arXiv subjects

Jaakko Akola

Publications and source records attributed to Jaakko Akola.

12 recordsLinked to original sources

arXiv2025

Local doping of an oxide semiconductor by voltage-driven splitting of anti-Frenkel defects

Layered oxides exhibit high ionic mobility and chemical flexibility, attracting interest as cathode materials for lithium-ion batteries and the pairing of hydrogen production and carbon capture. Recently, layered oxides emerged as highly tunable semiconductors. For example, by introducing anti-Frenkel defects, the electronic hopping conductance in hexagonal manganites was increased locally by orders of magnitude. Here, we demonstrate local acceptor and donor doping in Er(Mn,Ti)O$_3$, facilitated by the splitting of such anti-Frenkel defects under applied d.c. voltage. By combining density functional theory calculations, scanning probe microscopy, atom probe tomography, and scanning transmission electron microscopy, we show that the oxygen defects readily move through the layered crystal structure, leading to nano-sized interstitial-rich (p-type) and vacancy-rich (n-type) regions. The resulting pattern is comparable to dipolar npn-junctions and stable on the timescale of days. Our findings reveal the possibility of temporarily functionalizing oxide semiconductors at the nanoscale, giving additional opportunities for the field of oxide electronics and the development of transient electronics in general.

cond-mat.mtrl-sci↗

arXiv2024

Doping-Induced Enhancement of Hydrogen Evolution at MoS2 Electrodes

Rate theory and DFT calculations of hydrogen evolution reaction (HER) on MoS2 with Co, Ni and Pt impurities show the significance of dihydrogen (H2*) complex where both hydrogen atoms are interacting with the surface. Stabilization of such a complex affects the competing Volmer-Heyrovsky (direct H2 release) and Volmer-Tafel (H2* intermediate) pathways. The resulting evolution proceeds with a very small overpotential for all dopants ($η$ = 0.1 to 0.2 V) at 25% edge substitution, significantly reduced from the already low $η$ = 0.27 V for the undoped edge. At full edge substitution, Co-MoS2 remains highly active ($η$ = 0.18 V) while Ni- and Pt-MoS2 are deactivated ($η$ = 0.4 to 0.5 V) due to unfavorable interaction with H2*. Instead of the single S-vacancy, the site of intrinsic activity in the basal plane was found to be the undercoordinated central Mo-atom in threefold S-vacancy configurations, enabling hydrogen evolution with $η$ = 0.52 V via a H2* intermediate. The impurity atoms interact favorably with the intrinsic sulfur vacancies on the basal plane, stabilizing but simultaneously deactivating the triple vacancy configuration. The calculated shifts in overpotential are consistent with reported measurements, and the dependence on doping level may explain variations in experimental observations.

cond-mat.mtrl-sci↗

arXiv2023

Is the doped MoS2 basal plane an efficient hydrogen evolution catalyst? Calculations of voltage-dependent activation energy

Transition metal dichalcogenides are cheap and earth-abundant candidates for the replacement of precious metals as catalyst materials. Experimental measurements of the hydrogen evolution reaction (HER), for example, have demonstrated significant electrocatalytic activity of MoS2 but there is large variation depending on preparation method. In order to gain information about the mechanism and active sites for HER, we have carried out calculations of the reaction and activation energy for HER at the transition metal doped basal plane of MoS2 under electrochemical conditions, i.e. including applied electrode potential and solvent effects. The calculations are based on identifying the relevant saddle points on the energy surface obtained from density functional theory within the generalized gradient approximation, and the information on energetics is used to construct voltage-dependent volcano plots. Doping with 3d-metal atoms as well as Pt is found to enhance hydrogen adsorption onto the basal plane by introducing electronic states within the band gap, and in some cases (Co, Ni, Cu, Pt) significant local symmetry breaking. The Volmer-Heyrovsky mechanism is found to be most likely and the associated energetics show considerable dopant and voltage-dependence. While the binding free energy of hydrogen can be tuned to be seemingly favorable for HER, the calculated activation energy turns out to be significant, at least 0.7 eV at a voltage of -0.5 V vs. SHE, indicating low catalytic activity of the doped basal plane. This suggests that other sites are responsible for the experimental activity, possibly edges or basal plane defects.

cond-mat.mtrl-sci↗

arXiv2022

Machine-learned model Hamiltonian and strength of spin-orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb)

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds $\rm Mg_2Si, Mg_2Ge, Mg_2Sn$, and $\rm Mg_2Pb$ subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory (DFT) by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding model reproduces the key aspects of the valence band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the $3s$ orbital of $\rm Mg$ to the conduction bands and the outer shell $p$ orbitals of $\rm X~(X=Si,Ge,Sn,Pb)$ to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the $3s$ orbital of $\rm Mg$ and the outer shell $p$ orbitals of $\rm X$ to the conduction bands and valence bands, respectively. The $π$ bonding in the $\rm Mg_2X$ compounds is negligible compared to the $σ$ bonding components, which follow the hierarchy $|σ_{sp}|>|σ_{pp}|>|σ_{ss}|$, and the largest variation against strain belongs to $σ_{pp}$. The five-band model allows for estimating the strength of spin-orbit coupling (SOC) in $\rm Mg_2X$ and obtaining its dependence on the atomic number of $\rm X$ and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of $-10\%$ can open a band gap at the $Γ$ point in metallic $\rm Mg_2Pb$, whereas a tensile strain of $+10\%$ closes the semiconducting band gap of $\rm Mg_2Si$. A tensile strain of $+5\%$ removes the three-fold degeneracy of valence bands at the $Γ$ point in semiconducting $\rm Mg_2Ge$.

cond-mat.mtrl-sci↗

arXiv2021

Comparison of Optical Response from DFT Random Phase Approximation and Low-Energy Effective Model: Strained Phosphorene

The engineering of the optical response of materials is a paradigm that demands microscopic-level accuracy and reliable predictive theoretical tools. Here we compare and contrast the dispersive permittivity tensor, using both a low-energy effective model and density functional theory (DFT). As a representative material, phosphorene subject to strain is considered. Employing a low-energy model Hamiltonian with a Green's function current-current correlation function, we compute the dynamical optical conductivity and its associated permittivity tensor. For the DFT approach, first-principles calculations make use of the first-order random-phase approximation. Our results reveal that although the two models are generally in agreement within the low-strain and low-frequency regime, the intricate features associated with the fundamental physical properties of the system and optoelectronic device implementation such as band gap, Drude absorption response, vanishing real part, absorptivity, and sign of permittivity over the frequency range show significant discrepancies. Our results suggest that the random-phase approximation employed in widely used DFT packages should be revisited and improved to be able to predict these fundamental electronic characteristics of a given material with confidence. Furthermore, employing the permittivity results from both models, we uncover the pivotal role that phosphorene can play in optoelectronics devices to facilitate highly programable perfect absorption of electromagnetic waves by manipulating the chemical potential and exerting strain and illustrate how reliable predictions for the dielectric response of a given material are crucial to precise device design.

cond-mat.mtrl-sci↗

arXiv2021

Density functional simulations of pressurized Mg-Zn and Al-Zn alloys

The Mg-Zn and Al-Zn binary alloys have been investigated theoretically under static isotropic pressure. The stable phases of these binaries on both initially hexagonal-close-packed (HCP) and face-centered-cubic (FCC) lattices have been determined by utilizing an iterative approach that uses a configurational cluster expansion method, Monte Carlo search algorithm, and density functional theory (DFT) calculations. Based on 64-atom models, it is shown that the most stable phases of the Mg-Zn binary alloy under ambient condition are $\rm MgZn_3$, $\rm Mg_{19}Zn_{45}$, $\rm MgZn$, and $\rm Mg_{34}Zn_{30}$ for the HCP, and $\rm MgZn_3$ and $\rm MgZn$ for the FCC lattice, whereas the Al-Zn binary is energetically unfavorable throughout the entire composition range for both the HCP and FCC lattices under all conditions. By applying an isotropic pressure in the HCP lattice, $\rm Mg_{19}Zn_{45}$ turns into an unstable phase at P$\approx$$10$~GPa, a new stable phase $\rm Mg_{3}Zn$ appears at P$\gtrsim$$20$~GPa, and $\rm Mg_{34}Zn_{30}$ becomes unstable for P$\gtrsim$$30$~GPa. For FCC lattice, the $\rm Mg_{3}Zn$ phase weakly touches the convex hull at P$\gtrsim$$20$~GPa while the other stable phases remain intact up to $\approx$$120$~GPa. Furthermore, making use of the obtained DFT results, bulk modulus has been computed for several compositions up to pressure values of the order of $\approx$$120$~GPa. The findings suggest that one can switch between $\rm Mg$-rich and $\rm Zn$-rich early-stage clusters simply by applying external pressure. $\rm Zn$-rich alloys and precipitates are more favorable in terms of stiffness and stability against external deformation.

cond-mat.mtrl-sci↗

arXiv2020

Strain-Engineered Widely-Tunable Perfect Absorption Angle in Black Phosphorus from First-Principles

Using the density functional theory of electronic structure, we compute the anisotropic dielectric response of bulk black phosphorus subject to strain. Employing the obtained permittivity tensor, we solve Maxwell's equations and study the electromagnetic response of a layered structure comprising a film of black phosphorus stacked on a metallic substrate. Our results reveal that a small compressive or tensile strain, $\sim 4\%$, exerted either perpendicular or in the plane to the black phosphorus growth direction, efficiently controls the epsilon-near-zero response, and allows a perfect absorption tuning from low-angle of the incident beam $θ=0^\circ$ to high values $θ\approx 90^\circ$ while switching the energy flow direction. Incorporating a spatially inhomogeneous strain model, we also find that for certain thicknesses of the black phosphorus, near-perfect absorption can be achieved through controlled variations of the in-plane strain. These findings can serve as guidelines for designing largely tunable perfect electromagnetic wave absorber devices.

cond-mat.mtrl-sci↗

arXiv2020

Josephson effect in graphene bilayers with adjustable relative displacement

The Josephson current is investigated in a superconducting graphene bilayer where pristine graphene sheets can make in-plane or out-of-plane displacements with respect to each other. The superconductivity can be of an intrinsic nature, or due to a proximity effect. The results demonstrate that the supercurrent responds qualitatively differently to relative displacement if the superconductivity is due to either intralayer or interlayer spin-singlet electron-electron pairing, thus providing a tool to distinguish between the two mechanisms. Specifically, both the AA and AB stacking orders are studied with antiferromagnetic spin alignment. For the AA stacking order with intralayer and on-site pairing no current reversal is found. In contrast, the supercurrent may switch its direction as a function of the in-plane displacement and out-of-plane interlayer coupling for the cases of AA ordering with interlayer pairing and AB ordering with either intralayer or interlayer pairing. In addition to sign reversal, the Josephson signal displays many characteristic fingerprints which derive directly from the pairing mechanism. Thus, measurements of the Josephson current as a function of the graphene bilayer displacement open up the means achieve deeper insights into the superconducting pairing mechanism.

cond-mat.mes-hall↗

arXiv2019

CLEASE: A versatile and user-friendly implementation of Cluster Expansion method

Materials exhibiting a substitutional disorder such as multicomponent alloys and mixed metal oxides/oxyfluorides are of great importance in many scientific and technological sectors. Disordered materials constitute an overwhelmingly large configurational space, which makes it practically impossible to be explored manually using first-principles calculations such as density functional theory (DFT) due to the high computational costs. Consequently, the use of methods such as cluster expansion (CE) is vital in enhancing our understanding of the disordered materials. CE dramatically reduces the computational cost by mapping the first-principles calculation results on to a Hamiltonian which is much faster to evaluate. In this work, we present our implementation of the CE method, which is integrated as a part of the Atomic Simulation Environment (ASE) open-source package. The versatile and user-friendly code automates the complex set up and construction procedure of CE while giving the users the flexibility to tweak the settings and to import their own structures and previous calculation results. Recent advancements such as regularization techniques from machine learning are implemented in the developed code. The code allows the users to construct CE on any bulk lattice structure, which makes it useful for a wide range of applications involving complex materials. We demonstrate the capabilities of our implementation by analyzing the two example materials with varying complexities: a binary metal alloy and a disordered lithium chromium oxyfluoride.

cond-mat.mtrl-sci↗

arXiv2014

Density-functional investigation of molecular graphene: CO on Cu(111)

Man-made artificial graphene has attracted significant attention in the past few years due to the possibilities to construct designer Dirac fermions with unexpected topological properties and applications in nanoelectronics. Here we use a first-principles approach within density-functional theory to study molecular graphene similar to the experiment by Gomes~{\em et al.}, Nature {\bf 483}, 306 (2012). The system comprises carbon monoxide molecules arranged on a copper (111) surface in such a way that a hexagonal lattice is obtained with the characteristic electronic properties of graphene. Our results show in detail how carbon monoxide molecules modify the copper surface (and regions beneath) and create a hexagonal lattice of accumulated electrons between the adsorbate molecules. We also demonstrate how the properties of the formed Dirac fermions change as the CO density is tuned, and provide a direct comparison with experimental scanning tunneling microscope images.

cond-mat.mtrl-sci↗

arXiv2013

CPMD simulation of Cu2+ -- phenylalanine complex under micro-solvated environment

The study combines DFT calculations and CPMD simulations to investigate the structures of phenylalanine-copper (II) ([Phe-Cu]2+) complexes and the micro-solvation processes. ....It is found that the phenylalanine moiety appears to be in the neutral form in isolated and mono-hydrated complexes, but in the zwitterionic form in other hydrated complexes (with n no less than 2). .... The present CPMD simulations reveal that the maximum coordination of Cu2+ in the presence of the Phe ligand does not exceed four: the oxygen atoms from three water molecules and one carboxyl oxygen atom of Phe. Any excess water molecules will migrate to the second solvation shell. Moreover a unique structural motif is present in the lowest energy complexes, which is recognized to be significant in stabilizing the structures of the complexes. .....

physics.chem-ph↗

arXiv1999

Photoelectron spectra of aluminum cluster anions: Temperature effects and ab initio simulations

Photoelectron (PES) spectra from aluminum cluster anions (from 12 to 15 atoms) at various temperature regimes, were studied using ab-initio molecular dynamics simulations and experimentally. The calculated PES spectra, obtained via shifting of the simulated electronic densities of states by the self-consistently determined values of the asymptotic exchange-correlation potential, agree well with the measured ones, allowing reliable structural assignments and theoretical estimation of the clusters' temperatures.

physics.atm-clus↗