Session J01: Modeling of Complex Systems

Observing and Simulating Magnetic Microparticle Hopping on Permalloy Disks

Magnetic microspheres are commercially-available, fluid-borne particles made of iron oxide encapsulated in polystyrene. These microspheres are designed for bioseparation of cells, proteins, DNA, and RNA, whereas they can be specifically bound to these targets allowing for field gradients to separate the particles from a mixture. We study the transport of these particles about grids of permalloy disks, driven by varying, weak (<100 Oe) magnetic fields. Recent work has been done in the development of surface-based transport schemes and methods of applying tunable magnetic forces, showing promise for use in on-chip devices. I will discuss some of the applications of this chip-based technique as well as phenomena that arise during transport, for example variation in particle speed with external fields and transition from ordered to disordered transport. We use results from these experiments to develop competing computer models for the purpose of understanding magnetic characteristics of both the microparticles (i.e. susceptibility) as well as the permalloy disks (i.e. magnetization landscapes).

    

Exploring Cathodoluminescence Evident Features of Tungsten Disulfide, Molybdenum Disulfide, and Tungsten-Sulfide-Selenide

Cathodoluminescence (CL) microscopy can be used to characterize the quantum optical behaviors of two-dimensional nanostructures. Imaging using CL can demonstrate features of the underlying structure and behavior of nanoscale materials that would otherwise not be evident in the secondary-electron surveys used by traditional scanning electron microscopy. Other studies have shown that CL images can identify point defects in hexagonal boron nitride.  This finding led us to search for evidence of related behavior in monolayers of tungsten disulfide (WS₂) molybdenum disulfide (MoS₂) and tungsten-sulfide-selenide Janus structures (WSSe). To investigate this behavior, we mounted flakes of WS₂, MoS_2, and WSSe on a SiO₂ substrate and analyzed these samples under both high vacuum and low H₂O vacuum conditions using a scanning electron microscope. We then captured CL and secondary-electron images of the samples at multiple electron-beam energies and currents (5 keV to 30 keV, and 0.5 nA to 5 nA, respectively). We used a range of beam currents and energies to maximize image resolution while minimizing beam-induced sample damage. The images we secured were subjected to nonnegative matrix factorization and principal component analysis processing to identify and isolate areas of unique quantum optical behavior across various wavelengths. Artifacts in the CL images exposed flaws in this analysis, so we needed to develop a more robust image processing method to account for systematic flaws in images not caused by unique sample behavior. Analysis is underway now, but preliminary results showed evidence of non-uniform CL emission across flakes of WSSe and WS₂. Greater emission was observed at the edges of flakes, compared to the center of the flakes, despite their supposed uniform, two-dimensional composition. We are currently quantifying the phenomenon; we anticipate a need to acquire additional data through CL and photoluminescence images.