arXiv subjects

Sergei Manzhos

Publications and source records attributed to Sergei Manzhos.

49 records · Page 3Linked to original sources

arXiv2016

Grown-in beryllium diffusion in indium gallium arsenide: An ab initio, continuum theory and kinetic Monte Carlo study

A roadblock in utilizing InGaAs for scaled-down electronic devices is its anomalous dopant diffusion behavior; specifically, existing models are not able to explain available experimental data on beryllium diffusion consistently. In this paper, we propose a comprehensive model, taking self-interstitial migration and Be interaction with Ga and In into account. Density functional theory (DFT) calculations are first used to calculate the energy parameters and charge states of possible diffusion mechanisms. Based on the DFT results, continuum modeling and kinetic Monte Carlo simulations are then performed. The model is able to reproduce experimental Be concentration profiles. Our results suggest that the Frank-Turnbull mechanism is not likely, instead, kick-out reactions are the dominant mechanism. Due to a large reaction energy difference, the Ga interstitial and the In interstitial play different roles in the kick-out reactions, contrary to what is usually assumed. The DFT calculations also suggest that the influence of As on Be diffusion may not be negligible.

cond-mat.mtrl-sci↗

arXiv2016

Comparative Density Functional Theory and Density Functional Tight Binding Study of Arginine and Arginine-Rich Cell-Penetrating Peptide TAT Adsorption on Anatase TiO2

We present a comparative Density Functional Theory (DFT) and Density Functional Tight Binding (DFTB) study of geometries and electronic structures of arginine (Arg), arginine adsorbed on the anatase (101) surface of titania in several adsorption configurations, and of an arginine-rich cell penetrating peptide TAT and its adsorption on the surface of TiO2. Two DFTB parameterizations are considered, tiorg-0-1/mio-1-1 and matsci-0-3. While there is good agreement in the structures and relative energies of Arg and peptide conformers between DFT and DFTB, both adsorption geometries and energies are noticeably different for Arg adsorbed on TiO2. The tiorg-0-1/mio-1-1 parameterization performs better than matsci-0-3. We relate this difference to the difference in electronic structures resulting from the two methods (DFT and DFTB) and specifically to the band alignment between the molecule and the oxide. We show that the band alignment of TAT and of TiO2 modeled with DFTB is qualitatively correct but that with DFT using the PBE functional is not. This is specific to the modeling of large molecules where the HOMO is close to the conduction band of the oxide. We therefore report a case where the approximate DFT-based method - DFTB (with which the correct band structure can be effectively obtained) - performs better than the DFT itself with a functional approximation feasible for the modeling of large bio-inorganic interfaces, i.e. GGA (as opposed to hybrid functionals which are impractical at such a scale). Our results highlight the utility of the DFTB method for the modeling of bioinorganic interfaces not only from the CPU cost perspective but also from the accuracy point of view.

cond-mat.mtrl-sci↗

arXiv2016

Understanding the difference in cohesive energies between alpha and beta tin in DFT calculations

The transition temperature between the low-temperature alpha phase of tin to beta tin is close to the room temperature (Tab =13C), and the difference in cohesive energy of the two phases at 0 K of about dEcoh=0.02 eV/atom is at the limit of the accuracy of DFT (density functional theory) with available exchange-correlation functionals. It is however critically important to model the relative phase energies correctly for any reasonable description of phenomena and technologies involving these phases, for example, the performance of tin electrodes in electrochemical batteries. Here, we show that several commonly used and converged DFT setups using the most practical and widely used PBE functional result in dEcoh of about 0.04 eV/atom, with different types of basis sets and with different models of core electrons (all-electron or pseudopotentials of different types), which leads to a significant overestimation of Tab. We show that this is due to the errors in relative positions of s and p -like bands, which, combined with different populations of these bands in alpha and beta Sn, leads to overstabilization of alpha tin. We show that this error can be effectively corrected by applying a Hubbard +U correction to s -like states, whereby correct cohesive energies of both alpha and beta Sn can be obtained with the same computational scheme. We quantify for the first time the effects of anharmonicity on dEcoh and find that it is negligible.

cond-mat.mtrl-sci↗

arXiv2015

Comparative computational study of lithium and sodium insertion in van der Waals and covalent tetracyanoethylene (TCNE) -based crystals as promising materials for organic lithium and sodium ion batteries

We present a comparative ab initio study of Li and Na insertion in molecular (van der Waals) crystals of TCNE (tetracyanoethylene) as well as in covalent Li/Na-TCNE crystals. We confirm the structure of previously synthesized (covalent) Li-TCNE crystal as well as predict the existence of its Na-TCNE analogue. In the molecular/covalent TCNE crystals, insertion sites are identified with the binding energy of Li and Na up to 2.7/1.8 and 2.6/1.8 eV stronger than Li and Na cohesive energy, respectively, in dilute concentrations. Up to 5.5/2.5 and 3/2 Li and Na atoms per TCNE unit can be inserted in the molecular/covalent crystals, respectively, while preserving the structure, with maximum voltages, respectively, 3.5/2.2 and 3.3/2.7 V. Significantly, up to capacity of 418 mAh g-1 for both Li and Na in the molecular crystal and 198 mAh g-1 for Li and 177 mAh g-1 for Na in the covalent crystal, the insertion of Li and Na would not lead to reactions with common electrolytes. Tetracyanoethylene-based molecular and covalent crystals could therefore become efficient organic cathode and anode materials for Li and Na ion batteries.

cond-mat.mtrl-sci↗

arXiv2015

Comparison of alpha- and beta- tin for lithium, sodium, and magnesium storage: an ab initio study including phonon contributions

We present a comparative ab initio study of Li, Na, and Mg storage in tin, including phononic effects and phase competition between alpha and beta Sn. Mg doping at low concentration is found to stabilize the beta phase. On the contrary, Li and Na doping is shown to reverse the stability of the phases at room temperature: Li/Na-doped alpha-Sn is more stable than Li/Na-doped beta-Sn up to a temperature of around 380/400 K. This may rationalize the formation of alpha-Sn upon lithiation and delithiation of beta-Sn anodes reported in experimental studies. The changes in phase stability with Li/Na/Mg doping are directly related to the intercalation energies of Li/Na/Mg in one phase versus the other: at 300 K, Li/Na is easier intercalated in alpha-Sn (-0.37/-0.08 eV) than in beta-Sn (0.06/0.49 eV), while Mg intercalation energy is, although positive (i.e. unfavored intercalation), lower in beta-Sn (0.53 eV) than in alpha-Sn (0.66 eV). The temperature effect is found to affect significantly the intercalation energy, by up to 0.13 eV at 300 K. Analysis of diffusion barriers shows that Li, Na, and Mg diffusion in beta-Sn is anisotropic with migration barriers along the (001) direction (respectively 0.01, 0.22, and 0.07 eV) significantly lower than those in alpha-Sn (respectively 0.20, 0.52, and 0.40 eV).

cond-mat.mtrl-sci↗

arXiv2015

Lithium and sodium storage on tetracyanoethylene (TCNE) and TCNE-(doped)-graphene complexes: a computational study

Li and Na attachment to free tetracyanoethylene (TCNE) molecules and TCNE adsorbed on doped graphene is studied using density functional theory. While TCNE is adsorbed only weakly on ideal graphene, we identified a configuration in which TCNE is chemisorbed on Al-doped graphene via molecule C atom and a surface oxygen atom. Up to four (five) Li and Na atoms can be stored on both free (adsorbed) TCNE with binding energies stronger than cohesive energies of the Li and Na metals. When storing up to three atoms per molecule, it should be possible to avoid reduction of common battery electrolytes. TCNE immobilized on a conducting graphene-based substrate could therefore become an efficient anode material for organic Li and Na ion batteries. Importantly, there is no significant difference either in specific capacity (per unit mass of material excluding Li/Na) nor in voltage between Li and Na storage, which makes this kind of approach very promising for post-Li storage in general.

cond-mat.mtrl-sci↗

arXiv2015

Amorphous carbon a promising material for sodium ion battery anodes: a first principles study

We present a comparative ab initio computational study of sodium and lithium storage in amorphous (glassy) carbon (a-C) vs. graphite. Amorphous structures are obtained by fitting stochastically generated structures to a reference radial distribution function. Li insertion is thermodynamically favored in both graphite and a-C. While sodium insertion is thermodynamically unfavored in graphite, a-C possesses multiple insertion sites with binding energies stronger than Na cohesive energy, making it usable as anode material for Na-ion batteries.

cond-mat.mtrl-sci↗

arXiv2014

Highly Accurate Local Pseudopotentials of Li, Na, and Mg for Orbital Free Density Functional Theory

We present a method to make highly accurate pseudopotentials for use with orbital-free density functional theory (OF-DFT) with given exchange-correlation and kinetic energy functionals, which avoids the compounding of errors of Kohn-Sham DFT and OF-DFT. The pseudopotentials are fitted to reference (experimental or highly accurate quantum chemistry) values of interaction energies, geometries, and mechanical properties, using a genetic algorithm. This can enable routine large-scale ab initio simulations of many practically relevant materials. Pseudopotentials for Li, Na, and Mg resulting in accurate geometries and energies of different phases as well as of vacancy formation and bulk moduli are presented as examples.

cond-mat.mtrl-sci↗

arXiv2014

Aluminum doping improves the energetics of lithium, sodium, and magnesium storage in silicon

While Si is an effective insertion type anode for Li-ion batteries, crystalline Si has been shown to be unsuitable for Na and Mg storage due, in particular, to insufficient binding strength. It has recently been reported that Si nanowires could be synthesized with high-concentration (several atomic %) and dispersed Al doping. Here we show based on density functional theory calculations that Al doping significantly improves the energetics for Na and Mg insertion, specifically, making it thermodynamically favored versus vacuum reference states. For high Al concentrations, the energy of Mg in Al-doped Si approaches the cohesive energy of Mg. However, the migration barriers for the diffusion of Li (0.57-0.70 eV), Na (1.07-1.19 eV) and Mg (0.97-1.18 eV) in Al-doped Si are found to remain about as high as in pure Si, likely preventing effective electrochemical sodiation and magnesiation.

cond-mat.mtrl-sci↗

arXiv2014

Comparative Computational Study of the Energetics of Li, Na, and Mg Storage in Amorphous and Crystalline Silicon

To assess the potential of amorphous Si (a-Si) as an anode for Li, Na, and Mg-ion batteries, the energetics of Li, Na, and Mg atoms in a-Si are computed from first-principles and compared to those in crystalline Si (c-Si). It is shown that Si preamorphization increases the average anode voltage and reduces the volume expansion of the anode during the insertion of the metal atoms. Analysis of computed formation energies of Li, Na, and Mg defects in a-Si and c-Si suggests that the energetics of the single atoms into a-Si are thermodynamically more favorable. For instance, defect formation energies of Li, Na, and Mg defects in a-Si are respectively 0.71, 1.72, and 1.82 eV lower compared to those in c-Si. Moreover, the defect formation energies of Li, Na, and Mg defects (vs. vacuum reference states) in a-Si are comparable with the metal cohesive energies and consequently the insertion of the metal atoms might be possible with appropriate control of charging process. This is in contrast to c-Si, where the storage of Na and Mg atoms is limited due to high energy cost of Na and Mg insertion into c-Si.

cond-mat.mtrl-sci↗

arXiv2013

In search of high performance anode materials for Mg batteries: computational studies of Mg in Ge, Si, and Sn

We present ab initio studies of structures, energetics, and diffusion properties of Mg in Si, Ge, and Sn diamond structures to evaluate their potential as insertion type anode materials for Mg batteries. We show that Si could provide the highest specific capacities (3817 mAh g-1) and the lowest average insertion voltage (~0.15 eV vs. Mg) for Mg storage. Nevertheless, due to its significant percent lattice expansion (~216%) and slow Mg diffusion, Sn and Ge are more attractive; both anodes have lower lattice expansions (~120 % and ~178 %, respectively) and diffusion barriers (~0.50 and ~0.70 eV, respectively for single-Mg diffusion) than Si. We show that Mg-Mg interactions at different stages of charging can decrease significantly the diffusion barrier compared to the single atom diffusion, by up to 0.55 eV.

cond-mat.mtrl-sci↗

arXiv2013

Effects of Nuclear Vibrations on the Energetics of Polythiophene: Quantized Energy Molecular Dynamics

Effects of nuclear dynamics on the energetics of polythiophene relevant for the performance of organic solar cells are studied for the first time. Nuclear motions change the expectation values of frontier orbital energies and the band gap by about 0.1 eV vs. values at the equilibrium geometry, which is expected to have a significant effect on light absorption, charge separation, and donor regeneration. A new molecular dynamics (MD) algorithm, which accounts for the quantum nature of vibrations, is introduced. It reproduces effects of temperature and deuteration which are lost in the standard MD. Inclusion of quantized vibrations leads to a broadening of the band gap and of energy levels by about 20% at 300K, while having little effect on their expectation values (which change by up to 0.03 eV). Increase in temperature from 300K to 400K and deuteration cause an additional broadening of the spectrum by about 26% and 21%, respectively.

physics.chem-ph↗

arXiv2012

Roles of discount rate, risk premium, and device performance in estimating the cost of energy for photovoltaics

We show that different rates should be used for borrowing and discount rates, and that the risk-free rate should be used for discounting when assessing and comparing the cost of energy accross diffferent producers and technologies, on the example of photovoltaics. Recent quantitative models using the same rate for borrowing and discounting lead to an underestimation of the cost for risky borrowers and to distorted sensitivities of the cost to financial and non-financial factors. Specifically, it is shown that they may lead to gross underestimation of the importance of solar-to-electricity conversion efficiency. The importance of device efficiency is re-established under the treatment of the discount rate proposed here. The effects on the cost of energy of the installation efficiency and degradation rate, on the discount rate and risk premium as well as on the project lifetime are estimated.

q-fin.GN↗