Regulating the electronic properties of the WGe2N4 monolayer by adsorption of 4d transition metal atoms towards spintronic devices.

We study the regulation of the electronic and spin transport properties of the WGe2N4 monolayer by adsorbing 4d transition metal atoms (Y-Cd) using density functional theory combined with non-equilibrium Green's function. It is found that the adsorption of transition metal atoms (except Pd, Ag and Cd atoms) can introduce a magnetic moment into the WGe2N4 monolayer. Among the transition metal atoms, the adsorption of Nb and Rh atoms transforms WGe2N4 from a semiconductor to a half-metal and a highly spin-polarized semiconductor, respectively. The half-metallic Nb-adsorbed WGe2N4 system is selected to investigate the spin transport properties, and a high magnetoresistance ratio of 107% is achieved. In both parallel and antiparallel magnetization configurations, the spin filtering efficiency reaches close to 100% in the whole bias range, and the antiparallel magnetization configuration exhibits a dual spin filtering effect with a rectification ratio of up to 104. Our study predicts that the adsorption of 4d transition metal heteroatoms is an effective method to regulate the electronic and magnetic properties of WGe2N4 towards high-performance spintronic devices.

Schottky diodes based on blue phosphorene nanoribbon homojunctions.

Diodes have been widely studied as one of the most commonly used electronic components in circuits, and it is important to find diodes with an excellent rectification performance. Herein, we investigate the electronic and transport properties of Schottky contact diodes based on zigzag hydrogenated blue phosphorene nanoribbons, by employing density functional theory combined with the non-equilibrium Green's function. It is found that the adsorption of transition metal atoms Sc/Cr/Ti and Ni on the top site of blue phosphorene nanoribbons leads to metallic and semiconducting properties, respectively. Devices consisting of the planar contact of the metallic and semiconducting nanoribbons show rectifying behavior due to the Schottky barriers of the homojunctions. The current is preferential to flow from the semiconducting side to the metallic side. The rectification ratio of the Sc-Ni device and the Cr-Ni device can reach up to 108, which is much higher than that of traditional p-n junctions of about 105-107. The high rectification ratio at low bias regions, together with the low threshold voltages and negligible reverse currents, make blue phosphorene nanoribbon homojunctions ideal rectifier diodes.

An ultra-sensitive gas sensor based on a two-dimensional manganese porphyrin monolayer.

The development of highly sensitive, low-power consuming, stable and recyclable gas sensing devices at room temperature has become an important solution for environmental safety detection. Utilizing a two-dimensional metalloporphyrin monolayer for gas sensing is appealing due to its large specific surface area and high surface activity. A two-dimensional manganese porphyrin monolayer (2DMnPr) is selected from 2D metalloporphyrins with 3d metal centers due to its semi-metallicity to explore its gas sensing properties. Using first-principles calculations, we systematically investigate the electronic structures and adsorption characteristics of gas molecules with toxicity and greenhouse effect on the surface of 2DMnPr, including H2S, CO, CO2, SO2, NO and NO2. The strength of the interaction and charge transfer between the 2DMnPr surface and the adsorbed molecules have a direct effect on the electronic properties and the sensing properties of the adsorbent surface. The sensing performance of the 2DMnPr adsorbent is evaluated via two observable parameters: work function and electrical conductivity. The work functions of 2DMnPr after the adsorption of CO, SO2, NO and NO2 gas molecules increase by different degrees depending on the charge transfer, and those of the H2S and CO2 cases decrease. In our simulation, adsorption of CO, SO2, NO and NO2 gas molecules affects the electronic properties of 2DMnPr markedly, and current-voltage characteristics within a low bias range uncover the superior sensitivity of the conductivity of the 2DMnPr monolayer to these molecules. Besides, the sensing performance is demonstrated to be stable under strain and at room temperature. The desorption time of a gas is positively related to its adsorption energy. The recovery time of CO is predicted to be short enough to realize sustainable detection at room temperature, and the SO2, NO and NO2 gases can also be desorbed at higher temperatures. These results demonstrate that 2DMnPr enables the sensitive detection of these gases and predict the potential application of 2DMnPr as an ultra-sensitive, low-power, stable and recyclable gas sensor at room temperature.

Pervasive Ohmic contacts of monolayer 4-hT2 graphdiyne transistors.

Monolayer (ML) graphdiyne, a two-dimensional semiconductor with appropriate band gap and high carrier mobility, is a promising candidate for channel material in field effect transistors (FETs). Using density functional theory combined with non-equilibrium Green's function method, we systematically investigate the contact and transport properties of graphdiyne FETs with various electrodes, including metals (Cu, Au, Ni, Al and Ag) and MXenes (Cr2C, Ta2C and V2C). Strong interaction can be found between ML graphdiyne and the Cu, Ni and MXenes electrodes with indistinguishable band structure of ML graphdiyne, while weak or medium interaction exists in the contacts of ML graphdiyne and the Au, Al and Ag electrodes where the band structure of ML graphdiyne remains intact. Despite the different contact interactions, Ohmic contacts are generated with all considered electrode materials owing to the weak Fermi level pinning of graphdiyne. The linear I-V characteristic curve verifies the Ohmic contact between Au electrode and graphdiyne ultimately. The theoretically calculated Schottky barrier heights of graphdiyne with Cu electrode are consistent with the available experimental data. Our calculation suggests that graphdiyne is an excellent channel material of FETs forming desired Ohmic contacts with wide-ranging electrodes and thus is promising to fabricate high performance FETs.

Surface decoration of phosphorene nanoribbons with 4d transition metal atoms for spintronics.

The recent production of phosphorene nanoribbons provides a platform for designing phosphorene-based high-speed electronic devices. Introducing a magnetic moment to phosphorene nanoribbons for spintronics application is attractive. Based on density functional theory combined with the non-equilibrium Green's function method, the electronic, magnetic and spin-polarized transport properties of phosphorene nanoribbons modified by adsorption and substitutional doping of 4d transition metal atoms (Y, Zr, Nb and Mo) are investigated systematically. The results show that both the adsorption and the doping of 4d transition metal atoms can introduce a magnetic moment into phosphorene nanoribbons, except the Y- and Nb-doping cases. The adsorption shows superior performance in terms of modulating the electronic and magnetic properties of phosphorene nanoribbons compared to substitutional doping, exhibiting higher spin polarization near the Fermi level with a narrower band gap. This discrepancy originates from the different electronic redistribution in the adsorption and doping situations. Furthermore, the nanoribbons with adsorbed 4d transition metal atoms exhibit excellent spin-polarized transport properties: a giant magnetoresistance ratio of the Mo-adsorbed nanoribbon reaches over 108 under low bias; the Y-Mo-adsorbed nanoribbons with parallel spin configurations show a spin filtering effect of about 100% with the bias larger than 0.1 V, and those with antiparallel spin configurations exhibit a dual spin filtering effect in an applied bias range of (-0.2 V, 0.2 V). Our results demonstrate that 4d-transition-metal-atom adsorption is a favourable approach to modify the electronic, magnetic and transport properties of phosphorene nanoribbons, thus providing a reference for the rational design of spintronic devices based on phosphorene nanoribbons.

Anisotropic interfacial properties of monolayer C2N field effect transistors.

Monolayer C2N is promising for next-generation electronic and optoelectronic applications due to its appropriate band gap and high carrier efficiency. However, relative studies have been held back due to the lack of high-quality electrode contacts. Here, we comprehensively study the electronic and transport properties of monolayer C2N with a series of electrode materials (Al, Ti, Ni, Cu, Ag, Pt, V2C, Cr2C and graphene) by using the nonequilibrium Green's function (NEGF) method combined with density functional theory (DFT). The monolayer C2N forms Ohmic contacts with the Ti/Cu/Ag electrode material in both armchair and zigzag directions, whereas Ohmic contact is only formed in the zigzag direction of the C2N-Al field effect transistor. However, the C2N-Ni, -Pt, -V2C, -Mo2C, -graphene contact systems form n-type Schottky contacts in either the armchair or zigzag direction owing to the relatively strong Fermi level pinning (the pinning factor S = 0.32 in the armchair direction and S = 0.26 in the zigzag direction). By insertion of BN or graphene between the C2N and Pt electrode in the armchair direction of contact systems, the Fermi level pinning can be effectively weakened due to the suppression of metal-induced gap states. Conspicuously, an Ohmic contact is realized in the C2N field effect transistors with the BN-Pt electrode, suggesting a possible approach to fabricating high-performance devices. Our study is conducive to selecting appropriate electrode materials for C2N-based field effect transistors.

Surface engineering of phosphorene nanoribbons by transition metal heteroatoms for spintronics.

Modulating the electronic and magnetic properties of phosphorene is important for fabricating multi-functional electronic and spintronic devices. Employing density functional theory combined with the non-equilibrium Green's function, we systematically investigate the electronic, magnetic and transport properties of hydrogenated armchair phosphorene nanoribbons chemically modified by 3d transition metal atoms (Sc, Ti, V, Cr, Mn, Fe, Co and Ni). With chemical adsorption of transition metal atoms, the phosphorene nanoribbons exhibit excellent spin-polarized transport properties. A giant magnetoresistance effect is found with Ti, Fe and Mn adsorption, in which ratios higher than 102 for the Ti and Mn cases, and 105 for the Fe case, are exhibited. Moreover, in the bias range of (-0.2 V, 0.2 V), the Ti, V, Mn and Fe-adsorbed nanoribbons with parallel spin configurations demonstrate a remarkable bias-independent spin filtering efficiency at about 100%, while the Fe and Mn-adsorbed nanoribbons with antiparallel spin configuration show a dual spin filtering effect. The spin-polarized electronic transport properties are closely related to the band structures. Remarkable spin-polarization of the current occurs when the dispersed and flat bands near the Fermi level originate from different spin orientations. The magnetic moments of transition metal adatoms on nanoribbons are reduced by 0.2-2 µB relative to the isolated atoms due to electron rearrangement and charge transfer, which results in various degrees of spin polarization. These results provide a fundamental understanding of the electronic, magnetic and transport properties of transition metal modified hydrogenated armchair phosphorene nanoribbons, and suggest a referential approach to manufacture spintronic devices based on phosphorene.

Two-dimensional MoSi2As4-based field-effect transistors integrating switching and gas-sensing functions.

Multifunctional nanoscale devices integrating multiple functions are of great importance for meeting the requirements of next-generation electronics. Herein, using first-principles calculations, we propose multifunctional devices based on the two-dimensional monolayer MoSi2As4, where a single-gate field-effect transistor (FET) and FET-type gas sensor are integrated. After introducing the optimizing strategies, such as underlap structures and dielectrics with a high dielectric constant (κ), we designed a 5 nm gate-length MoSi2As4 FET, whose performance fulfilled the key criteria of the International Technology Roadmap for Semiconductors (ITRS) for high-performance semiconductors. Under the joint adjustment of the underlap structure and high-κ dielectric material, the on/off ratio of the 5 nm gate-length FET reached up to 1.38 × 104. In addition, driven by the high-performance FET, the MoSi2As4-based FET-type gas sensor showed a sensitivity of 38% for NH3 and 46% for NO2. Moreover, the weak interaction between NH3 (NO2) and MoSi2As4 favored the recycling of the sensor. Furthermore, the sensitivity of the sensor could be effectively improved by the gate voltage, and was increased up to 67% (74%) for NH3 (NO2). Our work provides theoretical guidance for the fabrication of multifunctional devices combining a high-performance FET and sensitive gas sensor.