Assessing student engagement with teamwork in an online, large-enrollment course-based undergraduate research experience in physics
Structural and Elastic Properties of Nanostructured Materials Extracted Via Nondestructive Coherent Extreme UV Scatterometry and Electron Tomography
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.
Unveiling the spontaneous blistering of graphene
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.
Nano-imaging functional materials at their elementary scale
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.
Imaging the electron wind force
Simultaneous Successive Twinning Captured by Atomic Electron Tomography
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.