First-principles investigation of copper diffusion barrier performance in defective 2D layered materials

This paper investigates the diffusion barrier performance of 2D layered materials with pre-existing vacancy defects using first-principles density functional theory. Vacancy defects in 2D materials may give rise to a large amount of Cu accumulation, and consequently, the defect becomes a diffusion path for Cu. Five 2D layered structures are investigated as diffusion barriers for Cu, i.e., graphene with C vacancy, hBN with B/N vacancy, and MoS2 with Mo/2S vacancy. The calculated energy barriers using climbing image - nudged elastic band show that MoS2-V2S has the highest diffusion energy barrier among other 2D layers, followed by hBN-VN and graphene. The obtained energy barrier of Cu on defected layer is found to be proportional to the length of the diffusion path. Moreover, the diffusion of Cu through vacancy defects is found to modulate the electronic structures and magnetic properties of the 2D layer. The charge density difference shows that there exists a considerable charge transfer between Cu and barrier layer as quantified by Bader charge. Given the current need for an ultra-thin diffusion barrier layer, the obtained results contribute to the field of application of 2D materials as Cu diffusion barrier in the presence of mono-vacancy defects.

Effect of Point Defects on Electronic Properties and Structure of Talc (001) Surface by First Principles

The surface structure and electronic properties of Mg vacancy defects on talc (001) and impurity defects with Fe, Mn, Ni, Al, and Ca replacing Mg atoms were calculated by using density functional theory. The calculation results show that the order of impurity substitution energy is Mn < Ni < Al < Ca < Fe. This indicates that Fe impurity defects are most easily formed in talc crystals. The covalent bonding between Si atoms and reactive oxygen atoms adjacent to impurity atoms is weakened and the ionic property is enhanced. The addition of Fe, Mn, and Ni atoms makes the surface of talc change from an insulator to a semiconductor and enhances its electrical conductivity. The analysis of electron state density shows that surface states composed of impurity atoms 4S orbital appear near the Fermi level.

Performance Improvement in E-Gun Deposited SiOx- Based RRAM Device by Switching Material Thickness Reduction

A performance improvement by reduction in switching material thickness in a e-gun deposited SiOx based resistive switching memory device was investigated. Reduction in thickness cause thinner filamentary path formation during ON-state by controlling the vacancy defects. Thinner filament cause lowering of operation current from 500 μA to 100 μA and also improves the reset current (from >400 μA to <100 μA). Switching material thickness reduction also cause the forming free ability in the device. All these electrical parametric improvements enhance the device reliability performances. The device show >200 dc endurance, >3-hour data retention and >1000 P/E endurance with 100 ns pulses.

Water Gas Shift Reaction Activity on Fe (110): A DFT Study

Metal Fe is one of the phases existing on iron-based catalysts for a high-temperature water gas shift reaction (WGSR), but research on the activity of metal Fe in WGSR is almost not reported. In this work, the density functional theory (DFT) method was used to systematically study the reaction activity and mechanisms of WGSR on metal Fe (110), including the dissociation of H2O, the transformation of CO and the formation of H2, as well as the analysis of surface electronic properties. The results show that (1) the direct dissociation of H2O occurs easily on Fe (110) and the energy barrier is less than 0.9 eV; (2) the generation of CO2 is difficult and its energy barrier is above 1.8 eV; (3) H migrates easily on the Fe surface and the formation of H2 also occurs with an energy barrier of 1.47 eV. Combined with the results of Fe3O4, it can be concluded that the active phase should be Fe3O4 with O vacancy defects, and the iron-rich region plays an important role in promoting the formation of H2 in WGSR.

Gas-Phase Fluorination of g-C3N4 for Enhanced Photocatalytic Hydrogen Evolution

Graphitic carbon nitride (g-C3N4) has attracted much attention because of its potential for application in solar energy conservation. However, the photocatalytic activity of g-C3N4 is limited by the rapidly photogenerated carrier recombination and insufficient solar adsorption. Herein, fluorinated g-C3N4 (F-g-CN) nanosheets are synthesized through the reaction with F2/N2 mixed gas directly. The structural characterizations and theoretical calculations reveal that fluorination introduces N vacancy defects, structural distortion and covalent C-F bonds in the interstitial space simultaneously, which lead to mesopore formation, vacancy generation and electronic structure modification. Therefore, the photocatalytic activity of F-g-CN for H2 evolution under visible irradiation is 11.6 times higher than that of pristine g-C3N4 because of the enlarged specific area, enhanced light harvesting and accelerated photogenerated charge separation after fluorination. These results show that direct treatment with F2 gas is a feasible and promising strategy for modulating the texture and configuration of g-C3N4-based semiconductors to drastically enhance the photocatalytic H2 evolution process.

Passivation Effect of CsPbI3 Quantum Dots on the Performance and Stability of Perovskite Solar Cells

The quality of active layer film is the key factor affecting the performance of perovskite solar cells. In this work, we incorporated CsPbI3 quantum dots (QDs) materials into the MAPbI3 perovskite precursor to form photoactive layer. On one hand, CsPbI3 QDs can be used as nucleation center to enhance the compactness of the perovskite film, and on the other hand, partially CsPbI3 QDs can be dissociated as anions and cations to passivate vacancy defects in the perovskite active layer. As a result, the film quality of the active layer was improved remarkably, thus exciton recombination was reduced, and carrier transfer increased accordingly. The devices based on doped-CsPbI3 QDs film had higher short circuit current, open circuit voltage and filling factor. Finally, the power conversion efficiency (PCE) was greatly enhanced from 14.85% to 17.04%. Furthermore, optimized devices also exhibited better stability. This work provides an effective strategy for the processing of high-quality perovskite films, which is of great value for the preparation and research of perovskite photoelectronic devices.

The Fingerprints of Resonant Frequency for Atomic Vacancy Defect Identification in Graphene

The identification of atomic vacancy defects in graphene is an important and challenging issue, which involves inhomogeneous spatial randomness and requires high experimental conditions. In this paper, the fingerprints of resonant frequency for atomic vacancy defect identification are provided, based on the database of massive samples. Every possible atomic vacancy defect in the graphene lattice is considered and computed by the finite element model in sequence. Based on the sample database, the histograms of resonant frequency are provided to compare the probability density distributions and interval ranges. Furthermore, the implicit relationship between the locations of the atomic vacancy defects and the resonant frequencies of graphene is established. The fingerprint patterns are depicted by mapping the locations of atomic vacancy defects to the resonant frequency magnitudes. The geometrical characteristics of computed fingerprints are discussed to explore the feasibility of atomic vacancy defects identification. The work in this paper provides meaningful supplementary information for non-destructive defect detection and identification in nanomaterials.