Cr Adatoms Research Articles

Favorable performance of WSe2 monolayer modified by chromium catalyst for sensing thermal runaway gases H2, CO and C2H4 using first-principles theory

Detections of thermal runaway gases in the Li-ion batteries (LIBs) is a workable method to evaluate its running status. In this work, first-principles theory is adopted to simulate adsorption and sensing performances of Cr-decorated WSe2 (Cr-WSe2) monolayer upon three typical gases, namely H2, CO and C2H4, in the thermal runaway LIBs. Results indicate that Cr-WSe2 monolayer performs physisorption upon H2 with Ead of −0.37 eV and chemisorption upon CO and C2H4 with Ead of −1.62 and −1.41 eV. The Cr adatom causes strong magnetic behavior to Cr-WSe2 monolayer, which impact the magnetic property of the adsorbed gas molecule as well. The analysis of electronic property suggests the strong potential of Cr-WSe2 monolayer as a high-sensitivity CO and C2H4 sensor, and can be applied as a reusable C2H4 sensor after gas desorption for 70.38 s at 398 K. The analysis of work function (WF) reveal its potential as a potential WF-based gas sensor upon H2 and CO as well. The findings in this work uncover the sensing potential of Cr-WSe2 monolayer upon three typical gases of thermal runaway in the LIBs, which can be explored as a nano gas sensor to perform thermal runaway monitoring using the nano-sensing method.

Unveiling the hybridization between the Cr-impurity-mediated flat band and the Rashba-split state of the α-Au/Si(111) surface.

Adsorption of foreign atoms onto 2D materials can either lead to ordinary electron doping or the emergence of new electronic effects including topology, superconductivity, and quantum anomalous Hall and Kondo states. We have investigated the effect of Cr doping on the electronic structure of the α-Au/Si(111)- surface and its adsorbate-modified family. It has been found that below a critical coverage of ∼0.05 monolayer, Cr adatoms penetrate beneath the Au and topmost Si layers and induce the occupied resonance flat band in the electronic spectrum as revealed by angle-resolved photoelectron spectroscopy. Further deposition of Cr leads to the growth of the 3D islands spoiling the surface homogeneity. Using density functional theory calculations, we have disclosed the effects of Cr doping on the electronic band structure and revealed the nature of hybridization between the Cr-induced magnetic-split band and the Au-induced Rashba-split surface state. We believe that the synthesized 2D phases and electronic effects produced by magnetic atom doping in the ultimate two-dimensional limit will stimulate further investigations related to the highly correlated phases and will find practical applications in nanoelectronic devices.

Surface termination and thickness dependent magnetic coupling of Cr adlayers on Ni2MnGa(001) surfaces: An ab initio study

Cr adlayers on magnetic and non-magnetic substrates have attracted attention in the recent years due to the interesting magnetic properties of these systems. In this work, we explore the magnetic properties of Cr adlayers on the (001) surface of a ferromagnetic Heusler alloy, namely Ni2MnGa, using density functional theory based electronic structure calculations. The magnitudes and orientation of the magnetic moments of the Cr adlayers on both the Mn-Ga and Ni terminated Ni2MnGa (001) surfaces have been investigated and we find that the magnetic ordering is sensitive to the substrate atoms and the adlayer thickness as well. For one monolayer of Cr adlayer on Mn-Ga (Ni) surface, the Cr atoms are ferromagnetically (anti-ferromagnetically) coupled within the layer. However, for two monolayers of Cr, the Cr adatoms are ferromagnetically coupled, irrespective of the surface termination. The ferromagnetic ordering of the Cr atoms has been explained from the Stoner criterion. The origin of different magnetic orderings of the Cr adatoms with the substrate has been understood by analyzing the Heisenberg exchange coupling parameters between the adatoms and the substrate atoms beneath the adlayer. Further, to obtain a chemical understanding of the observed magnetic behavior, we have employed the crystal orbital hamilton population bonding analyses. The results corroborate well with the results of Heisenberg exchange coupling parameters as well as spin-polarized atom-derived density of states.

DFT-Based Calculation of Dissolution Activation Energy and Kinetics of Ni–Cr Alloys

A density functional theory investigation of the process of dissolution of Ni and Cr adatoms from model Ni–Cr(111) alloy surfaces is presented, both in vacuum and with explicit water molecules. The goal is to understand how the electronic structures solved using DFT can provide insights as to changes in valence, energy and coordination of the adatoms in the process of dissolving from the alloy surface. It is found that nearby Cr solute atoms increase the dissolution activation energy of Ni. Cr adatoms have a similar dissolution activation energy as Ni adatoms, except for one particular surface configuration that has a much smaller dissolution activation energy, which might promote selective dissolution of Cr in Ni–Cr alloys. To interpret the first-principles modeling results for the potential energy trajectory along the dissolution reaction coordinate, we provide a thermodynamic breakdown of the various terms that contribute to the dissolution activation and total reaction energy. The role of addition of Cr is explored on the dissolution kinetics of a Ni–Cr alloys of varying composition by constructing a model for the corrosion current density with DFT-based activation energies. Finally, dissolution resistance index is proposed as a quantifiable descriptor of the dissolution resistance of corrosion resistant alloys.

Prospecting chiral multisite interactions in prototypical magnetic systems

Atomistic spin models have found enormous success in addressing the properties of magnetic materials, grounded on the identification of the relevant underlying magnetic interactions. The huge development in the field of magnetic skyrmions and other noncollinear magnetic structures is largely due to our understanding of the chiral Dzyaloshinskii-Moriya interaction. Recently, various works have proposed new types of chiral interactions, with seemingly different forms, but the big picture is still missing. Here, we present a systematic construction of a generalized spin model containing isotropic and chiral multi-site interactions. These are motivated by a microscopic model that incorporates local spin moments and the spin-orbit interaction, and their symmetry properties are established. We show that the chiral interactions arise solely from the spin-orbit interaction and that the multi-site interactions do not have to follow Moriya's rules, unlike the Dzyaloshinskii-Moriya and chiral biquadratic interactions. The chiral multi-site interactions do not vanish due to inversion symmetry, and comply with a generalized Moriya rule: If all sites connected by the interaction lie in the same mirror plane, the chiral interaction vector must be perpendicular to this plane. We then illustrate our theoretical considerations with density functional theory calculations for prototypical magnetic systems. These are triangular trimers built out of Cr, Mn, Fe and Co adatoms on the Re(0001), Pt(111) and Au(111) surfaces, for which $C_\mathrm{3v}$ symmetry applies, and Cr and Fe square tetramers on Pt(001) with $C_\mathrm{4v}$ symmetry. The multi-site interactions are substantial in magnitude and cannot be neglected when comparing the energy of different magnetic structures. Finally, we discuss the recent literature in light of our findings, and clarify several unclear or confusing points.

Robust Gapless Surface State against Surface Magnetic Impurities on (Bi_{0.5}Sb_{0.5})_{2}Te_{3} Evidenced by InSitu Magnetotransport Measurements.

Despite extensive experimental and theoretical efforts, the important issue of the effects of surface magnetic impurities on the topological surface state of a topological insulator (TI) remains unresolved. We elucidate the effects of Cr impurities on epitaxial thin films of (Bi_{0.5}Sb_{0.5})_{2}Te_{3}: Cr adatoms are incrementally deposited onto the TI held in ultrahigh vacuum at low temperatures, and insitu magnetoconductivity and Hall effect measurements are performed at each increment with electrostatic gating. In the experimentally identified surface transport regime, the measured minimum electron density shows a nonmonotonic evolution with the Cr density (n_{Cr}): it first increases and then decreases with n_{Cr}. This unusual behavior is ascribed to the dual roles of the Cr as ionized impurities and electron donors, having competing effects of enhancing and decreasing the electronic inhomogeneities in the surface state at low and high n_{Cr}, respectively. The magnetoconductivity is obtained for different n_{Cr} on one and the same sample, which yields clear evidence that the weak antilocalization effect persists and the surface state remains gapless up to the highest n_{Cr}, contrary to the expectation that the deposited Cr should break the time-reversal symmetry and induce a gap opening at the Dirac point.

Tuning structural, electronic, and magnetic properties of black-AsP monolayer by adatom adsorptions: A first principles study

Black Arsenic-phosphorus (AsP) monolayer is a novel two-dimensional nanomaterial with the characteristics of modest direct bandgap and superhigh carrier mobility. However, little is known about how the surface adsorption affects the property of AsP monolayer. Motivated by this, we researched systematically the geometry, adsorption energy, magnetic moment and electronic structure of 11 different adatoms adsorbed on AsP monolayer using first-principles calculations. The adatoms used in this study include light nonmetallic (C, N, O) adatoms, period-3 metal (Na, Mg, Al) adatoms, and transition-metal (Ti, V, Cr, Mn, and Fe) adatoms. The adatoms cause an abundant variety of structural, magnetic and electronic properties. This study shows that AsP binds strongly with all adatoms under study and the adsorption energies in all systems are much stronger than that on graphene, SiC, BN, or MoS2. The semiconductor property of AsP is affected by the introduction of adsorbed atoms, which can induce mid-gap states or cause n-type doping. Moreover, the adatom adsorptions cause various spintronic characteristics: N-, Ti-, and Fe-adsorbed AsP become bipolar semiconductors, while the Mn-decorated AsP becomes a bipolar spin-gapless semiconductor. Our results suggest that atomic adsorption on AsP monolayers has potential application in the field of nanoelectronics and spintronics.

Spin, atomic, and interatomic orbital magnetism induced by 3d nanostructures deposited on transition metal surfaces

We present a first-principles study of the surface magnetism induced by Cr, Mn, Fe and Co adatoms on the (111) surfaces of Rh, Pd, Ag, Ir, Pt and Au. We first describe how the different contributions to the surface magnetism enter the magnetic stray field, with special attention paid to the induced orbital moments. Then we present results for the spin and orbital magnetic moments of the adatoms, and for the induced surface spin and orbital magnetic moments, the latter being further divided into atomic and inter-atomic contributions. We investigate how the surface magnetism is determined by the chemical nature of the elements involved, such as the filling of the magnetic d-orbitals of the adatoms and the properties of the itinerant electrons at the surface (whether they are sp- or d-like, and whether the spin-orbit interaction is relevant), and how it is modified if the magnetic adatoms are brought together to form a cluster, with Cr, Mn, Fe, and Co trimers on Pt(111) as an example. We also explore the impact of computational approximations, such as the distance between the adatom and the Pt(111) surface, or confinement effects due to the finite thickness of the slab used to model it. Our discussion of the magnetic stray field generated by a single adatom and its environment suggests a possible way of disentangling the induced surface magnetism from the adatom one, which could be feasible with scanning NV-center microscopy.

Chromium Adsorption Reveals a Persistent Hydroxylation of Vacuum-Annealed α-Al2O3(0001)

We address herein the question of the termination of the Al-terminated α-Al 2 O 3 (0001) surface. Over decades, various analyses made by different groups repeatedly suggested the presence of a residual coverage of surface OH groups on Al 2 O 3 crystals and powders after annealing in vacuum. However, other authors came to contrary conclusions, thus maintaining a persistent blur on the issue. The present work examines the Cr/alumina interface via Cr K absorption edge analysis (Extended X-Ray Absorption Fine Structure and X-ray Absorption Near Edge Structure) and photoemission with the support of density functional (DFT) calculations. Experiments support the presence of surface OH groups to account for the observed environment as well as the oxidation state of Cr adatoms (Cr 3+). Following a comprehensive DFT-based analysis of Cr z+-O n H m configurations (z=0 to 6, n and m=0 to 3), the Cr 3+-O 2 H alumina-supported surface moieties are found to successfully fit the XAS edge calculations. Most importantly, the combination of experiment and theory that is developed unambiguously demonstrates the presence of surface OH groups on α-Al 2 O 3 (0001) after annealing in vacuum.

Theoretical prediction of stable cluster-assembled CdSe bilayer and its functionalization with Co and Cr adatoms

In this article, we present our results on bilayers assembled upon strategic placement of Cd6Se6 clusters. These bilayers are studied for their stability and electronic structure with the help of density functional theory and are further analyzed using Bardeen, Tersoff and Hamann formalism for their tunneling properties. Our calculations show that the hexagonal arrangement of these clusters prevails as the most stable geometry showing all real phonon modes. First-principles molecular dynamics studies on this 2D structure show that it remains intact even at room temperature. This bilayer shows an indirect semiconducting band gap of 1.28 eV with the current–voltage (I–V) characteristics similar to a tunnel diode. Furthermore, we functionalized this bilayer using transition metal atoms, Co and Cr. The aim was to see whether the bilayer sustains magnetism and how the concentration affects its electronic and magnetic properties. Co functionalization brings ferromagnetic ordering in the bilayer which turns near half-metallic upon increasing the concentration. On the other hand, Cr functionalization shows a transition from antiferro- to ferromagnetic ordering upon increasing the concentration. The I–V characteristics of all these functionalized bilayers show negative differential conductance similar to a tunnel diode.

First-principles calculations of initial Cr deposition on the Fe surface

The energy and electronic properties of Cr ad-atoms and Crn (n = 2–9) clusters deposition on the Fe surfaces were investigated by first principles calculations. There are two kinds of modes for atom deposition approach in our calculation which are the quasi-chain deposition mode and the quasi-aggregation mode, and there exists 2-dimentional to 3-dimentional structural evolution for the cluster deposition approach. To further understand the interaction between adsorbates and substrate, the electronic density of state (DOS) and Bader charge were calculated. The results indicate that the ad-atoms have lower surface reaction activity than the clusters, and all the adsorbates can serve as the electron donator to the surface Fe atoms. Moreover, the kinetic barrier and the transition state between two kinds of deposition configurations were analyzed, suggesting that the small size Cr clusters have less energy pathways and kinetic barriers than the large size Cr clusters.

Beating the exclusion rule against the coexistence of robust luminescence and ferromagnetism in chalcogenide monolayers

Monolayer chalcogenide semiconductors with both luminescent and ferromagnetic properties are dreamed for simultaneous polarization and detection of the valley degree of freedom in valleytronics. However, a conventional chalcogenide monolayer lacks these coexisting properties due to their mutually exclusive origins. Herein we demonstrate that robust ferromagnetism and photoluminescence (PL) could be achieved in a (Co, Cr)-incorporated single monolayer MoS2, where the ferromagnetic interaction is activated by Co ions, and the nonradiative recombination channels of excitons is cut off by Cr ions. This strategy brings a 90-fold enhancement of saturation magnetization and 35-fold enhancement of PL intensity than the pristine MoS2 monolayer. The main reasons for the coexisting ferromagnetism and PL are the electronic interactions between the impurity bands of atop Cr adatoms and substitutional Co atoms, as well as the increased content of neutral exciton. Our findings could extend the applications of two-dimensional chalcogenides into spintronics, valleytronic and photoelectric devices.

Half-metallic properties of transition metals adsorbed on WS2 monolayer: A first-principles study

Abstract Given the various applications of half-metals in the spintronics devises, we studied the structural, electronic and magnetic properties of transition-metal (TM) (Cr, Mn, Fe, Co, Ni, and Cu) adatoms on WS2 monolayer by performing first-principles calculations. Based on the adsorption energy, TM atoms prefer to occupy the TW site, above the W atoms, for all the cases. The results indicate that systems of Cr, Mn, Fe, and Co adsorbed on WS2 monolayer are magnetic, while in Ni and Cu atoms adsorbed don’t have any magnetic properties. The TM atoms are adsorbed chemically to the WS2 layer. The obtained values of the total magnetic moment vary from 4 μB to 1 μB in case of Cr to Co adsorption, respectively, which agree with the Slater-Pauling rule. Based on the magnetic moment of 0.00 μB, non-magnetic states are realized for Ni and Cu adsorption. More importantly, the high spin polarization of 100% at the Fermi level is achieved in the Cr, Mn, Fe and Co adsorption, which implies the potential for application in spintronic devices.

Interatomic orbital magnetism: The case of 3d adatoms deposited on the Pt(111) surface

The orbital magnetic moment is still surprisingly not well understood, in contrast to the spin part. Its description in finite systems, such as isolated atoms and molecules, is not problematic, but it was only recently that a rigorous picture was provided for extended systems. Here we focus on an intermediate class of systems: magnetic adatoms placed on a non-magnetic surface. We show that the essential quantity is the ground-state charge current density, in the presence of spin-orbit coupling, and set out its first-principles description. This is illustrated by studying the magnetism of the surface Pt electrons, induced by the presence of Cr, Mn, Fe, Co and Ni adatoms. A physically appealing partition of the charge current is introduced. This reveals that there is an important nonlocal contribution to the orbital moments of the Pt atoms, extending three times as far from each magnetic adatom as the induced spin and local orbital moments. We find that it is as sizable as the latter, and attribute its origin to a spin-orbital susceptibility of the Pt surface, different from the one responsible for the formation of the local orbital moments.

Influence of Magnetic Ordering between Cr Adatoms on the Yu-Shiba-Rusinov States of the β-Bi_{2}Pd Superconductor.

We show that the magnetic ordering of coupled atomic dimers on a superconductor is revealed by their intragap spectral features. Chromium atoms on the superconductor β-Bi_{2}Pd surface display Yu-Shiba-Rusinov bound states, detected as pairs of intragap excitations in tunneling spectra. By means of atomic manipulation with a scanning tunneling microscope's tip, we form Cr dimers with different arrangements and find that their intragap features appear either shifted or split with respect to single atoms. These spectral variations are associated with the magnetic coupling, ferromagnetic or antiferromagnetic, of the dimer, as confirmed by density functional theory simulations. The striking qualitative differences between the observed tunneling spectra prove that intragap Shiba states are extremely sensitive to the magnetic ordering on the atomic scale.

Planar model system of the Phillips (Cr/SiO2) catalyst based on a well-defined thin silicate film

The Phillips catalyst (Cr/SiO2) is successfully used in the large-scale production of polyethylene and has attracted a great interest in catalytic community over the last sixty years. However, the atomic structure of the active site(s) and the reaction mechanism remain controversial, in particular due to the structural complexity and surface heterogeneity of the amorphous silica. In this work, we used a well-defined, atomically flat silicate bilayer film grown on Ru(0001) as a support offering the opportunity to investigate mechanistic aspects at the atomic scale. To fabricate a planar Cr/SiO2 model system suitable for surface science studies, chromium was deposited using physical vapor deposition onto the hydroxylated silica film surface. Structural characterization and adsorption studies were performed by infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). Hydroxyls groups seem to serve as anchoring cites to Cr ad-atoms. As monitored by IRAS, hydroxyls consumption correlated with the appearance of the new band at ∼1007 cm−1 typical for CrO vibrations. In addition, CO titration experiments suggested also the presence of “naked” Cr, which transforms into mono- and di-oxo chromyl species and their aggregation upon oxidation treatments. TPD experiments of ethylene adsorption at low temperatures under UHV conditions showed the formation of butane as one of the main products. The resultant surfaces are thermally stable, at least, up to 400 K which allows to investigate ethylene polymerization further under more realistic conditions.

Chern insulators without band inversion in MoS2 monolayers with 3d adatoms

Electronic and topological properties of MoS2 monolayers endowed with 3d transition metal (TM) adatoms (V-Fe) are explored by using ab initio methods and k.p models. Without the consideration of the Hubbard U interaction, the V, Cr, and Fe adatoms tend to locate on the top of the Mo atoms, while the most stable site for the Mn atom is at the hollow position of the Mo-S hexagon. After the Hubbard U is applied, the most stable sites of all the systems become the top of the Mo atoms. Chern insulators without band inversion are achieved in these systems. The V and Fe adsorption systems are the best candidates to produce the topological states. The k.p model calculations indicate that these topological states are determined by the TM magnetism, the C3v crystal field from the MoS2 substrate, and the TM atomic spin-orbit coupling (SOC). The special two-meron pseudospin texture is found to contribute to the topology. The apparent difference between the Berry curvatures for the V and Fe adsorption systems are also explored. Our results widen the understanding to the Chern insulators and are helpful for the applications of the MoS2 monolayers in the future electronics and spintronics.

Two-dimensional ferromagnetism and spin filtering in Cr and Mn-doped graphdiyne

Ferromagnetism in half-metallic two-dimensional material can lead to unique spintronics application. In this work, Cr-doped single-layer graphdiyne is predicted to be two-dimensional ferromagnetic semiconductor, and Mn-doped graphdiyne is predicted to be ferromagnetic conductor using first-principle calculations. Cr and Mn adatoms could be stabilized on the corner sites of graphdiyne sheet due to high migration barriers. The currents through Cr- and Mn-doped graphdiyne possess spin polarization feature. For Cr-doped graphdiyne system, the transmission polarization is up to 100% and could be controlled by the gate voltage. Cr-/Mn-doped graphdiyne with spin-polarized semiconductor/conductor properties could be promising material for combined usage in spintronics application.

Surface electronic structure and evidence of plains-wave superconductivity in(Li0.8Fe0.2)OHFeSe

(Li0.8Fe0.2)OHFeSe is a newly-discovered intercalated iron-selenide superconductor with a Tc above 40 K, which is much higher than the Tc of bulk FeSe (8 K). Here we report a systematic study of (Li0.8Fe0.2)OHFeSe by low temperature scanning tunneling microscopy (STM). We observed two kinds of surface terminations, namely FeSe and (Li0.8Fe0.2)OH surfaces. On the FeSe surface, the superconducting state is fully gapped with double coherence peaks, and a vortex core state with split peaks near EF is observed. Through quasi-particle interference (QPI) measurements, we clearly observed intra- and inter-pocket scatterings in between the electron pockets at the M point, as well as some evidence of scattering that connects gamma and M points. Upon applying magnetic field, the QPI intensity of all the scattering channels are found to behave similarly. Furthermore, we studied impurity effects on the superconductivity by investigating intentionally introduced impurities and intrinsic defects. We observed that magnetic impurities such as Cr adatoms can induce in-gap states and suppress superconductivity. However, nonmagnetic impurities such as Zn adatoms do not induce visible in-gap states. Meanwhile, we show that Zn adatoms can induce in-gap states in thick FeSe films, which is believed to have an (s+-)wave pairing symmetry. Our experimental results suggest it is likely that (Li0.8Fe0.2)OHFeSe is a plain s-wave superconductor, whose order parameter has the same sign on all Fermi surface sections.

Observing spin excitations in3dtransition-metal adatoms on Pt(111) with inelastic scanning tunneling spectroscopy: A first-principles perspective

Spin excitations in atomic-scale nanostructures have been investigated with inelastic scanning tunneling spectroscopy, sometimes with conflicting results. In this work, we present a theoretical viewpoint on a recent experimental controversy regarding the spin excitations of Co adatoms on Pt(111). While one group [Balashov et al., Phys. Rev. Lett. 102, 257203 (2009)] claims to have detected them, another group reported their observation only after the hydrogenation of the Co adatom [Dubout et al., Phys. Rev. Lett. 114, 106807 (2015)]. Utilizing time-dependent density functional theory in combination with many-body perturbation theory, we demonstrate that, although inelastic spin excitations are possible for Cr, Mn, Fe, and Co adatoms, their efficiency differs. While the excitation signature is less pronounced for Mn and Co adatoms, it is larger for Cr and Fe adatoms. We find that the tunneling matrix elements or the tunneling cross-section related to the nature and symmetry of the relevant electronic states are more favorable for triggering the spin excitations in Fe than in Co. An enhancement of the tunneling and of the inelastic spectra is possible by attaching hydrogen to the adatom at the appropriate position.