Frederick U Nitta

All Publications

• Resolving the Electron Plume within a Scanning Electron Microscope. ACS nano Alcorn, F. M., Perez, C., Smoll, E. J., Hoang, L., Nitta, F. U., Mannix, A. J., Talin, A. A., Nakakura, C. Y., Chandler, D. W., Kumar, S. 2024

  Abstract

  Scanning electron microscopy (SEM), a century-old technique, is today a ubiquitous method of imaging the surface of nanostructures. However, most SEM detectors simply count the number of secondary electrons from a material of interest, and thereby overlook the rich material information contained within them. Here, by simple modifications to a standard SEM tool, we resolve the momentum and energy information on secondary electrons by directly imaging the electron plume generated by the electron beam of the SEM. Leveraging these spectroscopic imaging capabilities, our technique is able to image lateral electric fields across a prototypical silicon p-n junctions and to distinguish differently doped regions, even when buried beyond depths typically accessible by SEM. Intriguingly, the subsurface sensitivity of this technique reveals unexpectedly strong surface band bending within nominally passivated semiconductor structures, providing useful insights for complex layered component designs, in which interfacial dynamics dictate device operation. These capabilities for noninvasive, multimodal probing of complicated electronic components are crucial in today's electronic manufacturing but is largely inaccessible even with sophisticated techniques. These results show that seemingly simple SEM can be extended to probe complex and useful material properties.

  View details for DOI 10.1021/acsnano.4c10527

  View details for PubMedID 39601389

• Axon-like active signal transmission. Nature Brown, T. D., Zhang, A., Nitta, F. U., Grant, E. D., Chong, J. L., Zhu, J., Radhakrishnan, S., Islam, M., Fuller, E. J., Talin, A. A., Shamberger, P. J., Pop, E., Williams, R. S., Kumar, S. 2024

  Abstract

  Any electrical signal propagating in a metallic conductor loses amplitude due to the natural resistance of the metal. Compensating for such losses presently requires repeatedly breaking the conductor and interposing amplifiers that consume and regenerate the signal. This century-old primitive severely constrains the design and performance of modern interconnect-dense chips1. Here we present a fundamentally different primitive based on semi-stable edge of chaos (EOC)2,3, a long-theorized but experimentally elusive regime that underlies active (self-amplifying) transmission in biological axons4,5. By electrically accessing the spin crossover in LaCoO3, we isolate semi-stable EOC, characterized by small-signal negative resistance and amplification of perturbations6,7. In a metallic line atop a medium biased at EOC, a signal input at one end exits the other end amplified, without passing through a separate amplifying component. While superficially resembling superconductivity, active transmission offers controllably amplified time-varying small-signal propagation at normal temperature and pressure, but requires an electrically energized EOC medium. Operando thermal mapping reveals the mechanism of amplification-bias energy of the EOC medium, instead of fully dissipating as heat, is partly used to amplify signals in the metallic line, thereby enabling spatially continuous active transmission, which could transform the design and performance of complex electronic chips.

  View details for DOI 10.1038/s41586-024-07921-z

  View details for PubMedID 39261739

  View details for PubMedCentralID 1392413

• Toward Mass Production of Transition Metal Dichalcogenide Solar Cells: Scalable Growth of Photovoltaic-Grade Multilayer WSe2by Tungsten Selenization. ACS nano Neilson, K. M., Hamtaei, S., Nassiri Nazif, K., Carr, J. M., Rahimisheikh, S., Nitta, F. U., Brammertz, G., Blackburn, J. L., Hadermann, J., Saraswat, K. C., Reid, O. G., Vermang, B., Daus, A., Pop, E. 2024

  Abstract

  Semiconducting transition metal dichalcogenides (TMDs) are promising for high-specific-power photovoltaics due to their desirable band gaps, high absorption coefficients, and ideally dangling-bond-free surfaces. Despite their potential, the majority of TMD solar cells to date are fabricated in a nonscalable fashion, with exfoliated materials, due to the lack of high-quality, large-area, multilayer TMDs. Here, we present the scalable, thickness-tunable synthesis of multilayer WSe2 films by selenizing prepatterned tungsten with either solid-source selenium at 900 °C or H2Se precursors at 650 °C. Both methods yield photovoltaic-grade, wafer-scale WSe2 films with a layered van der Waals structure and superior characteristics, including charge carrier lifetimes up to 144 ns, over 14* higher than those of any other large-area TMD films previously demonstrated. Simulations show that such carrier lifetimes correspond to 22% power conversion efficiency and 64 W g-1 specific power in a packaged solar cell, or 3 W g-1 in a fully packaged solar module. The results of this study could facilitate the mass production of high-efficiency multilayer WSe2 solar cells at low cost.

  View details for DOI 10.1021/acsnano.4c03590

  View details for PubMedID 39177965

• Efficiency limit of transition metal dichalcogenide solar cells COMMUNICATIONS PHYSICS Nazif, K., Nitta, F. U., Daus, A., Saraswat, K. C., Pop, E. 2023; 6 (1)

  View details for DOI 10.1038/s42005-023-01447-y

  View details for Web of Science ID 001128803600002

• Efficiency Limit of Transition Metal Dichalcogenide Solar Cells Nazif, K., Nitta, F. U., Daus, A., Saraswat, K. C., Pop, E., IEEE IEEE. 2023

  View details for DOI 10.1109/PVSC48320.2023.10359865

  View details for Web of Science ID 001151676200341

• High-specific-power flexible transition metal dichalcogenide solar cells. Nature communications Nassiri Nazif, K., Daus, A., Hong, J., Lee, N., Vaziri, S., Kumar, A., Nitta, F., Chen, M. E., Kananian, S., Islam, R., Kim, K., Park, J., Poon, A. S., Brongersma, M. L., Pop, E., Saraswat, K. C. 2021; 12 (1): 7034

  Abstract

  Semiconducting transition metal dichalcogenides (TMDs) are promising for flexible high-specific-power photovoltaics due to their ultrahigh optical absorption coefficients, desirable band gaps and self-passivated surfaces. However, challenges such as Fermi-level pinning at the metal contact-TMD interface and the inapplicability of traditional doping schemes have prevented most TMD solar cells from exceeding 2% power conversion efficiency (PCE). In addition, fabrication on flexible substrates tends to contaminate or damage TMD interfaces, further reducing performance. Here, we address these fundamental issues by employing: (1) transparent graphene contacts to mitigate Fermi-level pinning, (2) MoOx capping for doping, passivation and anti-reflection, and (3) a clean, non-damaging direct transfer method to realize devices on lightweight flexible polyimide substrates. These lead to record PCE of 5.1% and record specific power of 4.4Wg-1 for flexible TMD (WSe2) solar cells, the latter on par with prevailing thin-film solar technologies cadmium telluride, copper indium gallium selenide, amorphous silicon and III-Vs. We further project that TMD solar cells could achieve specific power up to 46Wg-1, creating unprecedented opportunities in a broad range of industries from aerospace to wearable and implantable electronics.

  View details for DOI 10.1038/s41467-021-27195-7

  View details for PubMedID 34887383