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We provide a quantitative analysis on the charge-retention characteristics of sub-threshold operating In–Ga–Zn–O (IGZO) thin-film transistors (TFTs) with a defective gate-oxide for low-power synaptic applications. Here, a defective SiO 2 is incorporated as the synaptic gate-oxide in the fabricated IGZO TFTs, where a defect is physically playing the role as an electron trap. With this synaptic TFT, positive programming pulses for the electron trapping are applied to the gate electrode, followed by monitoring the retention characteristics as a function of time. And this set of the programming and retention-monitoring experiments is repeated in several times for accumulating effects of pre-synaptic stimulations. Due to these accumulated stimulations, electrons are expected to be getting occupied within a deeper trap-state with a higher activation energy, which can lead to a longer retention. To verify these phenomena, a stretched exponential function and respective inverse Laplace transform are employed to precisely estimate a retention time and trap activation-energy for transient experimental results.

Single-site isolated rhodium species anchored on zeolites or titanium dioxide are shown to catalyse the direct conversion of methane to methanol and acetic acid, using oxygen and carbon monoxide under mild conditions. Catalysing methane conversion The direct conversion of methane into liquid chemicals could be very valuable, but no catalysts are known that can do this efficiently and under mild conditions. Junjun Shan et al . now show that mononuclear rhodium species anchored on zeolite or titanium dioxide supports catalyse the direct conversion of methane to methanol and acetic acid, using oxygen and carbon monoxide and under mild conditions. Methanol and acetic acid, which are both widely used in the chemical industry, form through different pathways, so methane conversion can be tuned to produce either compound with selectivities of 70%–100%. The remarkable activity of the catalysts is still far from being technologically relevant, but could guide the development of next-generation catalysts and processes for directly converting methane into useful chemicals. An efficient and direct method of catalytic conversion of methane to liquid methanol and other oxygenates would be of considerable practical value. However, it remains an unsolved problem in catalysis, as typically it involves expensive 1 , 2 , 3 , 4 or corrosive oxidants or reaction media 5 , 6 , 7 , 8 that are not amenable to commercialization. Although methane can be directly converted to methanol using molecular oxygen under mild conditions in the gas phase, the process is either stoichiometric (and therefore requires a water extraction step) 9 , 10 , 11 , 12 , 13 , 14 , 15 or is too slow and low-yielding 16 to be practical. Methane could, in principle, also be transformed through direct oxidative carbonylation to acetic acid, which is commercially obtained through methane steam reforming, methanol synthesis, and subsequent methanol carbonylation on homogeneous catalysts 17 , 18 . However, an effective catalyst for the direct carbonylation of methane to acetic acid, which might enable the economical small-scale utilization of natural gas that is currently flared or stranded, has not yet been reported. Here we show that mononuclear rhodium species, anchored on a zeolite or titanium dioxide support suspended in aqueous solution, catalyse the direct conversion of methane to methanol and acetic acid, using oxygen and carbon monoxide under mild conditions. We find that the two products form through independent pathways, which allows us to tune the conversion: three-hour-long batch-reactor tests conducted at 150 degrees Celsius, using either the zeolite-supported or the titanium-dioxide-supported catalyst, yield around 22,000 micromoles of acetic acid per gram of catalyst, or around 230 micromoles of methanol per gram of catalyst, respectively, with selectivities of 60–100 per cent. We anticipate that these unusually high activities, despite still being too low for commercial application, may guide the development of optimized catalysts and practical processes for the direct conversion of methane to methanol, acetic acid and other useful chemicals.

In this paper, we present an empirical modeling procedure to capture gate bias dependency of amorphous oxide semiconductor (AOS) thin-film transistors (TFTs) while considering contact resistance and disorder effects at room temperature. From the measured transfer characteristics of a pair of TFTs where the channel layer is an amorphous In-Ga-Zn-O (IGZO) AOS, the gate voltage-dependent contact resistance is retrieved with a respective expression derived from the current–voltage relation, which follows a power law as a function of a gate voltage. This additionally allows the accurate extraction of intrinsic channel conductance, in which a disorder effect in the IGZO channel layer is embedded. From the intrinsic channel conductance, the characteristic energy of the band tail states, which represents the degree of channel disorder, can be deduced using the proposed modeling. Finally, the obtained results are also useful for development of an accurate compact TFT model, for which a gate bias-dependent contact resistance and disorder effects are essential.

We present a quantitative analysis of synaptic characteristics dependent on a read voltage (V read ) of a sub-threshold operating Hf-ZnO synaptic thin-film transistor (Syn-TFT). As an higher case with V read = 2.3 V near the threshold voltage (V T ), the output current is depressed with applying positive programming-pulses, which is due to a positive V T shift associated with the electron trapping into the gate insulator. For a lower case with V read = 0.6 V around the flat-band voltage (V FB ), negative programming-pulses are applied to get de-trapping electrons from the gate insulator, resulting in a facilitation of the output current associated with a negative V T shift. During the weight-update process, it is observed that the programming-speed for a lower V read is faster compared to a higher V read due to the steeper sub-threshold slope. After the programming processes, the retention characteristics is monitored with applying constant V read , showing that the effective retention time for a higher V read is much longer than that for a lower V read . This is because a higher positive bias stress can lead to a positive V T shift, suppressing the recovery of trapped electrons. Here, it is found that the bias stress can be intentionally used to improve the retention time.

The recent availability of shale gas has led to a renewed interest in C-H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C-H activation barriers, but their weaker binding to adsorbates offers resilience to coking. Using Pt/Cu single-atom alloys (SAAs), we examine C-H activation in a number of systems including methyl groups, methane and butane using a combination of simulations, surface science and catalysis studies. We find that Pt/Cu SAAs activate C-H bonds more efficiently than Cu, are stable for days under realistic operating conditions, and avoid the problem of coking typically encountered with Pt. Pt/Cu SAAs therefore offer a new approach to coke-resistant C-H activation chemistry, with the added economic benefit that the precious metal is diluted at the atomic limit.

A study on a programming-pulse width dependent synaptic characteristics of a sub-threshold operating hafnium doped zinc oxide (Hf-ZnO) thin-film transistor (TFT) is presented. For this, the static and pulsed characteristics of fabricated Hf-ZnO TFTs need to be monitored, respectively. Here, to achieve the memory capability (e.g. electron trapping and de-trapping) of this Hf-ZnO TFT, trap states in defective gate oxides, such as Al 2 O 3 and HfO x , caused by its low-temperature process with a thermal atomic-layer deposition can intentionally be used. Based on this memory phenomenon, when positive or negative programming-pulses are applied to the gate terminal, as decreasing the pulse width, it is observed that the programming speed is slower. However, in terms of the retention performance, a much longer retention time is confirmed from the mathematical modeling analysis with the stretched exponential function. In this vein, a trade-off relation between the programming speed and retention time is proposed. Besides these characteristics, such as the weight-update and retention characteristics, a paired-pulse depression and facilitation, and weight linearity are monitored to discuss the synaptic functionalities of Hf-ZnO TFTs further. Moreover, with this data at the device level, the crossbar simulation based on the analog accelerator is conducted, monitoring its performance (i.e. a classification accuracy).

1T-MoS 2 and single-atom modified analogues represent a highly promising class of low-cost catalysts for hydrogen evolution reaction (HER). However, the role of single atoms, either as active species or promoters, remains vague despite its essentiality toward more efficient HER. In this work, we report the unambiguous identification of Ni single atom as key active sites in the basal plane of 1T-MoS 2 (Ni@1T-MoS 2 ) that result in efficient HER performance. The intermediate structure of this Ni active site under catalytic conditions was captured by in situ X-ray absorption spectroscopy, where a reversible metallic Ni species (Ni 0 ) is observed in alkaline conditions whereas Ni remains in its local structure under acidic conditions. These insights provide crucial mechanistic understanding of Ni@1T-MoS 2 HER electrocatalysts and suggest that the understanding gained from such in situ studies is necessary toward the development of highly efficient single-atom decorated 1T-MoS 2 electrocatalysts. While single atom catalysis combines heterogeneous materials with molecular understanding, the role of the single atoms remains vague. Here, authors examine single Ni on MoS 2 via in situ X-ray absorption spectroscopy to reveal the intermediate and catalytically active species.

Copper-based catalyst is uniquely positioned to catalyze the hydrocarbon formations through electrochemical CO 2 reduction. The catalyst design freedom is limited for alloying copper with H-affinitive elements represented by platinum group metals because the latter would easily drive the hydrogen evolution reaction to override CO 2 reduction. We report an adept design of anchoring atomically dispersed platinum group metal species on both polycrystalline and shape-controlled Cu catalysts, which now promote targeted CO 2 reduction reaction while frustrating the undesired hydrogen evolution reaction. Notably, alloys with similar metal formulations but comprising small platinum or palladium clusters would fail this objective. With an appreciable amount of CO-Pd 1 moieties on copper surfaces, facile CO * hydrogenation to CHO * or CO-CHO * coupling is now viable as one of the main pathways on Cu(111) or Cu(100) to selectively produce CH 4 or C 2 H 4 through Pd-Cu dual-site pathways. The work broadens copper alloying choices for CO 2 reduction in aqueous phases. The inclusion of platinum-group metals for CO2 reduction electrocatalyst design may trigger the unwanted hydrogen evolution reaction. However, here the authors show that single-atom Pd and Pt on facet-selective Cu can selectively boost CO2 to CH4 or C2H4 conversion through dual-site pathways.

Despite the maximized metal dispersion offered by single-atom catalysts, further improvement of intrinsic activity can be hindered by the lack of neighboring metal atoms in these systems. Here we report the use of isolated Pt 1 atoms on ceria as “seeds” to develop a Pt-O-Pt ensemble, which is well-represented by a Pt 8 O 14 model cluster that retains 100% metal dispersion. The Pt atom in the ensemble is 100–1000 times more active than their single-atom Pt 1 /CeO 2 parent in catalyzing the low-temperature CO oxidation under oxygen-rich conditions. Rather than the Pt-O-Ce interfacial catalysis, the stable catalytic unit is the Pt-O-Pt site itself without participation of oxygen from the 10–30 nm-size ceria support. Similar Pt-O-Pt sites can be built on various ceria and even alumina, distinguishable by facile activation of oxygen through the paired Pt-O-Pt atoms. Extending this design to other reaction systems is a likely outcome of the findings reported here. Single-atom metal catalysts offer maximized material efficiency, but there is large room to improve the intrinsic activity per metal atom for many reactions. Here, the authors demonstrate that the solution for CO oxidation is to tackle the issue of lacking neighboring Pt atoms in the single-atom Pt1/CeO2 system.

We present a study on characteristics of operating region-dependent weight updates in a synaptic thin-film transistor (Syn-TFT) with an amorphous In–Ga–Zn–O (IGZO) channel layer. For a synaptic behavior (e.g. a memory phenomenon) of the IGZO TFT, a defective oxide (e.g. SiO 2 ) is intentionally used for a charge trapping due to programming pulses to the gate terminal. Based on this synaptic behavior, a conductance of the Syn-TFT is modulated depending on the programming pulses, thus weight updates. This weight update characteristics of the Syn-TFT is analyzed in terms of a dynamic ratio ( dr w ) for two operating regions (i.e. the above-threshold and sub-threshold regimes). Here, the operating region is chosen depending on the level of the gate read-voltage relative to the threshold voltage of the Syn-TFT. To verify these, the static and pulsed characteristics of the fabricated Syn-TFT are monitored experimentally. As experimental results, it is found that the dr w of the sub-threshold regime is larger compared to the above-threshold regime. In addition, the weight linearity in the sub-threshold regime is observed to be better compared to the above-threshold regime. Since it is expected that either the dr w or weight linearity can affect performances (e.g. a classification accuracy) of an analog accelerator (AA) constructed with the Syn-TFTs, the AA simulation is performed to check this with a crossbar simulator.