not only computationally expensive, but also require practitioners to get algorithmic training to optimize the parameters and obtain satisfactory results. These difficulties have thus far prevented CDI from being accessible to an even broader user community. Here we demonstrated deep learning CDI using convolutional neural networks (CNNs) trained only by simulated data. The CNNs are subsequently used to directly retrieve the phase images of monolayer graphene, twisted hexagonal boron nitride and a Au nanoparticle from experimental electron diffraction patterns without any iteration. Quantitative analysis shows that the phase images recovered by the CNNs have comparable quality to those reconstructed by a conventional iterative method and the resolution of the phase images by the CNNs is in the range of 0.71-0.53 Å. Looking forward, we expect that deep learning CDI could become an important tool for real-time, atomic-scale imaging of a wide range of samples across different disciplines.

D. J. Chang, C. M. O’Leary, C. Su, D. A. Jacobs, S. Kahn, A. Zettl, J. Ciston, P. Ercius, J. Miao, “Deep-Learning Electron Diffractive Imaging,” Physical Review Letters, 130, 016101, (2023). DOI: 10.1103/physrevlett.130.016101

Accurate quantification of lattice temperature dynamics from ultrafast electron diffraction of single-crystal films using dynamical scattering simulations

December 5, 2022|

In ultrafast electron diffraction (UED) experiments, accurate retrieval of time-resolved structural parameters, such as atomic coordinates and thermal displacement parameters, requires an accurate scattering model. In this article, we demonstrated dynamical scattering models that are suitable for matching ultrafast electron diffraction (UED) signals from single-crystal films and retrieving the lattice temperature dynamics. We first described the computational approaches used, including both a multislice and a Bloch wave method, and introduced adaptations to account for key physical parameters. We then illustrated the role of dynamical scattering in UED of single-crystal films by comparing static and temperature-dependent diffraction signals calculated using kinematical and dynamical models for gold films of varying thicknesses and rippling as well as varying electron probe energy. Lastly, we applied these models to analyze relativistic UED measurements of single-crystal gold films recorded at the High Repetition-rate Electron Scattering (HiRES) beamline of Lawrence Berkeley National Laboratory. Our results showed the importance of a dynamical scattering theory for quantitative analysis of UED and demonstrated models that can be practically applied to single-crystal materials and heterostructures.

D. B. Durham, C. Ophus, K. M. Siddiqui, A. M. Minor, D. Filippetto, “Accurate quantification of lattice temperature dynamics from ultrafast electron diffraction of single-crystal films using dynamical scattering simulations,” Structural Dynamics, 9, 064302, (2022). DOI: 10.1063/4.0000170

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 Research, 18, 020128, (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 Letters, 21, 10172-10177, (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 Nano, 16, 588-596, (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. Honarvar, J. L. Knobloch, T. D. Frazer, B. Abad, B. McBennett, M. I. Hussein, H. C. Kapteyn, M. M. Murnane, J. N. Hernandez-Charpak, "Directional thermal channeling: A phenomenon triggered by tight packing of heat sources," Proceedings of the National Academy of Sciences, 118, e2109056118, (2021). DOI: 10.1073/pnas.2109056118