Research Highlights

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.

Preterm birth is defined as delivery before 37 weeks of gestation and is a major factor in infant mortality worldwide. Premature birth can cause lifelong developmental disabilities for the child. Unfortunately, there is a significant lack of tools to assess preterm birth risk, which hinders patient care and the development of new treatments.

The aim of this effort was to develop a speculum-free, portable preterm imaging system (PPRIM) for cervical imaging; to test the polarization properties of birefringent samples using the PPRIM system; and to test the PPRIM under an IRB on healthy, non-pregnant volunteers for visualization and polarization analysis of cervical images.

The PPRIM can perform 4 × 3 Mueller-matrix imaging to characterize the remodeling of the uterine cervix during pregnancy. The PPRIM is built with a polarized imaging probe and a flexible insertable sheath made with a compatible flexible rubber-like material to maximize comfort and ease of use.

STROBE scientists demonstrated that the combination of Mueller-matrix imagery into a portable imaging system holds strong potential to aid in the early diagnosis and assistance for those at risk of preterm birth.

This device eliminates the need for a speculum used in standard clinical cervical examination. It can allow the system to be self-inserted, and future studies will focus on point-of-care testing.

The polarimetric results shown in this paper match well with earlier data on similar samples. This system can help reduce patient dropout during prenatal care. This tool’s ease of use and flexibility make it very suitable for in vivo polarization-sensitive imaging beyond the cervix. It could be added to various diagnostic tools focused on women’s health. With current healthcare trends emphasizing telemedicine and remote care, the PPRIM can provide better access and communication between healthcare providers and women through new information channels that weren’t available before.

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.

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.

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.

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.