Designing multifunctional CoOx layers for efficient and stable electrochemical energy conversion

Disordered and porous metal oxides are promising as earth-abundant and cost-effective alternatives to noble-metal electrocatalysts. Herein, we leverage non-saturated oxidation in plasma-enhanced atomic layer deposition to tune structural, mechanical, and optical properties of biphasic CoOx thin films, thereby tailoring their catalytic activities and chemical stabilities. To optimize the resulting film properties, we systematically vary the oxygen plasma power and exposure time in the deposition process. We find that short exposure times and low plasma powers incompletely oxidize the cobaltocene precursor to Co(OH)2 and result in the incorporation of carbon impurities. These Co(OH)2 films are highly porous and catalytically active, but their electrochemical stability is impacted by poor adhesion to the substrate. In contrast, long exposure times and high plasma powers completely oxidize the precursor to form Co3O4, reduce the carbon impurity incorporation, and improve the film crystallinity. While the resulting Co3O4 films are highly stable under electrochemical conditions, they are characterized by low oxygen evolution reaction activities. To overcome these competing properties, we applied the established relation between deposition parameters and functional film properties to design bilayer films exhibiting simultaneously improved electrochemical performance and chemical stability. The resulting biphasic films combine a highly active Co(OH)2 surface with a stable Co3O4 interface layer. In addition, these coatings exhibit minimal light absorption, thus rendering them well suited as protective catalytic layers on semiconductor light absorbers for application in photoelectrochemical devices.

Influence of low-Z impurity on the stabilization of m/n = 2/1 tearing/locked modes in EAST

The impact of the low-Z impurity concentration on the modes stabilization has been investigated in the EAST tokamak. Series of tearing modes (TMs) with multiple helicities are excited by the concentration of low-Z (carbon) impurity, and the dominant mode structure is featured by m/n = 2/1 magnetic islands that propagate in electron diamagnetic drift direction (m and n are poloidal and toroidal mode numbers respectively). The m/n = 2/1 locked modes (LMs) can be formed by the redistribution of low-Z impurity concentration, which is unlocked spontaneously for the decreasing of impurity concentration, where the width of magnetic islands can reach w ≅ 5 cm (w/a ≅ 0.1, a is minor radius). The increasing of electromagnetic brake torque is the primary reason for the mode locking, and the 'O'-point of m/n = 2/1 magnetic islands is locked by the tungsten protector limiter (toroidal position: -0.4π ≦ φ ≦ -0.3π) with separation of Δφ ≅ 0. The 3D asymmetric structure of m/n = 2/1 magnetic islands is formed for the interaction with the tungsten protector limiter, and the electromagnetic interaction decreases dramatically for the separation of Δφ ≧ 0.2π. The mechanisms for the mode excitation and locking can be illustrated by the "hysteresis effect" between the low-Z impurity concentration and the width of m/n = 2/1 magnetic islands, namely the growth of magnetic islands is modulated by the low-Z impurity concentration, and the rotation velocity is decelerated accordingly. However, the intrinsic mechanism for the unlocking of m/n = 2/1 LMs is complicated by considering the concentration of the low-Z impurity, and the possible unlocking mechanism is discussed. Therefore, understanding of the relationship between the impurities and magnetic islands is more important for optimizing the control techniques (RMP→LMs, ECRH→NTM, impurity seeding→major collapse, et al).

Spin-dependent vibronic response of a carbon radical ion in two-dimensional WS2

Atomic spin centers in 2D materials are a highly anticipated building block for quantum technologies. Here, we demonstrate the creation of an effective spin-1/2 system via the atomically controlled generation of magnetic carbon radical ions (CRIs) in synthetic two-dimensional transition metal dichalcogenides. Hydrogenated carbon impurities located at chalcogen sites introduced by chemical doping are activated with atomic precision by hydrogen depassivation using a scanning probe tip. In its anionic state, the carbon impurity is computed to have a magnetic moment of 1 μB resulting from an unpaired electron populating a spin-polarized in-gap orbital. We show that the CRI defect states couple to a small number of local vibrational modes. The vibronic coupling strength critically depends on the spin state and differs for monolayer and bilayer WS2. The carbon radical ion is a surface-bound atomic defect that can be selectively introduced, features a well-understood vibronic spectrum, and is charge state controlled.

Cathodoluminescence of carbon-related defects in hexagonal boron nitride

Hexagonal boron nitride is a wide band gap semiconductor exhibiting various luminescence bands in visible and near ultraviolet range, which can be used as single photon source. The luminescence band with zero phonon line at 4.1 eV is commonly ascribed to the carbon impurity introduced during crystal growth. In this paper we provide experimental evidence that carbon-related luminescent centers can be introduced in hBN by local electron irradiation in the chamber of scanning electron microscope at room temperature that can be used as a technique for the nanofabrication of single photon source devices with desired pattern.

Crystalline AlN Interfacial Layer on GaN Using Plasma-Enhanced Atomic Layer Deposition

In this study, we report on the deposition of a highly crystalline AlN interfacial layer on GaN at 330 °C via plasma-enhanced atomic layer deposition (PEALD). Trimethylaluminum (TMA) and NH3 plasma were used as the Al and N precursors, respectively. The crystallinity and mass density of AlN were examined using X-ray diffraction (XRD) and X-ray reflectivity (XRR) measurements, respectively, and the chemical bonding states and atomic concentrations of the AlN were determined by X-ray photoelectron spectroscopy (XPS). The AlN/n-GaN interface characteristics were analyzed using TOF-SIMS and STEM, and the electrical characteristics of the AlN were evaluated using metal-insulator-semiconductor (MIS) capacitors. The PEALD process exhibited high linearity between the AlN thickness and the number of cycles without any incubation period, as well as a low carbon impurity of less than 1% and high crystal quality even at a low deposition temperature of 330 °C. Moreover, the GaN surface oxidation was successfully suppressed by the AlN interfacial layer. Furthermore, enhanced electrical characteristics were achieved by the MIS capacitor with AlN compared to those achieved without AlN.