Research Highlights

Next-generation nano and quantum devices have increasingly complex 3D structure. As the dimensions of these devices shrink to the nanoscale, their performance is often governed by interface quality or precise chemical or dopant composition. A STROBE team from CU Boulder, UCLA, UC Berkeley, as well as laser and nanoelectronics industry partners, worked together for 4 years to design, construct and commission the first phase-sensitive extreme ultraviolet imaging reflectometer. It combines the excellent phase stability of tabletop coherent extreme UV (EUV) light sources, the unique chemical- and phase-sensitivity of coherent EUV imaging, and state-of-the-art algorithms. This tabletop microscope can non-destructively probe surface topography, layer thicknesses, and interface quality, as well as dopant concentrations and profiles. High-fidelity imaging was achieved by implementing phase sensitive imaging at different angles, by using advanced methods to mitigate noise and artifacts in the reconstructed image, and by using a high-brightness, EUV source with excellent intensity and wavefront stability. These measurements were validated through multiscale electron and atomic force microscopy imaging to show that this approach has unique advantages compared with others. Critical to this project were new photon and electron-based imaging methods, advanced algorithms, unique samples, as well STROBE advances in tabletop coherent imaging in transmission and reflection mode. Several STROBE trainees received awards for this effort.

Colloidal materials are a platform for studying self-assembly as well as the bottom-up creation of next generation hierarchical materials, and controllably perturbing their collective dynamics is an important step towards directing their assembly. In a liquid droplet, silica nanoparticles collect on the surface and organize to form an ordered 2D lattice. A STROBE research team led by Naomi Ginsberg (UC Berkeley) investigated these monolayers on a low vapor pressure ionic liquid, allowing experiments to be performed under the vacuum environment of a scanning electron microscope. Alongside imaging the particles, the electron beam serves as a perturbative tool for controllably charging the colloidal lattice. As particles charge, they sink into the droplet reducing the monolayer’s density and driving a melting transition. These findings will provide new insights for understanding phase transitions in soft materials and analogous atomic crystals.

The demand for faster, more efficient, and more compact nanoelectronic devices, like smartphone chips, requires engineers to develop increasingly complex designs. To achieve this, engineers use layer upon layer of very thin films – as thin as only a couple strands of DNA – with impurities added, to tailor the function. However, the presence of these necessary impurities and extreme thinness degrades the material strength, reducing its performance and making it more likely to fail. To date, it was simply not possible to test the stiffness or compressibility of the thinnest of these ultra-thin films. Now, by using laser-like beams at very short wavelengths – beyond the ultraviolet region of the spectrum – scientists were finally able to measure the mechanical properties of these films. What they learned was surprising: as the layers thinned, the mechanical properties dramatically deteriorated, becoming nearly 10 times flimsier than expected. Additionally, the presence of impurities can be more detrimental to the film’s strength than the effect of its thinning. These findings will influence the design of next generation electronic and other nanoscale devices.

Bacteriocins are contractile molecular syringes — nanomachines produced by one bacterium that can puncture the cell membrane of another bacterium to deliver a lethal punch. In this week’s issue of Nature and featured on the cover, STROBE UCLA scientist Hong Zhou and his colleagues present high-resolution structures of the bacteriocin pyocin R2 from P. aeruginosa – in both its preand post-contraction states. The results allow the researchers to suggest in detail how the molecular syringe works, offering insight into how R-type bacteriocins might be developed into a new class of antimicrobials. This work was featured in the April 2020 cover of Nature.

Due to the reduced dimensionality, the properties and functionality of 2D materials and van der Waals heterostructures are strongly influenced by atomic defects such as dopants, vacancies, dislocations, grain boundaries, strains, ripples and interfaces. Although x-ray diffraction can determine the 3D crystal structure of 2D materials at atomic resolution, it is blind to crystal defects. Aberration corrected electron microscopy and scanning probe microscopy allow us to see individual atoms without the constraint of crystal averaging. But, seeing atoms is not the same as knowing their 3D coordinates with high precision, which is required for an accurate prediction of properties using quantum mechanics. No ab initio calculations can take a 2D image of atoms as direct input to determine material properties.

A STROBE team led by John Miao (UCLA) in collaboration with scientists from Harvard University, ORNL and Rice University recently developed scanning atomic electron tomography (sAET) to determine the atomic positions and crystal defects in Re-doped MoS2 with a 3D precision down to 4 picometers. They observed dopants, vacancies and ripples, measured the full 3D strain tensor and quantified local strains induced by single dopants. By directly providing experimental 3D atomic coordinates to density functional theory (DFT), they obtained more truthful electronic band structures than those derived from conventional DFT calculations relying on relaxed 3D atomic models, which was confirmed by photoluminescence spectra measurements. Furthermore, they observed that the local strain induced by atomic defects along the z-axis is larger than that along the x- and y-axis and thus more strongly affects the electronic property of the 2D material. It is anticipated that sAET is not only generally applicable to the determination of the 3D atomic coordinates of 2D materials and heterostructures, but also could transform ab initio calculations by using experimental atomic coordinates as direct input to reveal more realistic physical, material, chemical and electronic properties.

The next generation of semiconducting materials that will facilitate energy transport and storage in the technologies around us is becoming increasingly complex. The ability of energy carriers to move between atoms and molecules underlies biochemical and material function. Understanding and controlling energy flow, however, requires observing it on ultrasmall and ultrafast spatio-temporal scales, where energetic and structural roadblocks dictate the fate of energy carriers.

A STROBE team led by Naomi Ginsberg (UCB) developed a novel time-resolved interferometric scattering microscope to visualize how energy navigates the intrinsically disordered landscapes in these materials on the nanoscale. With this high-throughput technique, they collected non-invasive stroboscopic movies in a variety of organic, inorganic, and hybrid materials to demonstrate its powerful versatility. Applied to other cutting-edge materials, we hope to inform the design of new functional devices for the semiconductor industry of tomorrow.

How molecules interact and transfer energy between each other dictates the performance in molecular electronics, organic light emitting diodes, photovoltaics, or in many biological processes. However, imaging the controlling underlying molecular order and associated wavefunction delocalization on the molecular scale has long remained a major challenge in imaging science.

A STROBE team from CU Boulder, UC Berkeley and LBNL, has overcome this challenge developing a new technique of nanoimaging in the infrared probing the delicate low-energy landscape of molecular interactions. Measuring coupled molecular vibrations with high precision provides for a new molecular ruler to resolve the effect of disorder with sub-nanometer resolution. In a representative organic electronic material of metal-porphyrin nano-crystals the researchers learned about the relationship between structure and function of energy transfer on molecular length scales. The new insights gained advance our understanding of light harvesting in photosynthesis and improve the design of next generation organic electronic and photonic devices.