Guigui Xu

Effects of surface alloying and orientation on work function of MoTa metal gate

Work functions of sub- and monolayers Mo on metal Ta of various orientations are investigated using first-principles methods based on density functional theory. The calculated results reveal that the work function has strongly orientation dependence. However, for a given surface orientation, the work function is insensitive to distributions of Ta/Mo atoms in the surface layer. Moreover, it is found that work functions of the (100) and (111) surfaces increase with increasing Mo composition in the surface layer but those of the (110) decrease. By analysis of surface dipole density, it is found that the metal work function is mainly determined by surface orientation and surface charge redistribution.

Effects of strain on effective work function for Ni/HfO2 interfaces

The effective work functions for Ni/HfO2 interfaces under two strain modes (uniaxial and triaxial strains) were studied by using first-principles methods based on density functional theory. The calculated results indicate that the effective work functions are strongly affected by the type of interface and the strain states (tensile and compressive strains). For the both above strain states, the changed value of the effective work functions linearly increases with increasing strain. Moreover, it is observed that for a certain strain, the variation of the effective work function for triaxial strain state is almost larger than that for uniaxial strain state. Finally, the electrons gas model, the interface dipole, and screening role of HfO2 were used to analyze and explain the strain and interface effects in metal-oxide interfaces. The evident difference between the effective work functions of Ni-Hf and Ni-O interfaces is found to be attributed to different metallic bondings and ionic bondings via the analysi...

First-principles investigation of the electronic and Li-ion diffusion properties of LiFePO4 by sulfur surface modification

We present a first-principles calculation for the electronic and Li-ion diffusion properties of the LiFePO4 (010) surface modified by sulfur. The calculated formation energy indicates that the sulfur adsorption on the (010) surface of the LiFePO4 is energetically favored. Sulfur is found to form Fe-S bond with iron. A much narrower band gap (0.67 eV) of the sulfur surface-modified LiFePO4 [S-LiFePO4 (010)] is obtained, indicating the better electronic conductive properties. By the nudged elastic band method, our calculations show that the activation energy of Li ions diffusion along the one-dimensional channel on the surface can be effectively reduced by sulfur surface modification. In addition, the surface diffusion coefficient of S-LiFePO4 (010) is estimated to be about 10−11 (cm2/s) at room temperature, which implies that sulfur modification will give rise to a higher Li ion carrier mobility and enhanced electrochemical performance.

A novel spin modulation of work function for C adsorbed Cr/Fe(001) metal gate

Work functions and magnetic moments of C adsorbed Cr/Fe(001) surfaces with different C coverages θ and magnetic alignments (parallel or antiparallel) between Cr and Fe atom moments are investigated using first-principles methods based on density functional theory. The calculated results reveal that the spin configuration plays a significant role in determining the work function of the systems. The work functions of the systems with parallel states are evidently larger than those with antiparallel states. Moreover, for θ≤0.5 ML, with increasing value of θ, the work function increases from 4.23 eV to 5.13 eV for antiparallel states and from 4.47 eV to 5.44 eV for parallel states. While for θ>0.5 ML, the work function decreases with increasing value of θ. It can be also found that, for θ≤0.5 ML, the smaller the Cr and Fe magnetic moments are, the lower the Fermi energy EF is and the larger the work functions of the systems are. Based on analysis and discussion, we conclude that the changes of the work functi...

An Ab Initio and Kinetic Monte Carlo Simulation Study of Lithium Ion Diffusion on Graphene

The Li+ diffusion coefficients in Li+-adsorbed graphene systems were determined by combining first-principle calculations based on density functional theory with Kinetic Monte Carlo simulations. The calculated results indicate that the interactions between Li ions have a very important influence on lithium diffusion. Based on energy barriers directly obtained from first-principle calculations for single-Li+ and two-Li+ adsorbed systems, a new equation predicting energy barriers with more than two Li ions was deduced. Furthermore, it is found that the temperature dependence of Li+ diffusion coefficients fits well to the Arrhenius equation, rather than meeting the equation from electrochemical impedance spectroscopy applied to estimate experimental diffusion coefficients. Moreover, the calculated results also reveal that Li+ concentration dependence of diffusion coefficients roughly fits to the equation from electrochemical impedance spectroscopy in a low concentration region; however, it seriously deviates from the equation in a high concentration region. So, the equation from electrochemical impedance spectroscopy technique could not be simply used to estimate the Li+ diffusion coefficient for all Li+-adsorbed graphene systems with various Li+ concentrations. Our work suggests that interactions between Li ions, and among Li ion and host atoms will influence the Li+ diffusion, which determines that the Li+ intercalation dependence of Li+ diffusion coefficient should be changed and complex.

Spin and surface orientation effects of work function for Cr/Ni(111), Cr/Ni(100) and Cr/Ni(110) metal gates

Work functions of Cr/Ni(111), Cr/Ni(100) and Cr/Ni(110) surfaces with different magnetic configurations for Cr atoms in the topmost Cr monolayer are investigated using first-principles methods based on density functional theory. The calculated results reveal that work functions vary with crystal orientations and magnetic configurations. The magnitude of the Cr magnetic moments for the three (111), (100) and (110) surfaces follows a change trend with MCr, Cr/Ni(111) < MCr, Cr/Ni(100) < MCr, Cr/Ni(110). Altering the magnetic configurations of the systems from an original ground state to an excited one will have the total energy and the Fermi level increase. Consequently, it will give rise to the reduction of the work function for the system. Moreover, the quite favorable variation range (4.92–4.41 eV) of the calculated work functions for Cr/Ni(100) system modulated by spin effect implies that the Cr/Ni(100) system may be a more promising candidate. Our work suggests that changing magnetic configurations can modulate the work functions of magnetic metal gates well.

Electronic structure and its external electric field modulation of PbPdO 2 ultrathin slabs with (002) and (211) preferred orientations

The Electronic structure of PbPdO2 with (002) and (211) preferred orientations were investigated using first-principles calculation. The calculated results indicate that, (002) and (211) orientations exhibit different electric field dependence of band-gap and carrier concentration. The small band gap and more sensitive electric field modulation of band gap were found in (002) orientation. Moreover, the electric field modulation of the resistivity up to 3–4 orders of magnitude is also observed in (002) slab, which reveals that origin of colossal electroresistance. Lastly, electric field modulation of band gap is well explained. This work should be significant for repeating the colossal electroresistance.

Engineering Topological Surface State of Cr-doped Bi 2 Se 3 under external electric field

External electric field control of topological surface states (SSs) is significant for the next generation of condensed matter research and topological quantum devices. Here, we present a first-principles study of the SSs in the magnetic topological insulator (MTI) Cr-doped Bi2Se3 under external electric field. The charge transfer, electric potential, band structure and magnetism of the pure and Cr doped Bi2Se3 film have been investigated. It is found that the competition between charge transfer and spin-orbit coupling (SOC) will lead to an electrically tunable band gap in Bi2Se3 film under external electric field. As Cr atom doped, the charge transfer of Bi2Se3 film under external electric field obviously decreases. Remarkably, the band gap of Cr doped Bi2Se3 film can be greatly engineered by the external electric field due to its special band structure. Furthermore, magnetic coupling of Cr-doped Bi2Se3 could be even mediated via the control of electric field. It is demonstrated that external electric field plays an important role on the electronic and magnetic properties of Cr-doped Bi2Se3 film. Our results may promote the development of electronic and spintronic applications of magnetic topological insulator.

Work function change of Ni, HfO2 films and Ni/HfO2 interfaces as a function of external electric field

In this paper, the effects of external electric field on the work functions for Ni(001), Ni(111), HfO2(001) and HfO2(111) films and the effective work functions for Ni(001)/HfO2(001) and Ni(111)/HfO2(111) interfaces were studied by first-principles methods based on density functional theory (DFT). It is found that the work functions for all the systems change linearly with the strength of external electric field. Comparing the slopes of the work function variation versus external electric field strength for Ni, HfO2 films and Ni/HfO2 interfaces, we have found that the response of the effective work function to external electric field for Ni/HfO2 interfaces is determined by the response of the HfO2 side to external electric field.

Effects of graphene intercalation on dielectric reliability of HfO 2 and modulation of effective work function for Ni/Gr/c-HfO 2 interfaces: first-principles study

We have investigated the effects of graphene intercalation on dielectric reliability of HfO2 for Ni/Gr/HfO2 interfaces, and the effects of graphene intercalation and interfacial atom vacancy on the effective work function (EWF) of Ni/Gr/HfO2 interfaces using first-principle calculation based on density functional theory. The calculated results indicate that graphene intercalation can improve dielectric reliability of HfO2 dielectric even for the interfaces having interfacial oxygen vacancy or a small amount carbon vacancy. Moreover, the calculated results indicate that, inserting graphene into Ni/HfO2 interface induces the EWF’s to decline, and controlling interfacial oxygen or carbon vacancy can effectively tune the EWF of Ni/Gr/HfO2 interface. Our work strongly suggests that the use of graphene synthesized into Ni/HfO2 interface is a very effective way to improve the interface quality, and controlling interfacial oxygen or carbon vacancy is also an attractive and promising way for modulating the EWF of Ni/Gr/HfO2 interfaces.