Correlation between morphology and local mechanical and electrical properties of van der Waals heterostructures

Properties of van der Waals (vdW) heterostructures strongly depend on the quality of the interface between two dimensional (2D) layers. Instead of having atomically flat, clean, and chemically inert interfaces without dangling bonds, top-down vdW heterostructures are associated with bubbles and intercalated layers (ILs) which trap contaminations appeared during fabrication process. We investigate their influence on local electrical and mechanical properties of MoS2/WS2 heterostructures using atomic force microscopy (AFM) based methods. It is demonstrated that domains containing bubbles and ILs are locally softer, with increased friction and energy dissipation. Since they prevent sharp interfaces and efficient charge transfer between 2D layers, electrical current and contact potential difference are strongly decreased. In order to reestablish a close contact between MoS2 and WS 2 layers, vdW heterostructures were locally flattened by scanning with AFM tip in contact mode or just locally pressed with an increased normal load. Subsequent electrical measurements reveal that the contact potential difference between two layers strongly increases due to enabled charge transfer, while local I/V curves exhibit increased conductivity without undesired potential barriers.

A solvable contact potential based on a nuclear model

We extend previous works on the study of a particle subject to a three-dimensional spherical singular potential including a $$\delta $$ δ –$$\delta '$$ δ ′ contact interaction. In this case, to have a more realistic model, we add a Coulombic term to a finite well and a radial $$\delta $$ δ –$$\delta '$$ δ ′ contact interaction just at the edge of the well, which is where the surface of the nucleus would be. We first prove that the we are able to define the contact potential by matching conditions for the radial function, fixing a self-adjoint extension of the non-singular Hamiltonian. With these matching conditions, we are able to find analytic solutions of the wave function and focus the analysis on the bound state structure characterizing and computing the number of bound states. For this approximation for a mean-field Woods–Saxon model, the Coulombic term enables us to complete the previous study for neutrons analyzing the proton energy levels in some doubly magic nuclei. In particular, we find the appropriate $$\delta '$$ δ ′ contribution fitting the available data for the neutron- and proton-level schemes of the nuclei $${}^{{208}}$$ 208 Pb, $${}^{{40}}$$ 40 Ca, and $${}^{{16}}$$ 16 O.

Adiabatic versus non-adiabatic electron transfer at 2D electrode materials

Abstract2D electrode materials are often deployed on conductive supports for electrochemistry and there is a great need to understand fundamental electrochemical processes in this electrode configuration. Here, an integrated experimental-theoretical approach is used to resolve the key electronic interactions in outer-sphere electron transfer (OS-ET), a cornerstone elementary electrochemical reaction, at graphene as-grown on a copper electrode. Using scanning electrochemical cell microscopy, and co-located structural microscopy, the classical hexaamineruthenium (III/II) couple shows the ET kinetics trend: monolayer > bilayer > multilayer graphene. This trend is rationalized quantitatively through the development of rate theory, using the Schmickler-Newns-Anderson model Hamiltonian for ET, with the explicit incorporation of electrostatic interactions in the double layer, and parameterized using constant potential density functional theory calculations. The ET mechanism is predominantly adiabatic; the addition of subsequent graphene layers increases the contact potential, producing an increase in the effective barrier to ET at the electrode/electrolyte interface.

High-entropy ZrTiCrNiCu coating

A magnetron target made of a high-entropy ZrTiCrNiCu alloy was synthesized by mechanical alloying methods followed by annealing in a vacuum furnace. Using this target, coatings were applied to steel samples with a thickness of 7-10 microns. After thermal annealing, the coatings were nanostructured. In terms of microhardness, the ZrTiCrNiCu coating is not inferior to, and in most cases exceeds the hardness of high-entropy equiatomic alloys. A high entropy coating has a low coefficient of friction. They turn out to be anti-friction, which, most likely, leads to energy savings. In this work, the surface energy, contact potential difference and work function of electrons for high-entropy coatings were determined for the first time.

A model study on superfluidity of a unitary Fermi gas of atoms interacting with a finite-ranged potential

We calculate Bardeen-Cooper-Schrieffer (BCS) state of a unitary Fermi gas of atoms interacting with the finite-ranged Jost-Kohn potential which has been recently shown to account for the resonant interactions [2019 {\rm J. Phys. B: At. Mol. Opt. Phys.} {\bf 52}, 165004]. Using exact scattering solution of the potential, we derive two-body ${\mathbf T}$-matrix element which is employed to construct the BCS Hamiltonian in momentum space. We present results on the energy- and range-dependence of the pairing gap and superfluid density and the range-dependence of the chemical potential for a wide variation of the scattering length including the \textcolor{red}{unitary} regime. In the zero range limit our calculated gap at the Fermi energy is found to be nearly equal to that calculated \textcolor{red}{in mean-field theory with contact potential}. The mean gap averaged over the full width at half maximum of the gap function in the zero range and unitary limits is found to be $0.42 E_F$ which is quite close to the recent result of the quantum Monte Carlo simulation [2018 {\rm Phys. Rev.A} {\bf 97}, 013601]. The chemical potential in the zero range limit also agrees well with that for the contact potential.

A Novel Approach for the Design of Fast-Settling Amplifiers for Biosignal Detection

The most common method used to pick up biomedical signals is through metallic electrodes coupled to the input of high-gain, low-noise amplifiers. Unfortunately, electrodes, amongst other effects, introduce an undesired contact resistance and a contact potential. The contact potential needs to be rejected since it would otherwise cause the saturation of the input stage of the amplifiers, and this is almost always obtained by inserting a simple RC high-pass filter in the input signal path. The contact resistance needs to be estimated to ensure that it does not impair correct measurements. Methods exist for estimating the contact resistance by dynamically modifying the input network configuration, but because of the presence of the input RC filter, long transients are induced any time a switch occurs between different input configurations, so that the measurement time may become unacceptably long. In this paper, we propose a new topology for a DC removal network at the input of the differential signal amplifier that results in an AC filter whose time constant can be continuously changed by means of a control voltage. As such, we can speed up the recovery from transients by setting very short time constants (during the input resistance estimation process) while maintaining the ability to obtain very low cut-in frequencies by setting a much larger time constant during actual measurements. A prototype of the system was built and tested in order to demonstrate the advantage of the approach we propose in terms of reduced measurement time.

Open-loop amplitude-modulation Kelvin probe force microscopy operated in single-pass PeakForce tapping mode

The open-loop (OL) variant of Kelvin probe force microscopy (KPFM) provides access to the voltage response of the electrostatic interaction between a conductive atomic force microscopy (AFM) probe and the investigated sample. The measured response can be analyzed a posteriori, modeled, and interpreted to include various contributions from the probe geometry and imaged features of the sample. In contrast to this, the currently implemented closed-loop (CL) variants of KPFM, either amplitude-modulation (AM) or frequency-modulation (FM), solely report on their final product in terms of the tip–sample contact potential difference. In ambient atmosphere, both CL AM-KPFM and CL FM-KPFM work at their best during the lift part of a two-pass scanning mode to avoid the direct contact with the surface of the sample. In this work, a new OL AM-KPFM mode was implemented in the single-pass scan of the PeakForce Tapping (PFT) mode. The topographical and electrical components were combined in a single pass by applying the electrical modulation only in between the PFT tip–sample contacts, when the AFM probe separates from the sample. In this way, any contact and tunneling discharges are avoided and, yet, the location of the measured electrical tip–sample interaction is directly affixed to the topography rendered by the mechanical PFT modulation at each tap. Furthermore, because the detailed response of the cantilever to the bias stimulation was recorded, it was possible to analyze and separate an average contribution of the cantilever to the determined local contact potential difference between the AFM probe and the imaged sample. The removal of this unwanted contribution greatly improved the accuracy of the AM-KPFM measurements to the level of the FM-KPFM counterpart.

THE USE OF A MONOMOLECULAR PROTECTIVE FILM ON THE SURFACE OF FRICTION UNITS TO INCREASE THE RELIABILITY OF AGRICULTURAL MACHINERY

One of the factors limiting the reliability of machines and mechanisms is the wear of the contacting surfaces of friction units, which affects their service life. To prevent it, the friction units are treated with lubricants. Surfactants containing fluorinated hydrocarbon radicals are of the greatest interest as an additive to lubricants. (Research purpose) The research purpose is in studying the effect of a protective monomolecular film of fluorinated surfactants on the processes of friction and wear occurring on the surfaces of friction units under conditions of boundary and hydrodynamic friction. (Materials and methods) The structure of the metal was studied before and after the application of a monomolecular protective film, the contact potential difference and the hardness of the samples were determined. The influence of surface energy on the oil absorption of materials was evaluated. (Results and discussion) It was revealed using the contact potential difference method, the process of applying a monomolecular protective film ends after 1.5-3.0 minutes and further exposure in the composition does not lead to a change in the contact potential difference. The values of the adhesion action and wetting energy for surfaces with this coating indicate that the surface energy does not depend on the material, but is determined by the coating of the monomolecular protective film of the test sample. The compositions of 0.05 percent of solutions of fluorinated surfactants form a more porous coating compared to the concentration of 0.5 percent. (Conclusions) Fluorinated surfactants have a high tribotechnical efficiency as antifriction and anti-wear nanomaterials. Their use makes it possible to protect the contact surfaces with a film 3-6 nm thick both under boundary and under hydrodynamic friction. The specified protective film performs the function of a "compensator" for various lubrication modes.

Optical Losses of Frontal Layers in Superstrate CdS/CdTe Solar Cells Using OPAL2

In this paper, optical losses in CdS/CdTe solar cells are calculated on the basis of the designated reflective index of various frontal layers using an OPAL2 calculator for the first time. Two types of glass (0.1 mm ultra-thin Schott and 1.1 mm standard borosilicate glass) were assumed to be coated by different Transparent-Conducting-Oxides (TCOs) such as SnO2:F, ZnO:Al, and ITO forming frontal layers for CdS/CdTe solar cells in superstrate configuration. Absorption, reflectance, transmittance, and consequently optical bandgap energies are calculated as a function of common thicknesses, used in the literature. The results show that an increase in TCO thickness led to a decrease in optical band gap as well as an enhancement in contact potential difference, which can deteriorate device performance. The optimum thickness of 100 nm for SnO2:F was calculated, while 200 nm for ZnO:Al and ITO show reasonable optical losses caused by reflections at the interfaces’ and the layer’s absorption. It is seen that 80 to 150 nm CdS on ITO might be an effective range to satisfy a high short circuit current and low defect densities at the CdS/CdTe interface. Finally, a minimum 2 μm thickness for the CdTe on the ultra-thin Schott glass coated by optimum layers can result in the highest short circuit current of 28.69 mA/cm2. This work offers a practical equivalent strategy to be applied for any superstrate solar cells containing TCO and CdS frontal layers.

Coupled Investigation of Contact Potential and Microstructure Evolution of Ultra-Thin AlOx for Crystalline Si Passivation

In this work, we report the same trends for the contact potential difference measured by Kelvin probe force microscopy and the effective carrier lifetime on crystalline silicon (c-Si) wafers passivated by AlOx layers of different thicknesses and submitted to annealing under various conditions. The changes in contact potential difference values and in the effective carrier lifetimes of the wafers are discussed in view of structural changes of the c-Si/SiO2/AlOx interface thanks to high resolution transmission electron microscopy. Indeed, we observed the presence of a crystalline silicon oxide interfacial layer in as-deposited (200 °C) AlOx, and a phase transformation from crystalline to amorphous silicon oxide when they were annealed in vacuum at 300 °C.