Xiangkang Meng

Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity

Dendritic Ag/Au bimetallic nanostructures have been synthesized via a galvanic replacement reaction (GRR) of Ag dendrites in a chlorauric acid (HAuCl4) solution. After short periods of time, one obtains structures with protruding flakes; these will mature into very porous structures with little Ag left over. The morphological, compositional, and crystal structural changes involved with reaction time t were analyzed by using scanning and transmission electron microscopy (SEM and TEM, respectively), energy-dispersive X-ray spectrometry (EDX), and X-ray diffraction. High-resolution TEM combined with EDX and selected area electron diffraction confirmed the replacement of Ag with Au. A proposed formation mechanism of the original Ag dendrites developing pores while growing Au flakes cover this underlying structure at longer reaction times is confirmed by exploiting surface-enhanced Raman scattering (SERS). Catalytic reduction of 4-nitrophenol (4-NP) by sodium borohydride (NaBH4) is strongly enhanced, implying promising applications in catalysis.

Field-induced resistive switching based on space-charge-limited current

Polycrystalline (Ba,Sr)(Zr,Ti)O3 thin films sandwiched between two Pt electrodes have been revealed to exhibit hysteretic current-voltage (I-V) characteristics and resistive switching at room temperature. High- and low-resistance states, as well as a less abrupt state transition, occur during the voltage cycle. The maximum ratio between these two resistance states is about 230. Analyses of I-V behaviors have been executed, and it is proposed that space-charge-limited-current conduction in higher voltage region caused by asymmetric electron trapping centers is responsible for such transition of resistance states.

A Review on Diverse Silver Nanostructures

This article reviews recent advances in the utilization of various water based synthesis routes towards the shape-controlled synthesis of silver nanoparticles and microstructures in a diverse range of shapes and sizes from several nanometers to micrometers. A variety of very simple one-pot methods, at times employing commercial microwave ovens, inexpensive low power ultrasound cleaners, or two-electrode electro-chemistry, can be surprisingly effective in the controlled synthesis of a wide range of nanostructured products, if only parameters are carefully chosen. Many approaches which are adopted include synthesis of Ag nanostructures with various shapes in solution, doping of Ag nanoparticles on unmodified silica and on/inside carbon spheres, kinetically controlled growth of Ag micro-particles with novel nanostructures on flat substrates, and galvanic replacement towards bimetallic Ag-Au dendrites and carbon composites. Characterizations of shape, composition and microstructure are carried out via scanning and transmission electron microscopy, various spectroscopy methods, N 2 absorption measurements and suchlike. The involved growth mechanisms are investigated in order to discover new means towards better control. Size, location and shape control, including micro- and nanostructure features, allows tuning the products properties towards desired applications. We focus on the optical properties and catalytic activities, but also the stability of compounds can be an issue of interest.

Structure, optical, and magnetic properties of sputtered manganese and nitrogen-codoped ZnO films

To realize the hole-mediated ferromagnetism, manganese and nitrogen-codoped ZnO (Zn1−xMnxO:N) films were prepared on sapphire (0001) by reactive radio-frequency (rf) magnetron sputtering from Zn0.97Mn0.03O ceramic targets using N2 gas. X-ray photon spectra reveal that the doped Mn ions are mainly in divalent states and the coexistence of O–Zn and N–Zn bonds in the films. According to the absorption spectra, the band gap of Zn0.97Mn0.03O:N films is about 3.15eV, which is slightly lower than that of ZnO films (3.20eV). Compared with Zn0.97Mn0.03O films, ferromagnetic behavior of Zn0.97Mn0.03O:N films were significantly changed with a coercivity of about 70Oe, a saturation magnetization of 0.92μB∕Mn2+ and a remanance over 0.15μB∕Mn2+ at 300K, while at 10K, they increased to be about 110Oe, 1.05μB∕Mn2+ and 0.23μB∕Mn2+, respectively. However, rapid thermal annealing treatment in pure oxygen results in a significant decrease on the magnetic properties of the films.

Highly catalytic spherical carbon nanocomposites allowing tunable activity via controllable Au–Pd doping

We report the synthesis of highly catalytic spherical carbon composite particles with Au-Pd bimetallic nanoparticle doping using a microwave-assisted technique that allows control over the location of the nanoparticles (NPs), putting them into stable interior, but still near-surface locations (within a 100 nm thick shell). First, composite particles with Pd NPs inside of nanoporous carbon spheres (CSs) were synthesized. Subsequent immersion of the composite particles in HAuCl(4) solutions containing PVP led to an addition of Au near the Pd. Au-Pd/CS composites with Au:Pd atomic ratios varying from 0.4 to 4.6 were prepared. The growth of Au and its location relative to the carbons surface and the Pd are discussed. The catalytic activity towards the reduction of 4-nitrophenol is tunable via the Au:Pd atomic ratio. Optimizing the composition increases the activity a hundredfold over that of the corresponding monometallic Pd/CS. The catalytic activity arises from the synergy between different contributing mechanisms, here especially the interaction between the carbon matrix and metals, metal-metal interfaces, and the hydrogen absorption capabilities of Pd.

Diffusion barrier properties of amorphous and nanocrystalline Ta films for Cu interconnects

In the present paper, the diffusion barrier properties of amorphous and nanocrystalline (NC) Ta films, and the interface microstructure of Ta/Cu were investigated as a function of annealing temperature. X-ray diffraction, scanning electron microscopy, cross-sectional transmission electron microscopy, and energy-dispersive spectrometer line scans were employed to study the microstructure evolution and diffusion behavior. It was found that an amorphous layer with a thickness of ∼5 nm formed at the interface of NC Ta/Cu at 450 °C annealing, while the interface of amorphous-Ta/Cu was still abrupt. Moreover, amorphous-Ta film acts as an effective diffusion barrier up to temperatures of 650 °C, which is higher than that for NC-Ta film. The fast diffusion along grain boundaries inside NC-Ta films is suggested to be responsible for the main failure of NC-Ta film.

Size-dependent ordering and Curie temperatures of FePt nanoparticles

The analytic models for size-dependent ordering and Curie temperatures of FePt nanoparticles have been proposed in terms of the size-dependent melting temperature. It is found that the order-disorder transition temperature TO and Curie temperature TC decrease with decreasing the particle size D, and the drop becomes dramatic once the size decreases to about 3 and 6 nm below for TO and TC, respectively. Moreover, the suppression in TC(D) is nearly twice as large as that in TO(D) when D is in the range of 5–20 nm. The accuracy of the developed model is verified by the recent experimental and computer simulation results.

A high energy density asymmetric all-solid-state supercapacitor based on cobalt carbonate hydroxide nanowire covered N-doped graphene and porous graphene electrodes

In order to achieve high energy densities, an asymmetric all-solid-state supercapacitor is developed by synthesizing a novel composite of cobalt carbonate hydroxide (CCH) nanowire covered N-doped graphene (NG) as positive and porous NG as negative electrodes. The CCH–NG composite is obtained from a one-step hydrothermal method, where optimization of the CCH content triples the specific capacitance of porous NG, reaching 1690 F g−1 at 1.0 A g−1. The optimal composite exhibits a remarkable cycling stability retaining 94.2% of the initial capacitance after 10 000 cycles, and good rate capability (still 1358 F g−1 at 10 A g−1). The assembled asymmetric supercapacitor based on the optimal composite has a high discharge areal capacitance of 153.5 mF cm−2 (at 1.0 mA cm−2), can cycle reversibly in the high-voltage region of 0–1.9 V, and thus provide superior energy and power densities (0.77 W h m−2 and 25.3 W m−2).

The Missing Memristor has Not been Found

In 1971, not only the theoretical and by definition already existing ‘ideal memristor’ concept was introduced, but a real memristor device was suggested on grounds of the already known real inductors. The latter is a scientifically significant hypothesis grounded in fundamental symmetries of basic physics, here electro-magnetism. 2008 claimed the discovery of the “missing memristor.” Controversy arose: The devices were not new, and the hypothesized device needs magnetism but has no material memory, while the available devices constitute analogue memory that would work in a world without magnetism. Nevertheless, even the originator of the prediction accepted the discovery. Defenders of the 2008 claim emphasize that the devices are not merely ‘memristive systems,’ which is however a distinction defined in 1976, not 1971. We clarify widely confused concepts and maintain that the originally hypothesized real memristor device is missing and likely impossible. The argument is illustrated also by finding an ideal mechanical memristor element and purely mechanical memristive systems, and hypothesizing a real mechanical memristor device that requires inert mass just like the 1971 implied device requires magnetic induction.

Buckling behavior of metal film/substrate structure under pure bending

Many studies on the thin film/substrate structure and its failure mechanism were reported in recent years. The direct experimental results of thin film/substrate structure by scanning electron microscopy presents an intriguing problem: there exists a buckling failure mechanism at the lateral edge of metal film under pure bending. The qualitative theoretical analysis has been done on such buckling failure of thin film/substrate structure. The experimental results and theoretical analysis are helpful to understand the extrinsic stresses or deformations that are induced by external physical effects.