Design of Single-Molecule Multiferroics for Efficient Ultrahigh-Density Nonvolatile Memories

It is known that an isolated single-molecule magnet tends to become super- paramagnetic even at an ultralow temperature of a few Kelvin due to the low spin switching barrier. Herein, single-molecule ferroelectrics/multiferroics is proposed, as the ultimate size limit of memory, such that every molecule can store 1 bit data. The primary strategy is to identify polar molecules that possess bistable states, moderate switching barriers, and polarizations fixed along the vertical direction for high-density perpendicular recording. First- principles computation shows that several selected magnetic metal porphyrin molecules possess buckled structures with switchable vertical polarizations that are robust at ambient conditions. When intercalated within a bilayer of 2D materials such as bilayer MoS2 or CrI3, the magnetization can alter the spin distribution or can be even switched by 180° upon ferroelectric switching, rendering efficient electric writing and magnetic reading. It is found that the upper limit of areal storage density can be enhanced by four orders of magnitude, from the previous super-paramagnetic limit of ≈40 to ≈106 GB in.−2, on the basis of the design of cross-point multiferroic tunneling junction array and multiferroic hard drive

STABILIZING ALKALI METAL ELECTRODEPOSITION VIA NANOSTRUCTURED HYBRID ELECTROLYTE AND INTERPHASE DESIGN FOR RECHARGEABLE METAL BASED BATTERIES

Significant advances in the amount of electrical energy that can be stored in electrochemical cells, such as rechargeable batteries require the adoption of high energy metallic anodes including Li, Na, Al, Zn, etc. Such anodes introduce as significant technical challenges because they are known to form rough electrodeposits, loosely termed dendrites, during the device operation. This produces irreversible active material (electrode and electrolyte) losses during normal cell operation and poses safety concerns because the dendrites can proliferate in the inter-electrode space, shorting the cell internally. Though similar phenomenon has been investigated in the more conventional context of metal electroplating, more complex effects can dominate in a battery configuration especially at current densities below the limiting current and in cells where the metal anodes undergo chemical reaction with electrolyte components. In this thesis, a comprehensive materials strategy involving structural and interfacial engineering is pursued to stabilize lithium metal electrodeposition. The strategy is based on guidelines defined by a theoretical linear stability analysis of metal electrodeposition in structured electrolytes. The origin of deposition instability is revealed to involve fundamental features of electrolytes and interfaces near metal anodes, which lead to electro- convective, morphological and chemical instability. I show that the first two instabilities can be addressed by using a nanostructured polymer/ceramic hybrid electrolyte, which exhibits high conductivity, high modulus and the ability to rectify ion transport through confinement. The well-defined nanoporous structure of the electrolytes also confine the length scale of the electrodeposit, which allows surface tension and other weaker forces at the interface to flatten rough electrodeposits, promoting dendrite-free operation. The chemical instability poses a more serious challenge because it is intrinsic to the chemistry of the electrode and electrolyte components; any exposure of one to the other can in principle drive a reaction cascade that ends in unconstrained growth in the cell impedance and premature failure. I show that this challenge can be overcome by the careful design of solid electrolyte interphases (SEIs) that regulate mass transport of reactive electrolyte ingredients and at the same time are able to flex to accommodate volume expansion of the anode. A significant finding is that these features can be realized using electrolyte additives designed to selectively break-down in-situ to form SEI with explicit composition set by the chemistry of the additive. A particularly important example are additives that break down to form halogen salts, which exhibit low surface diffusion barrier and fast interfacial transport. Such materials are shown to be highly effective in improve battery cycle lifetime. A second category of SEI explored in the study are so- called artificial SEI formed by pretreating the metallic electrode with polymer, metals, and metal oxide precursors prior to cell assembly

Two-Dimensional Metal-Free Organic Multiferroic Material for Design of Multifunctional Integrated Circuits

Coexistence of ferromagnetism and ferroelectricity in a single 2D material is highly desirable for integration of multifunctional units in 2D material-based circuits. We report theoretical evidence of C₆N₈H organic network as being the first 2D organic multiferroic material with coexisting ferromagnetic and ferroelectric properties. The ferroelectricity stems from multimode proton-transfer within the 2D C₆N₈H network, in which a long-range proton-transfer mode is enabled by the facilitation of oxygen molecule when the network is exposed to the air. Such oxygen-assisted ferroelectricity also leads to a high Curie temperature and coupling between ferroelectricity and ferromagnetism. We also find that hydrogenation and carbon doping can transform the 2D g-C₃N₄ network from an insulator to an <i>n</i>-type/<i>p</i>-type magnetic semiconductor with modest bandgap. Akin to the dopant induced <i>n</i>/<i>p</i> channels in silicon wafer, a variety of dopant created functional units can be integrated into the g-C₃N₄ wafer by design for nanoelectronic applications

Transcriptome Analysis Reveals Genetic Factors Related to Callus Induction in Barley

Barley is an important cereal crop worldwide. Its genetic transformation is now limited to very few cultivars because of the high genotype dependence of embryogenic callus. To reveal the key genes or factors controlling the callus induction and plantlet regeneration in barley, we compared the transcriptomic profiles of immature embryos of Golden Promise and ZU9, which differed dramatically in the efficiency of the genetic transformation. The samples were taken at 0, 5, 10 and 20 days of the culture, respectively. In total, 5386 up-regulated and 6257 down-regulated genes were identified in Golden Promise. Several genes, identified exclusively in GP callus, were selected for further investigation. These genes were mainly involved in protein metabolism, energy metabolism, stress response, detoxification and ubiquitin&ndash;proteasome. Four YUCCA flavin monooxygenases, one PIN-FORMED, one tryptophan aminotransferase related, three small auxin up RNA, three indole-3-acetic acid and one adenylate isopentenyl transferase, seven cytokinin oxidase/dehydrogenase, three Arabidopsis histidine kinase, three Arabidopsis histidine phosphotransfer protein, and one Arabidopsis response regulator were differentially expressed in the calli of the two barley genotypes, suggesting that biosynthesis, response and transport of auxin and cytokinin might be associated with cell reprogramming during callus induction. The current results provide insights into molecular mechanisms of callus induction at an early developmental stage and are helpful for optimizing the tissue culture system in barley

Highly Conductive, Sulfonated, UV-Cross-Linked Separators for Li–S Batteries

Metal (based on Li, Na, Mg, or Al)–sulfur batteries are promising candidates for rechargeable electrochemical energy storage devices capable of high charge storage. However, the success of metal–sulfur battery technology calls for solutions of fundamental problems associated with the inherently complex solution chemistry and interfacial reactivity of sulfur and polysulfide species in commonly used electrolytes. It is understood that the dissolution and shuttling of polysulfides induce rapid capacity degradation, poor cycling stability, and low efficiency of these cells. Herein, we report on the synthesis and transport properties of membranes containing sulfonate groups that are able to rectify transport of polysulfide species in liquid electrolytes. Composed of a cross-linked polyethylene glycol (PEG) framework containing pendant SO₃^2– groups, the membranes facilitate electrolyte wetting and Li⁺ ion transport, but are highly selective in preventing migration of negatively charged sulfur species (S_<i>n</i>^2–) dissolved in liquid electrolytes. We argue that the ion rectifying properties originate from two sources, the small tortuous pores originating from cross-linking small PEG molecules and from repulsive electrostatic interactions between the pendant SO₃^2– groups and large migrating S_<i>n</i>^2– species. Here we demonstrate the effectiveness of these membranes in Li–S batteries and we find that the materials enable high-efficiency (>98%) cycling in LiNO₃ additive-free electrolytes. Such membranes are also attractive in other electrochemical cell designs where they serve to decouple transport of positive and negative charged ions in the electrolyte to minimize polarization

Metal–Sulfur Battery Cathodes Based on PAN–Sulfur Composites

Sulfur/polyacrylonitrile composites provide a promising route toward cathode materials that overcome multiple, stubborn technical barriers to high-energy, rechargeable lithium–sulfur (Li–S) cells. Using a facile thermal synthesis procedure in which sulfur and polyacrylonitrile (PAN) are the only reactants, we create a family of sulfur/PAN (SPAN) nanocomposites in which sulfur is maintained as S₃/S₂ during all stages of the redox process. By entrapping these smaller molecular sulfur species in the cathode through covalent bonding to and physical confinement in a conductive host, these materials are shown to completely eliminate polysulfide dissolution and shuttling between lithium anode and sulfur cathode. We also show that, in the absence of any of the usual salt additives required to stabilize the anode in traditional Li–S cells, Li–SPAN cells cycle trouble free and at high Coulombic efficiencies in simple carbonate electrolytes. Electrochemical and spectroscopic analysis of the SPAN cathodes at various stages of charge and discharge further show a full and reversible reduction and oxidation between elemental sulfur and Li-ions in the electrolyte to produce Li₂S as the only discharge product over hundreds of cycles of charge and discharge at fixed current densities