Research Highlights

Home \ Research \ Research Highlights

High-fidelity imaging of highly periodic structures enabled by vortex high harmonic beams

September 20, 2023|

STROBE scientists solved a major imaging challenge by demonstrating high fidelity short wavelength imaging of highly-periodic structures for the first time, using engineered illumination via high harmonic extreme UV (EUV) beams carrying orbital angular momentum (OAM). This enables high-fidelity imaging and inspection of highly periodic structures for next-generation nano, energy and photonic devices.

Lensless imaging based on coherent diffractive imaging (CDI) enables near-perfect diffraction-limited microscopy at short wavelengths, overcoming the limits of imperfect and lossy optics. However, high fidelity imaging of highly periodic structures has been challenging. In CDI, a beam is scanned across a sample, and the scattered light is collected by a detector. A computer algorithm is then used to reconstruct an image of the sample. However, to retrieve high-fidelity images, the scatter patterns must change as the beam is scanned – which is not the case for highly periodic samples.

Graduate students Bin Wang and Nathan Brooks, working with Henry Kapteyn and Margaret Murnane, solved this long-standing challenge by using high harmonic beams carrying OAM. The high divergence and peak-intensity near their edges introduces strong interference fringes between adjacent diffraction orders in the far-field. These encode phase information into the scattered light as the beam is scanned, significantly enhancing diversity in the diffraction patterns so that the phase can be reliably retrieved. Moreover, defects in otherwise periodic grids can be more sensitively detected with improved signal-to-noise ratio > 100x, and with lower dose and sample damage than for scanning electron microscopy.

Visible laser beams carrying OAM (i.e. donut-shaped) beams revolutionized visible super-resolution microscopy. Now there is a path forward for bringing these powerful capabilities to shorter wavelengths.

B. Wang, N. J. Brooks, P. Johnsen, N. W. Jenkins, Y. Esashi, I. Binnie, M. Tanksalvala, H. C. Kapteyn, M. M. Murnane, "High-fidelity ptychographic imaging of highly periodic structures enabled by vortex high harmonic beams," Optica, 10, 1245-1252, (2023). DOI: 10.1364/optica.498619

Detecting, distinguishing, and spatiotemporally tracking photogenerated charge and heat at the nanoscale

September 18, 2023|

Since dissipative processes are ubiquitous in semiconductors, characterizing how electronic and thermal energy transduce and transport at the nanoscale is vital for understanding and leveraging their fundamental properties. For example, in low-dimensional transition metal dichalcogenides (TMDCs), excess heat generation upon photoexcitation is difficult to avoid since even with modest injected exciton densities, exciton-exciton annihilation still occurs. Both heat and photoexcited electronic species imprint transient changes in the optical response of a semiconductor, yet the distinct signatures of each are difficult to disentangle in typical spectra due to overlapping resonances. In response, it is necessary to simultaneously map both heat and charge populations in materials on relevant nanometer and picosecond length- and time scales.

By further honing stroboSCAT, a time-resolved optical scattering microscopy capable of capturing spatiotemporal energy flow in a wide range of materials, a STROBE team from UC Berkeley collaborated with Caltech to map both heat and exciton populations in few-layer TMDC MoS2 on the relevant length- and time scales and with 100-mK temperature sensitivity. We discern excitonic contributions to the signal from heat by combining observations close to and far from exciton resonances, characterizing photoinduced dynamics for each. Our approach is general and can be applied to any electronic material, including thermoelectrics, where heat and electronic observables spatially interplay, and lays the groundwork for direct and quantitative discernment of different types of coexisting energy without recourse to complex models or underlying assumptions. This work illustrates the ability to, finally, simultaneously observe and distinguish photogenerated heat from charge in a broad range of systems critical to the performance of next-generation energy conversion modules.

H. L. Weaver, C. M. Went, J. Wong, D. Jasrasaria, E. Rabani, H. A. Atwater, N. S. Ginsberg, “Detecting, Distinguishing, and Spatiotemporally Tracking Photogenerated Charge and Heat at the Nanoscale,” ACS Nano, 17, 19011-19021, (2023). DOI: 10.1021/acsnano.3c04607

MRS PREM Research Scholars Symposium

August 8, 2023|

STROBE Director of Outreach and Broadening Participation, Dr. Sarah Schreiner, worked with the NSF and other PREM (NSF Partnership for Research and Education in Materials) faculty to develop and organize an MRS PREM Research Scholars Symposium at the 2023 MRS Spring Meeting in San Francisco. This symposium hosted over 100 undergraduate PREM Research Scholars from around the US for two days to participate in professional development and networking activities. Dr. Schreiner ran two workshops at the symposium on Networking at Conferences and Turning Your Science into a Story. The symposium ended with a poster session for all participants. The STROBE-PREM partnership, PEAQS, had 10 students from Fort Lewis College and Norfolk State University participating in the symposium.

Ab initio structures from nanocrystal molecular lattices

July 22, 2023|

Electron diffraction has dramatically increased in popularity amongst chemists given its renewed application for ab initio structure determination from molecular nanocrystals. In one implementation, popularly referred to as 3D ED or MicroED, crystals nanocrystals orders of magnitude too small for conventional X-ray analysis are interrogated by an electron beam to determine atomic structures. However, these approaches are thwarted by disordered, overlapping, or otherwise poorly diffracting domains.

Spatially resolved diffraction mapping techniques can overcome some of these limitations, and have seen limited application in X-ray diffraction. In electron microscopy, such approaches, including 4D scanning electron microscopy (4D-STEM), have grown popular. We demonstrated that 4D-STEM can be used to determine ab initio structures of molecules by direct methods, from small ordered nanodomains of single microcrystals. In our approach 4D-STEM is used to generate diffraction scans that enable ex post facto reconstruction of digitally defined virtual apertures. The synthetic patterns derived from these scans are suitable for direct methods phasing of molecular structures.

In addition, this approach unveils that coherently diffracting zones (CDZs) in molecular crystals form unpredictably distributed striations. The observation of these zones and our ability to determine structures from these regions of nanocrystals empowers us to explore their atomic substructure and their response to radiolytic damage.

A. Saha, A. Pattison, M. Mecklenburg, A. Brewster, P. Ercius, J. Rodriguez, “Beyond MicroED: Ab Initio Structure Elucidation using 4D-STEM,” Microscopy and Microanalysis, 29, 309-310, (2023). DOI: 10.1093/micmic/ozad067.143

Operando Spectral Imaging of the Li-ion Battery’s Solid-Electrolyte Interphase

July 12, 2023|

Considering the scale of the lithium ion battery (LIB)  industry, it is surprising how poorly the function of LIBs is understood at the molecular level. While much is certainly known, this knowledge has been gained via inference and expensive trial-and-error because it is difficult to look inside a functioning LIB to “see” what is going on. The battery is a bulk device with a liquid, air-sensitive organic electrolyte. With use, there forms on the LIB electrodes an almost magical solid-electrolyte interphase (SEI) that is an insulator for electrons but a conductor for Li+ ions. The main mysteries of LIB function involve the chemical composition and structure of this layer. We present the first images of the LIB SEI acquired under room-temperature operando conditions with high spatial and spectroscopic resolution. This combination gives us an unprecedented view of the SEI’s development, where we can make chemical identifications localized to nanometer precision while the electrode is in the very act of intercalating. We image the bulk SEI, not just its surface, by contriving electrochemical fluid cells that are only 50 nm thick. With these thin cells we can map the Li itself by its unique spectroscopic fingerprint, an achievement described as “practically impossible” just a few years ago.

J. J. Lodico, M. Mecklenburg, H. Chan, Y. Chen, X. Ling, B. C. Regan, “Operando spectral imaging of the lithium ion battery’s solid-electrolyte interphase,” Science Advances, 9, (2023). DOI: 10.1126/sciadv.adg5135

Two-color high-harmonic generation from relativistic plasma mirrors

July 7, 2023|

Circularly polarized x-rays have a number of microscopy applications that leverage the rotational nature of some physical systems, one of the most common applications being magnetic dichroism of nanoscale magnetic devices. Generating circularly polarized x-rays, particularly coherent x-rays with ultrafast pulse durations is practically difficult and inefficient. One of the major successes of STROBE was the development of not just one, but two mechanisms for generating circularly polarized x-rays using light at moderate intensities in the strong field regime. In this work, we lay the foundation for scaling analogous mechanisms into the relativistic regime. In this regime, the energy cutoff can be much higher than in the strong field. Performing numerical simulations, we confirm that despite the physical mechanism being completely different at relativistic intensities, the same conservation laws observed in the earlier STROBE work are still valid in the relativistic regime. Experiments are being planned to demonstrate this mechanism in the lab, which can leverage new facilities such as the NSF ZEUS which would enable high single shot flux in dichroism experiments.

N. F. Beier, F. F. Dollar, “Two-color high-harmonic generation from relativistic plasma mirrors,” Physical Review E, 108, 015201, (2023). DOI: 10.1103/physreve.108.015201

Predicting heat flow in 3D semiconductor nanosystems

May 17, 2023|

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible, in principle, to engineer their thermal properties. However, this is currently not feasible because there is no general description for heat flow in 3D nanostructured semiconductors. In recent research, STROBE scientists used short wavelength extreme ultraviolet beams to study heat transport in a silicon metalattice with deep nanoscale features. They observed dramatically reduced thermal conductivity relative to bulk—about x50 times less than current model predictions. To explain this, they developed a new predictive theory that incorporates the idea that heat-carrying lattice vibrations can behave like a fluid—spreading out instead of just moving ballistically in straight lines. Moreover, this new theory of heat transport can be used to predict and engineer phonon transport in many other 3D nanosystems including nanowires and nanomeshes, that are of great interest for next-generation energy-efficient devices.

McBennett, A. Beardo, E. Nelson, B. Abad, T. Frazer, A. Adak, B. Li, H. Kapteyn, M. Murnane, J. Knobloch, "Universal Behavior of Highly Confined Heat Flow in Semiconductor Nanosystems: From Nanomeshes to Metalattices," Nano Letters 23, 2129 (2023).

Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga1−xZnx)(N1−xOx) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy

April 17, 2023|

Many ternary and quaternary semiconductors have been made in nanocrystalline forms for a variety of applications, but we have little understanding of how well their ensemble properties reflect the properties of individual nanocrystals. STROBE researchers at CU Boulder examined electronic structure heterogeneities in nanocrystals of (Ga1−xZnx)(N1−xOx), a semiconductor that splits water under visible illumination. They used valence electron energy loss spectroscopy (VEELS) in a scanning transmission electron microscope to map out electronic spectra of (Ga1−xZnx)(N1−xOx) nanocrystals with a spatial resolution of 8 nm. They examine three samples with varying degrees of intraparticle and interparticle compositional heterogeneity and ensemble optical spectra that range from a single band gap in the visible to two band gaps, one in the visible and one in the UV. The VEELS spectra resemble the ensemble absorption spectra for a sample with a homogeneous elemental distribution and a single band gap and, more interestingly, one with intraparticle compositional heterogeneity and two band gaps. They observe spatial variation in VEELS spectra only with significant interparticle compositional heterogeneity. Hence, they reveal the conditions under which the ensemble spectra reveal the optical properties of individual (Ga1−xZnx)(N1−xOx) particles. More broadly, they illustrate how VEELS can be used to probe electronic heterogeneities in compositionally complex nanoscale semiconductors.

B. F. Hammel, L. G. Hall, L. M. Pellows, O. M. Pearce, P. Tongying, S. Yazdi, G. Dukovic, “Relationships between Compositional Heterogeneity and Electronic Spectra of (Ga1-xZnX)(N1-xOX) Nanocrystals Revealed by Valence Electron Energy Loss Spectroscopy,” The Journal of Physical Chemistry C, 127, 7762-7771, (2023). DOI: 10.1021/acs.jpcc.3c00572

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

Three-dimensional topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice

January 23, 2023|

Topological magnetic monopoles (TMMs), also known as hedgehogs or Bloch points, are three-dimensional (3D) nonlocal spin textures that are robust to thermal and quantum fluctuations due to their topology. Understanding their properties is of fundamental interest and practical applications. However, it has been difficult to directly observe the 3D magnetization vector field of TMMs and probe their interactions at the nanoscale. Now, a STROBE team from UCLA, CU Boulder, UC Berkeley and LBNL collaborated with the Penn State MRSEC reports the creation of 138 stable TMMs at the specific sites of a ferromagnetic meta-lattice at room temperature. They developed 3D soft x-ray vector ptychography to determine the magnetization vector and emergent magnetic field of the TMMs with a 3D spatial resolution of 10 nm. This spatial resolution is comparable to the magnetic exchange length of transition metals, enabling them to probe monopole-monopole interactions. The team found that the TMM and anti-TMM pairs are separated by 18.3±1.6 nm, while the TMM and TMM, anti-TMM and anti-TMM pairs are stabilized at comparatively longer distances of 36.1±2.4 nm and 43.1±2.0 nm, respectively. They also observed virtual TMMs created by magnetic voids in the meta-lattice. This work demonstrates that ferromagnetic meta-lattices could be used as a new platform to create and investigate the interactions and dynamics of TMMs. Furthermore, it is expected that soft x-ray vector ptychography can be broadly applied to quantitatively image 3D vector fields in magnetic and anisotropic materials at the nanoscale.

A. Rana, C. Liao, E. Iacocca, J. Zou, M. Pham, X. Lu, E. Subramanian, Y. Lo, S. A. Ryan, C. S. Bevis, R. M. Karl, A. J. Glaid, J. Rable, P. Mahale, J. Hirst, T. Ostler, W. Liu, C. M. O’Leary, Y. Yu, K. Bustillo, H. Ohldag, D. A. Shapiro, S. Yazdi, T. E. Mallouk, S. J. Osher, H. C. Kapteyn, V. H. Crespi, J. V. Badding, Y. Tserkovnyak, M. M. Murnane, J. Miao, “Three-dimensional topological magnetic monopoles and their interactions in a ferromagnetic meta-lattice,” Nature Nanotechnology, 18, 227-232, (2023). DOI: 10.1038/s41565-022-01311-0
Go to Top