Phosphorene, a new elemental two-dimensional material, has attracted increasing attention owing to its intriguing electronic properties. In particular, pristine phospohorene, due to its ultrahigh surface-volume ratio and high chemical activity, has been shown to be promising for gas sensing (Abbas et al 2015 ACS Nano 9 5618). To further enhance its sensing ability, we perform first-principles calculations based on density functional theory to study substitutionally doped phosphorene with 17 different atoms, focusing on structures, energetics, electronic properties and gas sensing. Our calculations reveal that anionic X (X = O, C and S) dopants have a large binding energy and highly dispersive electronic states, signifying the formation of covalent X-P bonds and thus strong structural stability. Alkali atom (Li and Na) doping is found to donate most of the electrons in the outer s-orbital by forming ionic bonds with P, and the band gap decreases by pushing down the conduction band, suggesting that the optical and electronic properties of the doped phosphorene can be tailored. For doping with VIIIB-group (Fe, Co and Ni) elements, a strong affinity is predicted and the binding energy and charge transfer are correlated strongly with their electronegativity. By examining NO molecule adsorption, we find that these metal doped phosphorenes (MDPs) in general exhibit a significantly enhanced chemical activity compared with pristine phosphorene. Our study suggests that substitutionally doped phosphorene shows many intriguing electronic and optic properties different from pristine phosphorene and MDPs are promising in chemical applications involving molecular adsorption and desorption processes, such as materials growth, catalysis, gas sensing and storage.

Electrochemical water splitting is economically unviable due to the sluggish kinetics of the anodically uphill oxygen evolution reaction (OER).

Resistance switching characteristics of CeO2/Ti/CeO2 tri-layered films sandwiched between Pt bottom electrode and two different top electrodes (Ti and TaN) with different work functions have been investigated. RRAM memory cells composed of TaN/CeO2/Ti/CeO2/Pt reveal better resistive switching performance instead of Ti/CeO2/Ti/CeO2/Pt memory stacks. As compared to the Ti/CeO2 interface, much better ability of TaN/CeO2 interface to store and exchange plays a key role in the RS performance improvement, including lower forming/SET voltages, large memory window (~102) and no significant data degradation during endurance test of >104 switching cycles. The formation of TaON thinner interfacial layer between TaN TE and CeO2 film is found to be accountable for improved resistance switching behavior. Partial charge density of states is analyzed using density functional theory. It is found that the conductive filaments formed in CeO2 based devices is assisted by interstitial Ti dopant. Better stability and reproducibility in cycle-to-cycle (C2C) resistance distribution and Vset/Vreset uniformity were achieved due to the modulation of current conduction mechanism from Ohmic in low field region to Schottky emission in high field region.

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In this study, the resistive switching and synaptic properties of a complementary metal-oxide semiconductorcompatible Ti/a-BN/Si device are investigated for neuromorphic systems. A gradual change in resistance is observed in a positive SET operation in which Ti diffusion is involved in the conducting path. This operation is extremely suitable for synaptic devices in hardware-based neuromorphic systems. The isosurface charge density plots and experimental results confirm that boron vacancies can help generate a conducting path, whereas the conducting path generated by a Ti cation from interdiffusion forms is limited. A negative SET operation causes a considerable decrease in the formation energy of only boron vacancies, thereby increasing the conductivity in the low-resistance state, which may be related to RESET failure and poor endurance. The pulse transient characteristics, potentiation and depression characteristics, and good retention property of eight multilevel cells also indicate that the positive SET operation is more suitable for a synaptic device owing to the gradual modulation of conductance. Moreover, pattern recognition accuracy is examined by considering the conductance values of the measured data in the Ti/a-BN/Si device as the synaptic part of a neural network. The linear and symmetric synaptic weight update in a positive SET operation with an incremental voltage pulse scheme ensures higher pattern recognition accuracy.

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