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The rapid advancements in low-power portable/wearable electronic devices require concurrent development of technologies that can provide power without the need for bulky, heavy battery storage. Electronic ratchets, asymmetric transistor-based devices that can convert AC signals or electronic noise into DC power, have been proposed as one solution to this growing need. Here, the recent demonstration of conjugated polymer-based electronic ratchets offers a route toward lightweight, flexible power sources for portable applications. Here we demonstrate the fabrication of the first electronic ratchets where the active channel component consists of semiconducting single-walled carbon nanotubes (s-SWCNTs), which can transform electronic noise or AC signals to a stable DC current with higher output power (up to ca. 14 mW for a chemically doped device) than their polymer-based analogs. We also show that patterning of the dopant profile in the s-SWCNT channel enables reasonable power conversion performance (ca. 3.5 mW) with improved stability relative to homogeneously doped devices. Our findings demonstrate the promise for s-SWCNT electronic ratchets as energy harvesting devices for portable, low-power applications.

Monolayer molybdenum disulfide (MoS₂) is one of the most studied two-dimensional (2D) transition metal dichalcogenides that is being investigated for various optoelectronic properties, such as catalysis, sensors, photovoltaics, and batteries. One such property that makes this material attractive is the ease in which 2D MoS₂ can be converted between the semiconducting (2H) and metallic/semi-metallic (1T/1T') phases or heavily n-type doped 2H phase with ion intercalation, strain, or excess negative charge. Using n-butyl lithium (BuLi) immersion treatments, we achieve 2H MoS₂ monolayers that are heavily n-type doped with shorter immersion times (10–120 mins) or conversion to the 1T/1T' phase with longer immersion times (6–24 h); however, these doped/converted monolayers are not stable and promptly revert back to the initial 2H phase upon exposure to air. To overcome this issue and maintain the modification of the monolayer MoS₂ upon air exposure, we use BuLi treatments plus surface functionalization p-(CH₃CH₂)₂NPh-MoS₂ (Et₂N-MoS₂)—to maintain heavily n-type doped 2H phase or the 1T/1T' phase, which is preserved for over two weeks when on indium tin oxide or sapphire substrates. We also determine that the low sheet resistance and metallic-like properties correlate with the BuLi immersion times. These modified MoS₂ materials are characterized with confocal Raman/photoluminescence, absorption, x-ray photoelectron spectroscopy as well as scanning Kelvin probe microscopy, scanning electrochemical microscopy, and four-point probe sheet resistance measurements to quantify the differences in the monolayer optoelectronic properties. We will demonstrate chemical methodologies to control the modified monolayer MoS₂ that likely extend to other 2D transition metal dichalcogenides, which will greatly expand the uses for these nanomaterials.

The optoelectronic properties of lead halide perovskite thin films can be tuned through compositional variations and strain, but the associated nanocrystalline structure makes it difficult to untangle the link between composition, processing conditions, and ultimately material properties and degradation. Here, we study the effect of processing conditions and degradation on the local photoconductivity dynamics in [(CsPbI₃)_0.05(FAPbI₃)_0.85(MAPbBr₃)_0.15] and (FA_0.7Cs_0.3PbI₃) perovskite thin films using temporally and spectrally resolved microwave near-field microscopy with a temporal resolution as high as 5 ns and a spatial resolution better than 50 nm. For the latter FACs formulation, we find a clear effect of the process annealing temperature on film morphology, stability, and spatial photoconductivity distribution. After exposure of samples to ambient conditions and illumination, we find spectral evidence of halide segregation-induced degradation below the instrument resolution limit for the mixed halide formulation, while we find a clear spatially inhomogeneous increase in the carrier lifetime for the FACs formulation annealed at 180 degrees C.

Various processes lead to degradation of cathodes in lithium-ion batteries. Questions remain regarding mechanisms of many of these processes. Probe microscopy methods including scanning electrochemical microscopy (SECM) are capable of directly probing processes at the cathode/electrolyte interface. SECM uses a small electrode to perform rapid electrochemical analysis of transient species generated at an active cathode/electrolyte interface. This paper discusses the background of the SECM instrument and its application to lithium-ion battery research. We also describe the application of SECM methods to the study of cathode degradation processes observed in lithium-ion batteries. Specifically, we focus on characterizing the dissolution of manganese from LiMn2O4 (LMO) based on recent debate in the literature. We describe experiments to observe and characterize the electrochemical properties of manganese complexes emerging from degrading LMO electrode materials.

The approaching end of Moore's Law scaling has significantly accelerated multiple fields of research including neuromorphic-, quantum-, and photonic computing, each of which possesses unique benefits unobtained through conventional binary computers. One of the most compelling arguments for neuromorphic computing systems is power consumption, noting that computations made in the human brain are approximately 106 times more efficient than conventional CMOS logic. This review article focuses on the materials science and physical mechanisms found in metal chalcogenides that are currently being explored for use in neuromorphic applications. We begin by reviewing the key biological signal generation and transduction mechanisms within neuronal components of mammalian brains and subsequently compare with observed experimental measurements in chalcogenides. With robustness and energy efficiency in mind, we will focus on short-range mechanisms such as structural phase changes and correlated electron systems that can be driven by low-energy stimuli, such as temperature or electric field. We aim to highlight fundamental materials research and existing gaps that need to be overcome to enable further integration or advancement of metal chalcogenides for neuromorphic systems.

Abstract Electronic ratchets are energy‐harvesting devices that can utilize spatially asymmetric potential distributions to convert nondirectional/random sources of energy into direct current. The potential asymmetry can be generated in a number of ways, but one purported mechanism is to redistribute ions directly within the active material. Utilizing the known propensity for ion migration in lead‐halide perovskites (LHP), the first LHP flashing electronic ratchet is demonstrated by using a voltage stress to intentionally redistribute halide ions within a prototypical 2D perovskite. The resulting asymmetric potential distribution across the 2D perovskite allows for conversion of both electronic noise and unbiased square‐wave potentials into current. Furthermore, simultaneous application of light illumination and voltage stress enhances the asymmetric potential distribution, enabling higher current than the nonilluminated device. This work presents an electronic ratchet system that exploits facile ion migration, which can be modified by both electrical and optical stimuli, providing a model system with the potential to test outstanding mechanistic questions for electronic ratchets.

The Foundations of an Electric Mobility Strategy for the city of Mexicali aligns with numerous energy, environmental, and transport plans and will help Mexicali meet multiple related goals. Mexicali’s energy mix, with 28% renewables, already enables plugin electric vehicles (PEVs) to reduce the mass of greenhouse gases (GHGs) per km driven 2/3 below that of their conventional counterparts. This GHG benefit will increase should Mexicali take steps to further increase their share of renewables in their electricity supply. Beyond increasing renewables, Mexicali could possibly deploy PEVs so that electric load is added in the right location (depending on further analysis of substations and feeders) and at the right time (between 21:00 and 11:00) in order to minimize grid upgrade costs. There are a handful of charge timing control mechanisms –at various stages of development– that Mexicali could implement. Transport electrification can facilitate mass transit by powering buses, trains, and small vehicles that get people from their homes or work to the transit stations and vice versa. Mexicali could utilize fleets as early PEV adopters in order to gain acceptance and add electric vehicle supply equipment (EVSE). Recommended prioritization of different types of fleets are suggested in this report: transit buses, school buses, airport ground support equipment (GSE), refuse trucks, taxis, shuttle buses, campus vehicles, delivery trucks, utility trucks, and finally semitrailers. There are a handful of policy options that Mexicali could use to incentivize fleets to purchase PEVs, including mandates, economic incentives, energy performance contracts, waivers to access restrictions, electricity discounts, and EVSE requirements in building codes. Mexicali’s taxi fleet was an early adopter of PEVs and had experienced some challenges—mostly related to the insufficient range of the taxis due to hot weather. In this report, we strategize ways to extend the range of the current electric taxis, including ways to make charging more convenient to the drivers, and more suggestions for appropriate vehicles to purchase in the future. This report also includes the groundwork of geotracking Mexicali’s taxi fleet so that more detailed recommendations can be made in the future. Once fleets have increased PEV acceptance and EVSE installations, the market will be ready to expand to private vehicle owners. In order to do this, more EVSE needs to be installed in the right locations. This mobility strategy lays out general local areas where EVSE could be well utilized, based on traffic patterns, land use, and demographic data. Mexicali could approach businesses within these areas that would likely make suitable hosts, based on how well they can profit from the additional business that EVSE would bring. Mexicali could then adopt a series of purchase incentives (including sales tax waivers or access to high-occupancy vehicles [HOV] lanes) that would encourage private vehicle owners to purchase PEVs. Purchase incentives run the risk of creating equity issues, which can be countered by promoting electrification in mass transit, creating more HOV lanes that have PEV exemptions, and installing EVSEs in underserved communities. Private PEV ownership will require a set of experts that Mexicali can help train, including PEV repair technicians, EVSE installation electricians, and first responders.

Lithium transition-metal oxides (LiMn₂O₄ and LiMO₂ where M = Ni, Mn, Co, etc.) are widely applied as cathode materials in lithium-ion batteries due to their considerable capacity and energy density. However, multiple processes occurring at the cathode/electrolyte interface lead to overall performance degradation. One key failure mechanism is the dissolution of transition metals from the cathode. This work presents results combining scanning electrochemical microscopy with inductively coupled plasma (ICP) and electron paramagnetic resonance (EPR) spectroscopies to examine cathode degradation products. Our effort employs a LiMn₂O₄ (LMO) thin film as a model cathode to monitor the Mn dissolution process without the potential complications of conductive additive and polymer binders. We characterize the electrochemical behavior of LMO degradation products in various electrolytes, paired with ICP and EPR, to better understand the properties of Mn complexes formed following metal dissolution. We find that the identity of the lithium salt anions in our electrolyte systems [ClO₄^–, PF₆^–, and (CF₃SO₂)₂N^–] appears to affect the Mn dissolution process significantly as well as the electrochemical behavior of the generated Mn complexes. This implies that the mechanism for Mn dissolution is at least partially dependent on the lithium salt anion.

Colloidal halide perovskite nanocrystals or quantum dots (QDs) show similar defect tolerance as thin film perovskite materials with added nanoscale phenomena. Perovskite QD solar cells have demonstrated efficiencies of 16.6%, greater than that of any other QD material system. While the efficiency lags behind the best thin-film perovskite devices, these solar cells could have advantages over the thin-film versions in terms of processability, phase stability, and high open-circuit voltages. However, some operating principles behind perovskite quantum dot device stacks and the associated electric field properties are still unknown. Here, we characterize the junction structure within perovskite QD solar cells, by exposing functioning cross-sections and using nanometer-scale Kelvin probe force microscopy to offer insight into the selection and performance of charge selective contacts. We also evaluated various solar cell device architectures with different selective contacts to isolate the role of each junction in device performance. We show that in high-performance n-i-p architectures, both electron- and hole-transport layer (HTL) interfaces possess a strong electric field, but in the case of the inverted p-i-n architecture, we find that high interfacial recombination at the HTL/QD junction is responsible for subpar device performance. Perovskite QD and thin film materials can synergistically be combined to offer more design flexibility in PV devices, and here we demonstrate that the interface between perovskite thin films and QDs are relatively benign and amenable for synergistic device design.

Monocrystalline Si (c-Si) solar cells with passivated contacts based on the ultrathin SiOx and doped polycrystalline Si (poly-Si) layers in a poly-Si/SiOx/c Si structure show high solar cell efficiencies that are ~26%. Excellent surface passivation using these contacts is achieved via the combined effects of chemical passivation of the SiOx/c-Si interface by the SiOx layer and field-effect passivation from the heavily doped poly-Si layer. These contacts give best performance only when annealed to temperatures higher than 850 degrees C. Structural changes in the SiOx layer and dopant diffusion from poly-Si into the underlying c-Si wafer occur during this step which are hard to investigate using conventional characterization techniques. In this work we investigate poly-Si/SiOx contacts with both a 1.5 (tunneling transport) and 2.2 (pinhole transport) nm SiOx layer using atomic force microscopy techniques. Conductive AFM on n+-poly-Si/SiOx/p-Si structures show significant spatial variations for both contact types, likely due to non-uniformities in the poly-Si layer itself. The electrical and structural variations deeper into the contact were revealed by scanning Kelvin probe microscopy after precisely etching away the poly-Si and SiOx layers and few nanometers of c-Si surface. This etching was performed using tetramethylammonium hydroxide and dilute HF solutions. The resulting surface potential maps appear similar for both contacts, and show less than 500 nm size heavily-doped regions. However, further etching of the c-Si surface reveals these heavily-doped regions to be less than 200 nm deep for the 2.2 nm SiOx contact and greater than 200 nm deep for the 1.5 nm SiOx contact.