Research Highlights

Polymeric organic mixed ionic–electronic conductors (OMIECs) underpin several technologies where their electrochemical properties are desirable. These properties however depend on the microstructure that develops in their aqueous operational environment. In relevant experimental conditions, electrolyte-induced swelling amounts to up 20% in volume. We investigated the structure of a model OMIEC across multiple length-scales using cryogenic four-dimensional scanning transmission electron microscopy (cryo-4D-STEM) in both dry and hydrated states. 4D STEM allows us to identify the prevalent defects in the polymer crystalline regions and to analyze the liquid-crystalline nature of the polymer. The orientation maps of the dry and hydrated polymer show that swelling-induced disorder is mostly localized within discrete regions, thus largely preserving liquid crystalline order. Therefore, the liquid crystalline mesostructure makes electronic transport robust to electrolyte ingress. This study demonstrates that cryo-4D-STEM provides multiscale structural insights into complex, hierarchical structures such as polymeric OMIECs, even in their hydrated operating state.

Computational imaging reconstructions from multiple measurements that are captured sequentially often suffer from motion artifacts if the scene is dynamic. We propose a neural space–time model (NSTM) that jointly estimates the scene and its motion dynamics, without data priors or pre-training. Hence, we can both remove motion artifacts and resolve sample dynamics from the same set of raw measurements used for the conventional reconstruction. We demonstrate NSTM in three computational imaging systems: differential phase-contrast microscopy, three-dimensional structured illumination microscopy and rolling-shutter DiffuserCam. We show that NSTM can recover subcellular motion dynamics and thus reduce the misinterpretation of living systems caused by motion artifacts.

Uterine cervical remodeling is a fundamental feature of pregnancy, facilitating the delivery of the fetus through the cervical canal. Yet, we still know very little about this process due to the lack of methodologies that can quantitatively and unequivocally pinpoint the changes the cervix undergoes during pregnancy. We utilize polarization-resolved second harmonic generation to visualize the alterations the cervix extracellular matrix, specifically collagen, undergoes during pregnancy with exquisite resolution. This technique provides images of the collagen orientation at the pixel level (0.4 μm) over the entire murine cervical section. They show tight and ordered packing of collagen fibers around the os at the early stage of pregnancy and their disruption at the later stages. Furthermore, we utilize a straightforward statistical analysis to demonstrate the loss of order in the tissue, consistent with the loss of mechanical properties associated with this process. This work provides a deeper understanding of the parturition process and could support research into the cause of pathological or premature birth.

At the ultimate scaling limit, electronic memory would store bits of information by shifting atoms back and forth inside individual crystalline unit cells. Ferroelectric materials exhibit the electronic hysteresis required to realize this ideal. However, despite the perfect alignment between a material class and an economically important application, ferroelectric computer memory has almost zero commercial presence.

The materials properties that are possible in principle are not realized in practice. Strain, defects, phase competition, and inhomogenieties all confuse the experimental picture. Previously it has been difficult to look inside a ferroelectric to see what is going on.  The standard high-resolution imaging techniques, piezo-force microscopy (PFM) and transmission electron microscopy (TEM), struggle to visualize the electric and polarization fields that are the hallmarks of ferroelectricity. Understanding why ferroelectric materials have not yet lived up to their potential is widely recognized open problem.

Using scanning TEM (STEM) electron beam-induced current (EBIC) imaging, we map the electric fields in a Hf_0.5Zr_0.5O₂ capacitor, obtaining a view of the material’s ferroelectric properties that is unprecedented in its completeness. We map the whole device and  inside nanoscale domains, correlating global free currents with local polarization reversals. In individual domains we isolate and measure the remanent background E-field that does not switch, and we show that this field determines the coercive E-field required to switch the domain. These measurements connect the nanoscopic crystal structure to the mesoscopic materials properties that ultimately determine device function.

Strong coupling refers to the coherent interaction between quantum state transitions and optical modes. These hybrid states present new possibilities for applications such as single-molecule sensing, single photon emitters, and low-threshold solid state lasers. However, due to the fundamental limitation arising from the weak transition dipole moments in the infrared (IR), reaching the strong coupling regime has been limited to macroscopic ensembles. Recently, multi-quantum-wells (MQW) with quantum engineered electronic states offer a promising route towards mid-IR electronic strong coupling. However, with traditional diffraction-limited mode volumes, even for high Q-factor resonators, the strong coupling of a single quantum emitter has historically necessitated operation at cryogenic temperatures to counteract dissipation.

Here, a STROBE team from CU Boulder with collaborators from Sandia National Lab, the Walter Schottky Institute (Munich), Texas A&M, and Colgate University, achieved control of nonlinear IR light-matter interaction between a single nano-antenna and quantum well intersubband transitions. The team combined broadband synchrotron infrared nano-spectroscopy (SINS) at the ALS-LBL with intense fs/ps-pulsed IR s-SNOM imaging developed at CU Boulder for the dynamic manipulation of the antenna-quantum well hybrid states on the nanoscale. The results demonstrate the potential for localized and dynamic modification of quantum states and excitation pathways as a new regime of coherent and tunable IR electronic strong coupling in open nano-cavity systems, with the perspective of nano-scale sensing and nano-optical control of power limiters or saturable absorbers.

Aluminum nanocrystals created by catalyst-driven colloidal synthesis support excellent plasmonic properties, due to their high level of elemental purity, monocrystallinity, and controlled size and shape. Reduction in the rate of nanocrystal growth enables the synthesis of highly anisotropic Al nanowires, nanobars, and singly twinned “nanomoustaches”. Electron energy loss spectroscopy was used to study the plasmonic properties of these nanocrystals, spanning the broad energy range needed to map their plasmonic modes. The coupling between these nanocrystals and other plasmonic metal nanostructures, specifically Ag nanocubes and Au films of controlled nanoscale thickness, was investigated. Al nanocrystals show excellent long-term stability under atmospheric conditions, providing a practical alternative to coinage metal-based nanowires in assembled nanoscale devices.

Nanoscale metrology using coherent extreme-ultraviolet (EUV) or soft x-ray (SXR) light has unique advantages for a broad range of science and technology. Short EUV/SXR wavelengths have high sensitivity to small features, elemental composition, as well as electronic and magnetic orders. Tabletop high harmonic sources (HHG) have high spatial and temporal coherence, enabling precise phase-sensitive measurements of nanoscale functional properties (e.g. transport and mechanical), as well as diffraction-limited imaging with both amplitude and phase contrast. However, to date, most EUV HHG measurements were performed on hard materials, where damage is not significant concern.

STROBE scientists demonstrated that EUV HHG can rapidly and nondestructively characterize dose-sensitive materials such as polymer-based structures, with higher spatial resolution than visible light, and with far less damage than electron imaging. They collaborated with scientists from 3M to characterize polymer metamaterials, that have 2D periodic features less than the wavelength of visible light. Using HHG scatterometry, they extracted layer thicknesses, densities and top-surface geometry, without the need to coat or cut the sample. In contrast, SEM imaging of this polymer metamaterial requires that the sample be coated, and the high-energy electron beam can cause shrinking (see Fig.). Here, the significantly lower photon energy (~42eV) of EUV HHG compared with electron beams (~1-30keV) is key to lowering the dose, while maintaining high spatial resolution (<60nm transverse and <nm axial). Finally, correlative electron imaging, which requires highly specialized sample preparation to avoid sample damage, was implemented to validate the EUV HHG findings.

Many functional properties of molecular systems sensitively depend the local chemical environment seen by each molecule. In that regard, intermolecular coupling plays a pivotal role in controlling energy and charge transfer on molecular length scales. However, determining molecular structure and disorder and with nanometer resolution has notoriously been difficult. Conventional crystallography techniques based on the diffraction of high energy photons and electrons are not sensitive to this low-frequency intermolecular energy landscape.

STROBE teams have recently demonstrated that coupling between molecular vibrations and the resulting collective vibrational states have spectral features that allows one to derive not only the local molecular disorder and nano-scale domain formation, but also enables spectroscopic access to the low-frequency intermolecular energy landscape itself. The spatio-spectral nano-imaging of these collective vibrations in IR nano-spectroscopy has provided a new crystallography technique of vibrational coupling nano-crystallography (VCNC), which offers information on molecular order, disorder, and defects with nano-scale resolution.

In the new work, a STROBE team from CU Boulder collaborating with scientists from the University of Oklahoma now provides a solid theoretical foundation and benchmark measurements to make VCNC quantitative and predictive. This work advances VCNC from a qualitative tool capable of measuring changes in local molecular order to a quantitative technique able to measure and image precise vibrational wavefunction delocalization lengths and intermolecular interaction distances. The technique can now be applied to a wide range of functional molecular systems to image molecular order and disorder on their fundamental length scales.

Biological structures are often characterized by patterns that are self-similar, fractal, or periodic, over a hierarchy of length scales serving specific metabolic, skeletal, or locomotory functions. Many of these motifs have inspired human engineering designs including photonic devices based on butterfly wings, aerospace materials based on avian bone structures, or reduced hydrodynamic drag by emulating shark skin. Further, biological motives can serve as inspiration to address societal challenges including carbon sequestration, bone implants, and dental remineralization. However, understanding biomineralization relies on imaging chemically heterogeneous organic-inorganic interfaces across a hierarchy of spatial scales from atomic structure to nano- and micrometer crystallite dimensions, up to decimeter-size mollusk shells.

Here, a STROBE team from CU Boulder and PNNL collaborating with scientists in oceanography from the University of Washington combine nanoscale secondary ion mass spectroscopy (NanoSIMS) with spectroscopic nano-IR imaging (IR s-SNOM) for simultaneous chemical, molecular, and elemental nanoimaging. At the example of the black-lip pearl oyster mollusk shells they identified for the first time from the morphology of ~50 nm interlamellar protein sheets to aragonite subdomains encapsulated in the prism-covering organic membrane. The results help explain how mollusk shells as complex organic-inorganic composites gain their remarkable combination of stiffness, strength, and toughness unmatched by most manmade materials.