RSS 1.0Computational Physics and Materials Science (CPMS)
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Recent Submissions
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Potential of AlP and GaN as barriers in magnetic tunnel junctions.(Nanoscale, 2023-09-13) [Article]AlP and GaN are wide band gap semiconductors used industrially in light emitting diodes. We investigate their potential as tunnel barriers in magnetic tunnel junctions, employing density functional theory and the non-equilibrium Green's function method for ground state and quantum transport calculations, respectively. We show that the valence band edges are dominated by pz orbitals and the conduction band edges are dominated by s orbitals. Both materials filter Bloch states of Δ1 symmetry at the Γ-point of the Brillouin zone. In the zero bias limit, we find for the Co/AlP/Co junction a high tunnel magnetoresistance of ∼200% at the Fermi energy and for the Co/GaN/Co junction a tunnel magnetoresistance of even ∼300% about 1.4 eV below the Fermi energy.
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Stable Organic Solar Cells Enabled by Interlayer Engineering(Authorea, Inc., 2023-09-11) [Preprint]The development of high-performance organic solar cells (OSCs) with high operational stability is essential to accelerate their commercialization. Unfortunately, there is currently a lack of detailed understanding of the origin of instabilities in state-of-the-art OSCs based on bulk heterojunction (BHJ) featuring non-fullerene acceptors (NFAs). Herein, we developed NFA-based OSCs using different charge extraction interlayer materials and studied their storage, thermal, and operational stabilities. Despite the high power conversion efficiency (PCE) of the OSCs (17.54%), we found that cells featuring self-assembled monolayers (SAMs) as hole-extraction interlayers exhibited poor stability. The time required for these OSCs to reach 80% of their initial performance (T80) was only 6 h under continuous thermal stress at 85 °C in a nitrogen atmosphere and 1 h under maximum power point tracking (MPPT) in a vacuum. Inserting MoOx between ITO and SAM enhanced the T80 to 50 h and ~15 h after the thermal and operational stability tests, respectively, while maintaining a PCE of 16.9%. Replacing the organic PDINN electron transport layer with ZnO NPs further enhances the cells’ thermal and operational stability, boosting the T80 to 1000 and 170 h, respectively. Our work reveals the synergistic role of charge interlayers and device architecture in developing efficient and stable OSCs.
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Ferromagnetism and ferroelectricity in a superlattice of antiferromagnetic perovskite oxides without ferroelectric polarization(npj Computational Materials, Springer Science and Business Media LLC, 2023-09-07) [Article]We study the structural, electronic, and magnetic properties of the SrCrO3/YCrO3 superlattice and their dependence on epitaxial strain. We discover that the superlattice adopts A-type antiferromagnetic (A-AFM) ordering in contrast to its constituents (SrCrO3: C-AFM; YCrO3: G-AFM) and retains it under compressive strain while becoming ferromagnetic (5 μB per formula unit) at +1% strain. The obtained ferroelectric polarization is significantly higher than that of the R2NiMnO6/La2NiMnO6 (R = Ce to Er) series of superlattices [Nat. Commun. 5, 4021 (2014)] due to a large difference between the antipolar displacements of the Sr and Y cations. The superlattice is a hybrid-improper multiferroic material with a spontaneous ferroelectric polarization (13.5 μC/cm2) approaching that of bulk BaTiO3 (19 μC/cm2). The combination of ferromagnetism with ferroelectricity enables multistate memory applications. In addition, the charge-order-driven p-type semiconducting state of the ferromagnetic phase (despite the metallic nature of SrCrO3) is a rare property and interesting for spintronics. Monte Carlo simulations demonstrate a magnetic critical temperature of 90 K for the A-AFM phase without strain and of 115 K for the ferromagnetic phase at +5% strain, for example.
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Functionalized Carbon Honeycomb Membranes for Reverse Osmosis Water Desalination(Advanced Materials Interfaces, Wiley, 2023-08-30) [Article]Reverse osmosis desalination is a common technique to obtain fresh water from saltwater. Conventional membranes suffer from a trade-off between salt rejection and water permeability, raising a need for developing new classes of membranes. C-based membranes with porous graphene and carbon nanotubes offer high salt rejection, water permeability, and fouling resistance. However, controlling the pore size of these membranes is challenging. Therefore, a carbon honeycomb membrane is studied using classical molecular dynamics simulations. It is reported that functionalization with −COO– groups provides 100% salt rejection with around 1000 times higher water permeability than conventional polyamide membranes. Atomic-level understanding of the effect of the functional groups' location on salt rejection and water permeability is developed.
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Strain-Induced Sulfur Vacancies in Monolayer MoS2(ACS Materials Letters, American Chemical Society (ACS), 2023-08-25) [Article]The tuning of two-dimensional (2D) materials offers significant potential to overcome nanoelectronic limitations. As strain engineering is a nondestructive approach, we examine in this study the influence of biaxial strain on the chalcogen vacancy formation energy in transition metal dichalcogenides, employing a combination of calculations and experiments, specifically density functional theory, spherical-corrected scanning transmission electron microscopy, X-ray photoelectron spectroscopy, Raman and photoluminescence spectroscopy, Kelvin probe force microscopy, and I–V characterization. We demonstrate that compressive/tensile biaxial strain decreases/increases the chalcogen vacancy formation energy, increasing/decreasing the probability of creating chalcogen vacancies during the growth. Thus, differently strained areas within a sample can have different chalcogen vacancy densities, opening up a way to customize the work function and a route for defect engineering.
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Fluctuation mediated spin-orbit torque enhancement in the noncollinear antiferromagnet Mn3Ni0.35Cu0.65N(Research Square Platform LLC, 2023-08-10) [Preprint]The role of spin fluctuations near magnetic phase transitions is crucial for generating various exotic phenomena, including anomalies in the extraordinary Hall effect, excess spin-current generation through the spin-Hall effect (SHE), and enhanced spin-pumping, amongst others. In this study, we experimentally investigate the temperature dependence of spin-orbit torques (SOTs) generated by Mn3Ni0.35Cu0.65N (MNCN), a member of the noncollinear antiferromagnetic family that exhibits unconventional magnetotransport properties. Our work uncovers a strong and nontrivial temperature dependence of SOTs, peaking near the Néel temperature of MNCN, which cannot be explained by conventional intrinsic and extrinsic scattering mechanisms of the SHE. Notably, we measure a maximum SOT efficiency of 30%, which is substantially larger than that of commonly studied nonmagnetic materials such as Pt. Theoretical calculations confirm a negligible SHE and a strong orbital Hall effect which can explain the observed SOTs. We propose a previously unidentified mechanism wherein fluctuating antiferromagneticmoments trigger the generation of substantial orbital currents near the Néel temperature. Our findings present an approach for enhancing SOTs, which holds promise for magnetic memory applications by leveraging antiferromagnetic spin fluctuations to amplify both orbital and spin currents.
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Bandgap Engineering of Melon Using Highly Reduced Graphene Oxide for Enhanced Photoelectrochemical Hydrogen Evolution(Wiley, 2023-08-07) [Article]The uncondensed form of polymeric carbon nitrides (PCN), generally known as melon, is a stacked two-dimensional structure of poly(aminoimino)heptazine. Melon is used as a photocatalyst in solar energy conversion applications, but suffers from a poor photoconversion efficiency due to weak optical absorption in the visible spectrum, high activation energy, and inefficient separation of photoexcited charge carriers. We report experimental and theoretical studies to engineer the bandgap of melon with highly reduced graphene oxide (HRG). Three HRG@melon nanocomposites with different HRG:melon ratios (0.5%, 1%, and 2%) were prepared. The 1% HRG@melon nanocomposite showed a higher photocurrent density (71 μA cm−2) than melon (24 μA cm−2) in alkaline conditions. The addition of a hole scavenger further increased the photocurrent density to 630 μA cm−2 relative to the reversible hydrogen electrode (RHE). These experimental results were validated by calculations using density functional theory (DFT), which revealed that HRG results in a significant charge redistribution and an improved photocatalytic hydrogen evolution reaction (HER).
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Anomalous Ultralow Lattice Thermal Conductivity in Mixed-Anion Ba4Sb2Se and Ba4Sb2Te(ACS Applied Electronic Materials, American Chemical Society (ACS), 2023-07-17) [Article]Mixed-anion compounds currently attract great attention, and there are recent indications that they may also be of interest for thermoelectric applications. In this context, the lattice thermal conductivities of mixed-anion Ba4Sb2Se and Ba4Sb2Te are investigated using density functional theory and Boltzmann transport theory. We observe a 24% increase in the lattice thermal conductivity at room temperature when the atomic mass increases from Se to Te, which is counterintuitive given that lighter atoms typically result in higher phonon group velocities and lower phonon scattering rates. This anomalous behavior is attributed to a specific weak Ba–Se bond in Ba4Sb2Se as compared to the corresponding Ba–Te bond in Ba4Sb2Te, which generates numerous low-frequency optical phonons with low group velocities and enhances the phonon scattering. These findings provide avenues to customize the lattice thermal conductivity without the usual reliance on heavy atoms.
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Engineering of Carbon Anodes by Laser Irradiation for Advanced Sodium-Ion Batteries(Elsevier BV, 2023-06-07) [Preprint]Sodium-ion batteries (NIBs) represent an attractive alternative to Li-ion due to sodium-abundant sources, lower cost, and better safety characteristics. However, the limited sodium-ion intercalation into graphite anodes has motivated researchers to search for new anode materials of NIBs. Herein, we demonstrate that laser-irradiated fly ash carbon (FAC), which is a waste by-product of the oil-burning process, can be a promising carbon anode of NIBs. The laser irradiation approach yields three synergistic benefits: (1) introducing carbonyl and carboxylic functional groups that increase adsorption capacity, (2) structurally transforming amorphous carbon into highly graphitic nanoplatelets to boost dynamic intercalation, and (3) enabling an additive and binder-free electrode to increase gravimetric energy density. The laser-irradiated fly ash carbon (LFAC) anode exhibits an excellent specific charge capacity of 356 mAh g–1 at 0.1 A g–1, which is significantly higher than that of the FAC anode (119 mAh g–1). At the end of the cycling measurement, the LFAC anode delivers a capacity of 250 mAh g−1, which is three times higher than the untreated FAC anode (80 mAh g−1).
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Graphene foam membranes with tunable pore size for next-generation reverse osmosis water desalination.(Nanoscale horizons, 2023-05-31) [Article]The development of carbon-based reverse osmosis membranes for water desalination is hindered by challenges in achieving a high pore density and controlling the pore size. We use molecular dynamics simulations to demonstrate that graphene foam membranes with a high pore density provide the possibility to tune the pore size by applying mechanical strain. As the pore size is found to be effectively reduced by a structural transformation under strain, graphene foam membranes are able to combine perfect salt rejection with unprecedented water permeability.
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ZnSe and ZnTe as tunnel barriers for Fe-based spin valves(Physical chemistry chemical physics : PCCP, 2023-05-03) [Article]Owing to their use in the optoelectronic industry, we investigate whether ZnSe and ZnTe can be utilised as tunnel barrier materials in magnetic spin valves. We perform ab initio electronic structure and linear response transport calculations based on self-interaction-corrected density functional theory for both Fe/ZnSe/Fe and Fe/ZnTe/Fe junctions. In the Fe/ZnSe/Fe junction the transport is tunneling-like and a symmetry-filtering mechanism is at play, implying that only the majority spin electrons with Δ1 symmetry are transmitted with large probability, resulting in a potentially large tunneling magnetoresistance (TMR) ratio. As such, the transport characteristics are similar to those of the Fe/MgO/Fe junction, although the TMR ratio is lower for tunnel barriers of similar thickness due to the smaller bandgap of ZnSe as compared to that of MgO. In the Fe/ZnTe/Fe junction the Fermi level is pinned at the bottom of the conduction band of ZnTe and only a giant magnetoresistance effect is found. Our results provide evidence that chalcogenide-based tunnel barriers can be utilised in spintronics devices.
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Revealing the higher-order spin nature of the Hall effect in non-collinear antiferromagnet Mn3Ni0.35Cu0.65N(arXiv, 2023-04-21) [Preprint]Ferromagnets generate an anomalous Hall effect even without the presence of a magnetic field, something that conventional antiferromagnets cannot replicate but noncollinear antiferromagnets can. The anomalous Hall effect governed by the resistivity tensor plays a crucial role in determining the presence of time reversal symmetry and the topology present in the system. In this work we reveal the complex origin of the anomalous Hall effect arising in noncollinear antiferromagnets by performing Hall measurements with fields applied in selected directions in space with respect to the crystalline axes. Our coplanar magnetic field geometry goes beyond the conventional perpendicular field geometry used for ferromagnets and allows us to suppress any magnetic dipole contribution. It allows us to map the in-plane anomalous Hall contribution and we demonstrate a 120∘ symmetry which we find to be governed by the octupole moment at high fields. At low fields we subsequently discover a surprising topological Hall-like signature and, from a combination of theoretical techniques, we show that the spins can be recast into dipole, emergent octupole and noncoplanar effective magnetic moments. These co-existing orders enable magnetization dynamics unachievable in either ferromagnetic or conventional collinear antiferromagnetic materials.
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FeC6N monolayer with ideal properties for water splitting(Applied Surface Science, Elsevier BV, 2023-04-17) [Article]Electrocatalytic water splitting is an environmentally friendly way to prepare hydrogen as alternative energy source to replace fossil fuels. Therefore, the development of high-performance and low-cost catalysts is an urgent need. In the present work, we design two-dimensional catalysts based on earth-abundant elements. Particularly, the FeC6N monolayer with planar 6-coordinated Fe and planar 3-coordinated N shows high structural stability and inherent metallicity. Interestingly, both the Fe and N atoms are catalytically active for hydrogen evolution, resulting in a combined activity comparable to that of Pt. It turns out that this remarkable performance is associated with the 6-coordination of Fe and delocalization of the N lone pair electrons. Overall, the FeC6N monolayer emerges as an ideal catalyst for water splitting.
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Elucidating the Role of Contact-Induced Gap States and Passivation Molecules at Perovskite/Metal Contacts(ACS Applied Energy Materials, American Chemical Society (ACS), 2023-04-11) [Article]Metal halide perovskite solar cells hold great promise as an efficient and cost-effective photovoltaic technology. However, carrier recombination at their contacts impedes progress toward this goal. In this study, considering the archetypical MAPbI3 perovskite and Au as a model electrode, we employ first-principles calculations to show how the mere presence of a metal near the perovskite induces in-gap states that may impair electronically this contact because of carrier recombination and Fermi level pinning. The states are not associated with any defect. We then investigate the suppression of the contact-induced gap states by introducing various passivation molecules to displace the metal from the perovskite surface. Our results highlight from a fundamental perspective the importance of contact displacement and passivation for efficient perovskite solar cells, thereby elucidating further the role of thin molecular interlayers in experimental devices. The elimination of contact-induced gap states can greatly aid perovskite solar cells in fulfilling their promise as a future mainstream source of renewable electricity.
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Inhibitor and Activator: Dual Role of Subsurface Sulfide Enables Selective and Efficient Electro-Oxidation of Methanol to Formate on CuS@CuO Core-Shell Nanosheet Arrays(Small, Wiley, 2023-04-03) [Article]Selective electro-oxidation of aliphatic alcohols into value-added carboxylates at lower potentials than that of the oxygen evolution reaction (OER) is an environmentally and economically desirable anode reaction for clean energy storage and conversion technologies. However, it is challenging to achieve both high selectivity and high activity of the catalysts for the electro-oxidation of alcohols, such as the methanol oxidation reaction (MOR). Herein, a monolithic CuS@CuO/copper-foam electrode for the MOR with superior catalytic activity and almost 100% selectivity for formate is reported. In the core-shell CuS@CuO nanosheet arrays, the surface CuO directly catalyzes MOR, while the subsurface sulfide not only serves as an inhibitor to attenuate the oxidative power of the surface CuO to achieve selective oxidation of methanol to formate and prevent over-oxidation of formate to CO2 but also serves as an activator to form more surface O defects as active sites and enhances the methanol adsorption and charge transfer to achieve superior catalytic activity. CuS@CuO/copper-foam electrodes can be prepared on a large scale by electro-oxidation of copper-foam at ambient conditions and can be readily utilized in clean energy technologies.
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Observation of cnoidal wave localization in nonlinear topolectric circuits(Physical Review Research, American Physical Society (APS), 2023-03-21) [Article]We observe a localized cnoidal (LCn) state in an electric circuit network. Its formation derives from the interplay of nonlinearity and the topology inherent to a Su-Schrieffer-Heeger (SSH) chain of inductors. Varicap diodes act as voltage-dependent capacitors, and create a nonlinear on-site potential. For a sinusoidal voltage excitation around midgap frequency, we show that the voltage response in the nonlinear SSH circuit follows the Korteweg-de Vries equation. The topological SSH boundary state, which relates to a midgap impedance peak in the linearized limit is distorted into the LCn state in the nonlinear regime, where the cnoidal eccentricity decreases from edge to bulk.
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Ti3C2Tx MXene van der Waals gate contact for GaN high electron mobility transistors(Advanced Materials, Wiley, 2023-03-20) [Article]Gate controllability is a key factor that determines the performance of GaN high electron mobility transistors (HEMTs). However, at traditional metal-GaN interface, direct chemical interaction between metal and GaN can result in fixed charges and traps, which can significantly deteriorate the gate controllability. In this study, Ti3C2Tx MXene films were integrated into GaN HEMTs as the gate contact, wherein van der Waals heterojunctions were formed between MXene films and GaN without direct chemical bonding. The GaN HEMTs with enhanced gate controllability exhibited an extremely low off-state current (IOFF) of 10−7 mA/mm, a record high ION/IOFF current ratio of ∼1013 (which is six orders of magnitude higher than conventional Ni/Au contact), a high off-state drain breakdown voltage of 1085 V, and a near-ideal subthreshold swing of 61 mV/dec. This work shows the great potential of MXene films as gate electrodes in wide-bandgap semiconductor devices.
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Ca2FeOsO6/Sr2FeOsO6 superlattice: Multiferroicity above room temperature with giant hybrid-improper ferroelectric polarization and high photovoltaic efficiency(Physical Review B, American Physical Society (APS), 2023-03-15) [Article]Using first-principles calculations, the structure and electronic properties of the room-temperature ferrimagnetic Ca2FeOsO6/Sr2FeOsO6 superlattice are investigated. We show that the superlattice hosts hybrid-improper ferroelectricity despite the fact that bulk Sr2FeOsO6 realizes an a0a0c− tilting pattern of the O octahedra. The magnitude is comparable to that of conventional ferroelectric materials and is found to increase under both compressive and tensile strain. In contrast to competing superlattices, a ferrimagnetic critical temperature above room temperature is realized. An indirect-to-direct band-gap transition is observed between n +1% and +2% strain, coming along with localization of the valence and conduction states on different transition-metal sublattices, which enables efficient electron-hole separation upon photoexcitation. The potential gradient due to the ferroelectric polarization supports the electron-hole separation and a spectroscopic limited maximum efficiency of 27% confirms excellent potential in solar cell applications. The tunable room-temperature ferroelectricity, high critical temperature of the ferrimagnetic ordering with high magnetization, and favorable photoabsorption properties of the Ca2FeOsO6/Sr2FeOsO6 superlattice open up a broad range of technological applications.
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Nucleation Stage for the Oriented Growth of Tantalum Sulfide Monolayers on Au(111)(The Journal of Physical Chemistry C, American Chemical Society (ACS), 2023-03-13) [Article]We study the nucleation stage in the epitaxial growth of monolayer TaS2 as a model system for monolayer transition-metal sulfides. The growth was done under ultrahigh-vacuum conditions with Au(111) as a substrate on which the metal atoms are evaporated, and the sulfur is provided from a background of H2S. Using scanning tunneling microscopy, we find atomic-scale protrusions with a well-defined triangular shape that act as nuclei for the further growth of extended tantalum sulfide monolayers. We identify these protrusions as TaS3 using density functional theory. We propose that their unique orientation is the cause of the well-defined orientation of a complete TaS2 layer found under favorable growth conditions.
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Two-dimensional borocarbonitrides for photocatalysis and photovoltaics(Journal of Materials Chemistry C, Royal Society of Chemistry (RSC), 2023-03-08) [Article]We have designed two-dimensional borocarbonitrides (poly-butadiene-cyclooctatetraene framework BC2N) with hexagonal unit cells, which are stable according to the cohesive energy, phonon dispersion, ab initio molecular dynamics, and elastic modulus results. They are n-type semiconductors with strain-tunable direct band gaps (1.45–2.20 eV), an ultrahigh electron mobility (5.2 × 104 cm2 V−1 s−1 for β-BC2N), and strong absorption (an absorption coefficient of up to 105 cm−1). The intrinsic electric field due to the Janus geometry of α-BC2N reduces the recombination of photo-generated carriers. The band edge positions of α-BC2N and β-BC2N are suitable for photocatalytic hydrogen production, achieving high solar-to-hydrogen efficiencies of 17% and 12%, respectively, in excess of the typical target value of 10% for industrial application. Both γ-BC2N and δ-BC2N can be used as electron donors in type-II heterostructures with two-dimensional transition metal dichalcogenides, and the power conversion efficiency of a solar cell based on these heterostructures can be as high as 21%, approaching the performance of perovskite-based solar cells.
