Owing to their small physical size and low power consumption, resistive random access memory (RRAM) devices are potential for future memory and logic applications in microelectronics. In this study, a new resistive switching material structure, TiOx/silver nanoparticles/TiOx/AlTiOx, fabricated between the fluorine-doped tin oxide bottom electrode and the indium tin oxide top electrode is demonstrated. The device exhibits excellent memory performances, such as low operation voltage (<±1 V), low operation power, small variation in resistance, reliable data retention, and a large memory window. The current-voltage measurement shows that the conducting mechanism in the device at the high resistance state is via electron hopping between oxygen vacancies in the resistive switching material. When the device is switched to the low resistance state, conducting filaments are formed in the resistive switching material as a result of accumulation of oxygen vacancies. The bottom AlTiOx layer in the device structure limits the formation of conducting filaments; therefore, the current and power consumption of device operation are significantly reduced.
Using MoS2 nanosheets or composites of MoS2 nanosheets for the anode materials is beneficial to the energy storage performance of lithium ion batteries (LIBs). In order to understand the stability of MoS2 nanosheets as the anode material in LIBs (Li–MoS2 batteries), we study the structural evolution of MoS2 nanosheets during the lithiation process by means of in situ scanning electron microscopy (SEM) and ex situ microstructural analyses. By adjusting chemical vapor deposition growth parameters, monolayer MoS2 atomic sheets and continuous multilayer MoS2 nanosheets are respectively prepared for the experiments. In situ SEM analysis of the monolayer MoS2 atomic sheets shows that phase transformations occur at some voltages during the linear sweep voltammetry measurement. These reactions are further validated in Li–MoS2 batteries by cyclic voltammetry. Microstructural analyses of the MoS2 nanosheet anode confirm that the morphology of MoS2 anode significantly changes during the initial lithiation process from the open circuit voltage to 1.1 V. In addition to the reversible intercalation of Li+ ions, another irreversible reaction between MoS2 and Li+ ions also occurs in the lithiation process. This irreversible phase transformation plays an important role in battery performance when the MoS2 anode material is scaled down.
The absence of an epitaxial relation of material growth with silicon substrates usually results from the formation of native oxide on the substrate surface, especially in the lengthy solution process at elevated temperature. An effective method to impede surface oxidation and the consequent inheritance of the lattice structure of the substrate are beneficial for the epitaxial growth. In this study, we use the two-step halogenation/methylation process to create a methylated Si(111) surface for the growth of ZnO nanorods in a chemical bath deposition (CBD) process. The grafted methyl groups effectively passivate the Si(111) surface, and the deposited ZnO nanorods exhibit an epitaxial relation with the silicon substrateZnO[0001]//Si[111]; ZnO{11̅00}//Si{22̅0}. Because no chemical bonding is formed between the ZnO nanorods and the Si(111) substrate, this solution process exhibits the characteristics of the van der Waals epitaxial (vdWE) growth. The approach developed in this study paves a way for vdWE growth of nanostructures on Si substrates by the CBD process.
Focused ion beam (FIB) is a powerful tool for making site-specific and uniform transmission electron microscopy (TEM) specimens out of almost all kinds of solid materials. For thin-film materials, the TEM specimens are usually prepared for cross-sectional and plan-view observations. FIB-based preparation methods for the cross-section specimens have been developed for a long time, and highquality TEM specimens that are suitable for retrieving information with the atomic resolution can be conveniently obtained [1]. On the other hand, reliable plan-view specimen preparation approaches using FIB are seldom reported yet. Using FIB for plan-view specimen preparation is especially suitable for moisture-sensitive materials and two-dimensional atomic layers, such as graphene, MoS2, etc. However, special attentions have to be paid as the thin surface layer can easily be damaged by the ion beam or contaminated by re-deposited material and gallium during preparation [2][3][4][5][6].In this study, we develop fabrication procedures for preparing plan-view TEM specimens using the FEI Helios 600i DualBeam TM FIB system, equipped with platinum and carbon gas injection systems and Omniprobe TM 200 manipulating system. The procedures contain three parts: making protecting walls and lid on the sample, lifting out the protected specimen, and final specimen thinning. We apply this method to prepare a plan-view specimen of a single-layer graphene sheet with a graphene adlayer on it. The graphene sheet was grown on a Cu foil by chemical vapor deposition and transferred to an oxidized Si substrate. The thickness of the oxide layer is 300 nm.Firstly, the protecting walls and lid are prepared using the material in arbitrary areas on the substrate; they are prepared by making empty wedges surrounding a square area, as shown in Figs. 1(a) and 1(b), respectively, and an additional square hole is milled to make the protecting walls. The lid is lifted out by the manipulator and rotated to an appropriate angle for ion beam milling to shape it into a plate ( Fig. 1(c)), and then it is laid onto the protecting walls ( Fig. 1(d)). After sealing the gap between the lid and walls, the whole piece is lifted up for ion beam milling to remove the bottom wedge and reveal the walls, as shown in Fig. 1(e). The protecting cap is laid on the graphene sheet ( Fig. 1(f)). The cap protects the graphene sheet from ion-beam bombardment and the re-deposition of materials in the following specimen cutting and thinning procedures. The thinning procedures are similar to those in regular FIB cross-section specimen preparation, including lifting out and mounting the specimen onto a Cu supporting washer (Fig. 1(g)) and ion-beam thinning. When the substrate is thinned down to ~2 μm, the manipulator is inserted. Its tip is welded with the lid by platinum, and the lid is removed by extracting the manipulator, as shown in Fig. 1(h). The specimen is then further thinned down. Since the ion beam voltage and current (5 KV, 15 pA) in final thinning procedure is relatively gen...
Nicotinamide adenine dinucleotide (NAD+) is a critical cofactor essential for various cellular processes. Abnormalities in NAD+ metabolism have also been associated with a number of metabolic disorders. The regulation and interconnection of NAD+ metabolic pathways are not yet completely understood. By employing an NAD+ intermediate-specific genetic system established in the model organism S. cerevisiae, we show that histone deacetylases (HDACs) Hst1 and Rpd3 link the regulation of the de novo NAD+ metabolism-mediating BNA genes with certain aspects of the phosphate (Pi)-sensing PHO pathway. Our genetic and gene expression studies suggest that the Bas1–Pho2 and Pho2–Pho4 transcription activator complexes play a role in this co-regulation. Our results suggest a model in which competition for Pho2 usage between the BNA-activating Bas1–Pho2 complex and the PHO-activating Pho2–Pho4 complex helps balance de novo activity with PHO activity in response to NAD+ or phosphate depletion. Interestingly, both the Bas1–Pho2 and Pho2–Pho4 complexes appear to also regulate the expression of the salvage-mediating PNC1 gene negatively. These results suggest a mechanism for the inverse regulation between the NAD+ salvage pathways and the de novo pathway observed in our genetic models. Our findings help provide a molecular basis for the complex interplay of two different aspects of cellular metabolism.
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