arXiv subjects

Yubo Qi

Publications and source records attributed to Yubo Qi.

25 records · Page 2Linked to original sources

arXiv2019

Sr-induced dipole scatter in BST: Insights from MD simulations using a transferable bond valence-based interatomic potential

In order to design next-generation ferroelectrics, a microscopic understanding of their macroscopic properties is critical. One means to achieving an atomistic description of ferroelectric and dielectric phenomena is classical molecular dynamics simulations. Previously, we have shown that interatomic potentials based on the bond valence molecular dynamics (BVMD) method can be used to study structural phase transitions, ferroelectric domain nucleation, and domain wall migration in several perovskite oxides and fixed-composition binary and ternary alloys. Most modern devices, however, use variable-composition perovskite oxide alloys such as Ba$_x$Sr$_{1-x}$TiO$_3$ (BST). In this paper, we extend our bond valence approach to BST solid solutions and, in so doing, show that the potential parameters for each element are transferable between materials with different $x$. Using this potential, we perform BVMD simulations investigating the temperature and composition dependence of the lattice constants, Ti displacements, and ferroelectric polarization of BST and find that our predictions match experiments and first-principles theory. Additionally, based on a detailed analysis of local dipole distributions, we demonstrate that substitution of Sr for Ba scrambles dipoles, reduces global polarization, and enhances the order-disorder character of the ferroelectric-paraelectric phase transition.

cond-mat.mtrl-sci↗

arXiv2017

Tribopolymer Formation Mechanism on the RuO$_2$(110) Surface

Tribopolymer formed on the contacts of microelectromechanical and nanoelectromechanical system (MEMS/NEMS) devices is a major concern hampering their practical use in information technology. Conductive metal oxides, such as RuO$_2$ and ReO$_3$, have been regarded as promising candidate materials for MEMS/NEMS contacts due to their conductivity, hardness, and relatively chemically inert surfaces. However, recent experimental works demonstrate that trace amounts of polymer could still form on RuO$_2$ surfaces. In this study, we demonstrate the mechanism of this class of unexpected tribopolymer formation by conducting density functional theory based computational compression experiments with benzene as the contamination gas. First, mechanical force during compression changes the benzene molecules from slightly physisorbed to strongly chemisorbed. Further compression causes deformation and chemical linkage of the benzene molecules. Finally, the two contacts detach, with one having a complex organic molecule attached and the other with a more reactive surface. The complex organic molecule, which has an oxabicyclic segment, can be viewed as the rudiment of tribopolymer, and the more reactive surface can trigger the next adsorption--reaction--tribopolymer formation cycle. Based on these results, we also predict tribopolymer formation rates by using transition--state theory and the second--order rate law. This study deepens our understanding of tribopolymer formation (especially on metal oxides) and provides strategies for suppressing tribopolymerization.

cond-mat.mtrl-sci↗

arXiv2017

Ultra-fast electric field control of giant electrocaloric effect in ferroelectrics

There is a surge of interest in developing environmentally friendly cooling technology based on the solid--state electrocaloric effect (ECE). Here, we point out that negative ECE with a fast cooling rate ($\approx$10$^{11}$ K/s) can be achieved by driving solid crystals to a high--temperature phase with a properly designed electric field pulse. Specifically, we predict that an ultrafast electric field pulse can cause a negative ECE up to 35 K in PbTiO$_3$ occurring on few picosecond time scales. We acquire and analyze these results by clarifying the mechanism of ECE during an adiabatic irreversible process; In addition to the conventional explanation of the ECE with entropy reallocation, we simply portray the ECE with the concept of internal energy redistribution. Electric field does work on a ferroelectric crystal and redistributes its internal energy. How the kinetic energy is redistributed determines the temperature change and strongly depends on the electric field temporal profile. This concept is supported by our all--atom molecular dynamics simulations of PbTiO$_3$ and BaTiO$_3$. Based on this improved understanding of ECE, we propose strategies for inducing both giant and negative ECE. This work offers a more general framework to understand the ECE and highlights the opportunities of electric--field engineering for controlled design of fast and efficient cooling technology.

cond-mat.mtrl-sci↗

arXiv2016

Adsorption of Benzene on the RuO2(110) Surface

Hydrocarbon tribopolymer, a type of polymer formed due to friction between surfaces, is a major impediment to the development of micro- and nanoelectromechanical systems (MEMS/NEMS) devices for industrial application. Tribopolymer buildup can prevent MEMS and NEMS from making or breaking electrical contact. We describe the adsorption of benzene (C6H6) on the RuO2(110) surface using density functional theory. This adsorption is an important initial step in the mechanism of hydrocarbon tribopolymer layer formation on MEMS and NEMS devices. The adsorption interaction is studied by considering three oxygen coverages of RuO2(110) and all the possible adsorption sites for benzene. We find that adsorption of benzene on O-poor RuO2(110) via C-Ru bonds is stronger than adsorption on the O-rich RuO2(110) via H-O bonds. For an in-depth study of the adsorption behavior, we include the van der Waals interaction for a holistic investigation. By incorporating the thermodynamic chemical potentials into the adsorption simulations, we describe a model that can provide guidance for realistic situations.

cond-mat.mtrl-sci↗

arXiv2016

Atomistic Description for Temperature-Driven Phase Transitions in BaTiO$_3$

Barium titanate (BaTiO$_3$) is a prototypical ferroelectric perovskite that undergoes the rhombohedral-orthorhombic-tetragonal-cubic phase transitions as the temperature increases. In this work, we develop a classical interatomic potential for BaTiO$_3$ within the framework of the bond-valence theory. The force field is parameterized from first-principles results, enabling accurate large-scale molecular dynamics (MD) simulations at finite temperatures. Our model potential for BaTiO$_3$ reproduces the temperature-driven phase transitions in isobaric-isothermal ensemble (NPT) MD simulations. This potential allows the analysis of BaTiO$_3$ structures with atomic resolution. By analyzing the local displacements of Ti atoms, we demonstrate that the phase transitions of BaTiO$_3$ exhibit a mix of order-disorder and displacive characters. Besides, from detailed observation of structural dynamics during phase transition, we discover that the global phase transition is associated with changes in the equilibrium value and fluctuations of each polarization component, including the ones already averaging to zero, Contrary to the conventional understanding that temperature increase generally causes bond-softening transition, the x polarization component exhibits a bond-hardening character during the orthorhombic to tetragonal transition. These results provide further insights about the temperature-driven phase transitions in BaTiO$_3$.

cond-mat.mtrl-sci↗

arXiv2015

Theoretical Modeling of Tribochemical Reaction on Pt and Au Contacts: Mechanical Load and Catalysis

Micro-electro-mechanical system and nano-electro-mechanical system (MEMS and NEMS) transistors are considered promising for size-reducing and power-maximizing electronic devices. However, the tribopolymer which forms due to the mechanical load to the surface contacts affects the conductivity between the contacts dramatically. This is one of the challenging problems that prevent widespread practical use of these otherwise promising devices. Here, we use density functional theory (DFT) to investigate the mechanisms of tribopolymer formation, including normal mechanical loading, the catalytic effect, as well as the electrochemical effect of the metal contacts. We select benzene select as the background gas, because it is one of the most common and severe hydrocarbon contaminants. Two adsorption cases are considered: one is benzene on the reactive metal surface, Pt(111), and the other is benzene on the noble metal, Au(111). We demonstrate that the formation of tribopolymer is induced both by the mechanical load and by the catalytic effect of the contact. First, benzene molecules are adsorbed on the Pt surfaces. Then, due to the closure of the Pt contacts, stress is applied to the adsorbates, making the C-H bonds more fragile. As the stress increases further, H atoms are pressed close to the Pt substrate and begin to bond with Pt atoms. Thus Pt acts as a catalyst, accelerating the dehydrogenation process. When there is voltage applied across the contacts, the catalytic effect is enhanced by electrochemistry. Finally, due to the loss of H atoms, C atoms become more reactive and link together or pile up to form tribopolymer. By understanding these mechanisms, we provide guidance on design strategies for suppressing tribopolymer formation.

cond-mat.mtrl-sci↗

arXiv2015

Low Operation Voltage Ferroelectric Field-Effect Transistor Based on Polarization Rotation Effect

The effect of polarization rotation on the performance of metal oxide semiconductor field-effect transistors was investigated with a Landau-Ginzburg-Devonshire theory based model. In this analytical model, depolarization field, polarization rotations and the electrostatic properties of the doped silicon substrate are considered to illustrate the size effect of ferroelectric oxides and the stability of polarization in each direction. Based on this model, we demonstrate that MOSFET operation could be achieved by polarization reorientation with a low operating voltage, if the thickness of ferroelectric oxide is properly selected. Polarization reorientation can boost the surface potential of the silicon substrate, leading to a subthreshold swing S lower than 60 mV/decade. We believe that this model could provide guidance in designing electronic logic devices with low operating voltages and low active energy consumption.

cond-mat.mtrl-sci↗