Molecularly-induced roughness and oxidation in cobalt/organodithiol/cobalt nanolayers synthesized by sputter-deposition and molecular sublimation.

Integrating interfacial molecular nanolayers (MNL) with inorganic nanolayers is of interest for understanding processing-structure/chemistry correlations in hybrid nanolaminates. Here, we report the synthesis of Co/biphenyldithiol (BPDT)/Co nanolayer sandwiches by metal sputter-deposition and molecular sublimation. The density and surface roughness of the Co layers deposited on the native oxide are invariant with the Ar pressure pAr during deposition. In contrast, the Co layer roughness rCo deposited on top of the BPDT MNL increases with pAr, and correlates with a higher degree of Co oxidation. Increased roughening is attributed to MNL-accentuated self-shadowing of low mobility Co atoms at high pAr, which consequently increases Co oxidation. These results indicating MNL-induced effects on the morphology and chemistry of the inorganic layers should be of importance for tailoring nanolayered hybrid interfaces and laminates.

Reflective, polarizing, and magnetically soft amorphous neutron optics with 11B-enriched B4C.

The utilization of polarized neutrons is of great importance in scientific disciplines spanning materials science, physics, biology, and chemistry. However, state-of-the-art multilayer polarizing neutron optics have limitations, particularly low specular reflectivity and polarization at higher scattering vectors/angles, and the requirement of high external magnetic fields to saturate the polarizer magnetization. Here, we show that, by incorporating 11B4C into Fe/Si multilayers, amorphization and smooth interfaces can be achieved, yielding higher neutron reflectivity, less diffuse scattering, and higher polarization. Magnetic coercivity is eliminated, and magnetic saturation can be reached at low external fields (>2 militesla). This approach offers prospects for substantial improvement in polarizing neutron optics with nonintrusive positioning of the polarizer, enhanced flux, increased data accuracy, and further polarizing/analyzing methods at neutron scattering facilities.

Toward an increased reliability of chemical bonding assignment in insulating samples by x-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) spectra from solid samples are conventionally referenced to the spectrometer Fermi level (FL). While, in the case of metallic samples, alignment of the sample and the spectrometer FLs can be directly verified from the measured Fermi edge position, thus allowing to assess the surface electrical potential, this is not a workable option for insulators. When applied, it generates a large spread in reported binding energy values that often exceed involved chemical shifts. By depositing insulating amorphous alumina thin films on a variety of conducting substrates with different work functions, we show not only that FL referencing fails but also that the Al2O3 energy levels align instead to the vacuum level, as postulated in the early days of XPS. Based on these model experiments that can be repeated for all sorts of thin-film insulators, a solution to the binding energy reference problem is proposed for reliable assessment of chemical bonding.

Selectivity Control of Oxygen Reduction Reaction over Mesoporous Transition Metal Oxide Catalysts for Electrified Purification Technologies.

Direct electrification of oxygen-associated reactions contributes to large-scale electrical storage and the launch of the green hydrogen economy. The design of the involved catalysts can mitigate the electrical energy losses and improve the control of the reaction products. We evaluate the effect of the interface composition of electrocatalysts on the efficiency and productivity of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), both mechanistically and at device levels. The ORR and OER were benchmarked on mesoporous nickel(II) oxide and nickel cobaltite (NiO and NiCo2O4, respectively) obtained by a facile template-free hydrothermal synthesis. Physicochemical characterization showed that both NiO and NiCo2O4 are mesoporous and have a cubic crystal structure with abundant surface hydroxyl species. NiCo2O4 showed higher electrocatalytic activity in OER and selectivity to water as the terminal product of ORR. On the contrary, ORR over NiO yielded hydroxyl radicals as products of a Fenton-like reaction of H2O2. The product selectivity in ORR was used to construct two electrolyzers for electrified purification of oxygen and generation of hydroxyl radicals.

Electronic excitation of transition metal nitrides by light ions with keV energies.

We investigated the specific electronic energy deposition by protons and He ions with keV energies in different transition metal nitrides of technological interest. Data were obtained from two different time-of-flight ion scattering setups and show excellent agreement. For protons interacting with light nitrides, i.e. TiN, VN and CrN, very similar stopping cross sections per atom were found, which coincide with literature data of N2 gas for primary energies ⩽25 keV. In case of the chemically rather similar nitrides with metal constituents from the 5th and 6th period, i.e. ZrN and HfN, the electronic stopping cross sections were measured to exceed what has been observed for molecular N2 gas. For He ions, electronic energy loss in all nitrides was found to be significantly higher compared to the equivalent data of N2 gas. Additionally, deviations from velocity proportionality of the observed specific electronic energy loss are observed. A comparison with predictions from density functional theory for protons and He ions yields a high apparent efficiency of electronic excitations of the target for the latter projectile. These findings are considered to indicate the contributions of additional mechanisms besides electron hole pair excitations, such as electron capture and loss processes of the projectile or promotion of target electrons in atomic collisions.

Compromising Science by Ignorant Instrument Calibration-Need to Revisit Half a Century of Published XPS Data.

X-ray photoelectron spectroscopy (XPS) is an indispensable technique in modern materials science for the determination of chemical bonding as evidenced by more than 10 000 XPS papers published annually. A literature survey reveals that in the vast majority of cases an incorrect referencing of the binding energy scale is used, neglecting warnings that have been formulated from the early days of the technique. Consequences for the data reliability are disastrous and decades of XPS work require revisiting. The purpose of this Viewpoint is to highlight the existing problems, review the criticism and suggest ways forward.

Enhanced Ti0.84Ta0.16N diffusion barriers, grown by a hybrid sputtering technique with no substrate heating, between Si(001) wafers and Cu overlayers.

We compare the performance of conventional DC magnetron sputter-deposited (DCMS) TiN diffusion barriers between Cu overlayers and Si(001) substrates with Ti0.84Ta0.16N barriers grown by hybrid DCMS/high-power impulse magnetron sputtering (HiPIMS) with substrate bias synchronized to the metal-rich portion of each pulse. DCMS power is applied to a Ti target, and HiPIMS applied to Ta. No external substrate heating is used in either the DCMS or hybrid DCMS/HiPIMS process in order to meet future industrial thermal-budget requirements. Barrier efficiency in inhibiting Cu diffusion into Si(001) while annealing for 1 hour at temperatures between 700 and 900 °C is investigated using scanning electron microscopy, X-ray diffraction, four-point-probe sheet resistance measurements, transmission electron microscopy, and energy-dispersive X-ray spectroscopy profiling. Ti0.84Ta0.16N barriers are shown to prevent large-scale Cu diffusion at temperatures up to 900 °C, while conventional TiN barriers fail at ≤700 °C. The improved performance of the Ti0.84Ta0.16N barrier is due to film densification resulting from HiPIMS pulsed irradiation of the growing film with synchronized Ta ions. The heavy ion bombardment dynamically enhances near-surface atomic mixing during barrier-layer deposition.

Functionalization of bacterial cellulose wound dressings with the antimicrobial peptide ε-poly-L-Lysine.

Wound dressings based on bacterial cellulose (BC) can form a soft and conformable protective layer that can stimulate wound healing while preventing bacteria from entering the wound. Bacteria already present in the wound can, however, thrive in the moist environment created by the BC dressing which can aggravate the healing process. Possibilities to render the BC antimicrobial without affecting the beneficial structural and mechanical properties of the material would hence be highly attractive. Here we present methods for functionalization of BC with ε-poly-L-Lysine (ε-PLL), a non-toxic biopolymer with broad-spectrum antimicrobial activity. Low molecular weight ε-PLL was cross-linked in pristine BC membranes and to carboxymethyl cellulose functionalized BC using carbodiimide chemistry. The functionalization of BC with ε-PLL inhibited growth of S. epidermidis on the membranes but did not affect the cytocompatibility to cultured human fibroblasts as compared to native BC. The functionalization had no significant effects on the nanofibrous structure and mechanical properties of the BC. The possibility to functionalize BC with ε-PLL is a promising, green and versatile approach to improve the performance of BC in wound care and other biomedical applications.

C 1s Peak of Adventitious Carbon Aligns to the Vacuum Level: Dire Consequences for Material's Bonding Assignment by Photoelectron Spectroscopy.

The C 1s signal from ubiquitous carbon contamination on samples forming during air exposure, so called adventitious carbon (AdC) layers, is the most common binding energy (BE) reference in X-ray photoelectron spectroscopy studies. We demonstrate here, by using a series of transition-metal nitride films with different AdC coverage, that the BE of the C 1s peak EBF varies by as much as 1.44 eV. This is a factor of 10 more than the typical resolvable difference between two chemical states of the same element, which makes BE referencing against the C 1s peak highly unreliable. Surprisingly, we find that C 1s shifts correlate to changes in sample work function ϕSA , such that the sum EBF+ϕSA is constant at 289.50±0.15 eV, irrespective of materials system and air exposure time, indicating vacuum level alignment. This discovery allows for significantly better accuracy of chemical state determination than offered by the conventional methods. Our findings are not specific to nitrides and likely apply to all systems in which charge transfer at the AdC/substrate interface is negligible.