Mn Dimer Research Articles

The dual-transition-metal dichalcogenide monolayers anchored with Mn dimer: Promising electrocatalysts for efficient nitrogen reduction reaction

Due to its great efficiency, the electrocatalytic nitrogen reduction reaction (NRR) presents itself as a viable and eco-friendly substitute for the traditional Haber-Bosch ammonia production process. However, it is still a formidable task to find electrocatalysts with high activity and selectivity. In this study, a series of transition metals (TM = Ti–Ni, Zr–Mo, Ru–Pd, and Hf–Pt) anchored on 1T′-dual-transition-metal dichalcogenides (1T′-d-TMDs) monolayers (TM2@1T′-CrCoS4) were systematically investigated as electrocatalysts for NRR using first-principles calculations based on density functional theory. Based on a thorough examination of selectivity, high activity, and stability, Mn2@1T′-CrCoS4 and Co2@1T′-CrCoS4 demonstrate exceptional NRR performance. With a limiting potential of −0.11 V, Mn2@1T′-CrCoS4 stood out among them in terms of catalytic activity, favoring the enzymatic pathway. Furthermore, ab initio molecular dynamics (AIMD) simulations were used to assess the dynamic stability of Mn2@1T′-CrCoS4 and Co2@1T′-CrCoS4. To determine the source of increased activities, the density of states (DOS), charge density difference, and crystal orbital Hamilton population analysis were used. According to our research, the 1T′-d-TMDS is a viable substrate for the development of effective NRR catalysts and offers a platform for electrocatalyst experimentation.

Integrating Host Design and Tailored Electronic Effects of Yolk–Shell Zn−Mn Diatomic Sites for Efficient CO2 Electroreduction

AbstractModulating the surface and spatial structure of the host is associated with the reactivity of the active site, and also enhances the mass transfer effect of the CO2 electroreduction process (CO2RR). Herein, we describe the development of two‐step ligand etch–pyrolysis to access an asymmetric dual‐atomic‐site catalyst (DASC) composed of a yolk–shell carbon framework (Zn1Mn1‐SNC) derived from S,N‐coordinated Zn−Mn dimers anchored on a metal–organic framework (MOF). In Zn1Mn1‐SNC, the electronic effects of the S/N−Zn−Mn−S/N configuration are tailored by strong interactions between Zn−Mn dual sites and co‐coordination with S/N atoms, rendering structural stability and atomic distribution. In an H‐cell, the Zn1Mn1‐SNC DASC shows a low onset overpotential of 50 mV and high CO Faraday efficiency of 97 % with a low applied overpotential of 343 mV, thus outperforming counterparts, and in a flow cell, it also reaches a high current density of 500 mA cm−2 at −0.85 V, benefitting from the high structure accessibility and active dual sites. DFT simulations showed that the S,N‐coordinated Zn−Mn diatomic site with optimal adsorption strength of COOH* lowers the reaction energy barrier, thus boosting the intrinsic CO2RR activity on DASC. The structure‐property correlation found in this study suggests new ideas for the development of highly accessible atomic catalysts.

CO2 Activation with Manganese Tricarbonyl Complexes by an H-Atom Responsive Benzimidazole Ligand.

Herein, we report the synthesis and characterization of two manganese tricarbonyl complexes, MnI (HL)(CO)3 Br (1 a-Br) and MnI (MeL)(CO)3 Br (1 b-Br) (where HL=2-(2'-pyridyl)benzimidazole; MeL=1-methyl-2-(2'-pyridy)benzimidazole) and assayed their electrocatalytic properties for CO2 reduction. A redox-active pyridine benzimidazole ancillary ligand in complex 1 a-Br displayed unique hydrogen atom transfer ability to facilitate electrocatalytic CO2 conversion at a markedly lower reduction potential than that observed for 1 b-Br. Notably, a one-electron reduction of 1 a-Br yields a structurally characterized H-bonded binuclear Mn(I) adduct (2 a') rather than the typically observed Mn(0)-Mn(0) dimer, suggesting a novel method for CO2 activation. Combining advanced electrochemical, spectroscopic, and single crystal X-ray diffraction techniques, we demonstrate the use of an H-atom responsive ligand may reveal an alternative, low-energy pathway for CO2 activation by an earth-abundant metal complex catalyst.

Mn(II), Fe(II), and Co(II) Aryloxides: Steric and Dispersion Effects and the Thermal Rearrangement of a Cobalt Aryloxide to a Co(II) Semiquinone Complex.

A series of Mn(II), Fe(II), and Co(II) bisaryloxide dimers ([M(OC6H2-2,4,6-Cy3)2]2 {M = Mn (1), Fe (2), and Co (3)} were synthesized by the addition of 2,4,6-tricyclohexylphenol (HOC6H2-2,4,6-Cy3) to the silyl amido dimers [M(N(SiMe3)2)2]2 (M = Mn, Fe, Co; Cy = cyclohexyl). An unexpected and unique Co(II) phenoxide derivative (4), [Co(OC6H2-2,4,6-Cy3)(O2C6H-3,5,6-Cy3)]2, was obtained via ligand rearrangement of 3 at ca. 180 °C. This yielded 4 in which there are two unchanged, bridging phenoxide ligands as well as a terminal bidentate semiquinone ligand bound to each cobalt. Complexes 1 and 2 did not undergo such a rearrangement under the same conditions; both are thermally stable to temperatures exceeding 250 °C and feature numerous short-contact (<2.5 Å) H···H interactions consistent with the presence of dispersion stabilization. Use of the aryloxide ligand -OC6H3-2,6-Pri2 (Pri = isopropyl), which is sterically similar to -OC6H2-2,4,6-Cy3 but produces fewer close H···H interactions, gave the trimeric species [M(OC6H3-2,6-Pri2)2]3 {M = Fe (5) or Co (6)} which feature a linear array of three metal atoms bridged by aryloxides. The higher association number in 5 and 6 in comparison to that of 1-3 is due to the lower dispersion energy donor properties of the -OC6H3-2,6-Pri2 ligand and the lower stabilization it produces.

Tuning a Two-Impurity Kondo System by a Moiré Superstructure.

Two-impurity Kondo models are paradigmatic for correlated spin-fermion systems. Working with Mn atoms on Au(111) covered by a monolayer of MoS_{2}, we tune the interadatom exchange via the adatom distance and the adatom-substrate exchange via the location relative to a moiré structure of the substrate. Differential-conductance measurements on isolated adatoms exhibit Kondo peaks with heights depending on the adatom location relative to the moiré structure. Mn dimers spaced by a few atomic lattice sites exhibit split Kondo resonances. In contrast, adatoms in closely spaced dimers couple antiferromagnetically, resulting in a molecular-singlet ground state. Exciting the singlet-triplet transition by tunneling electrons, we find that the singlet-triplet splitting is surprisingly sensitive to the moiré structure. We interpret our results theoretically by relating the variations in the singlet-triplet splitting to the heights of the Kondo peaks of single adatoms, finding evidence for coupling of the adatom spin to multiple conduction electron channels.

Systematic study of Mn atoms, artificial dimers, and chains on superconducting Ta(110)

Magnetic adatoms coupled to an $s$-wave superconductor give rise to local bound states, so-called Yu-Shiba-Rusinov states. Focusing on the ultimate goal of tailoring chains of such adatoms into a topologically superconducting phase, we investigate basic building blocks - single Fe and Mn adatoms and Mn dimers on clean superconducting Ta(110) - using scanning tunneling microscopy and spectroscopy. We perform a systematic study of the hybridizations and splittings in dimers, and their dependence on the crystallographic directions and interatomic spacings, in order to identify potentially interesting chain geometries for this novel sample type. Subsequently, we study the spin structure as well as the length dependent Shiba band structure in Mn chains of those geometries using spin-resolved scanning tunneling spectroscopy. All results are compared to the according properties of structurally identical dimers and chains on the previously studied Nb(110), which has almost identical surface structure and electronic properties, but an about three times smaller spin-orbit interaction.

Highly efficient Mn–Mn dimer activated phosphors for high-power near-infrared LED application

Broadband NIR phosphors as light sources are receiving increasing attention.

Synergetic effects of the co-doping transition metal ions and the silica-shell coating for enhancing the photoluminescence and stability of Mn: CsPbCl3 nanocrystals and their application

Doping Mn2+ ions into cesium lead halide CsPbX3 (X = Cl, Br, I) perovskite nanocrystals (PNCs) has been proven to be an effective strategy to reduce the Pb2+ content. However, excessive Mn2+ ions may form the Mn–Mn dimers in PNCs, leading to concentration quenching effects and low photoluminescence quantum yields (PLQYs). Herein, secondary metal ions (Ni2+, Cu2+, Zn2+) were introduced to substitute Pb2+ ions as co-doped (Mn, M):CsPbCl3 PNCs (M = Ni, Cu, Zn), which were prepared by a facile method under ambient conditions, aiming for positively passivating the surface defects and improving the Mn2+ emission. Also, to protect the co-doped (Mn, M):CsPbCl3 PNCs from moisture damage, we performed silica encapsulation, which further passivated surface defects due to the effective hydrolysis of silane. The evolution of optical properties and stabilities of three series of (Mn, M):CsPbCl3 PNCs was compared. The PLQYs of co-doped (Mn, M):CsPbCl3 PNCs were larger than those of single-doped Mn: CsPbCl3 PNCs, especially for (Mn, Ni):CsPbCl3 and (Mn, Cu):CsPbCl3, which can reach nearly 30%. The PLQYs of (Mn, Ni):CsPbCl3 and (Mn, Zn):CsPbCl3 PNCs decreased dramatically during the encapsulation process, while the PLQY values of (Mn, Cu):CsPbCl3 remained 30% even after the same treatment process. Consequently, the prototypes of white light-emitting devices (WLEDs) were successfully fabricated based on orange-red (Mn, Ni):CsPbCl3@SiO2 and (Mn, Zn):CsPbCl3@SiO2 composites. The synergetic effects of co-doping transition metal ions and silica-shell coating on improving optical properties and stabilities were investigated. It has potential for a wide range of applications in the field of solid-state light emission.

Ultralow-Temperature NOx Reduction over SmMn2O5 Mullite Catalysts Via Modulating the Superficial Dual-Functional Active Sites

The development of highly efficient catalysts for low-temperature NOx abatement still existed as a scabrous issue. An acid-etched mullite-type SmMn2O5 catalyst (SM-E) was developed and applied in ultralow-temperature selective catalytic reduction of NOx with NH3 (NH3-SCR) to meet the increasingly rigorous demands of emission control in the nonelectric industry, which can convert more than 90% NOx in a wide operating window (90–200 °C). It has been demonstrated that NO can be preferably adsorbed and activated on the unique Mn–Mn dimer sites and further react with the adsorbed NH3 on adjacent Mn–O sites over the SM-E catalyst. The synergistic effects of these abundant exposed dual-functional active sites on the SM-E catalyst contributed to the extraordinary NH3-SCR performance compared to the pristine SmMn2O5 catalyst exposing deficient active sites as well as the Sm–Mn composite oxides containing sole Mn–O sites. This work sheds light on a novel strategy to deeply study the structure–performance relationship and provides deep insights into the rational design of NH3-SCR catalysts for ultralow-temperature NOx abatement.

Yu-Shiba-Rusinov states in two-dimensional superconductors with arbitrary Fermi contours

Magnetic impurities on a superconductor induce subgap Yu-Shiba-Rusinov (YSR) bound states, localized at the impurity site, decaying over distances dependent on microscopical details. Here, the authors present an effective model providing analytical expressions of their spatial distribution for a two-dimensional superconductor with an arbitrary Fermi contour. These results prove that vector nesting in anisotropic Fermi contours focuses YSR states into one-dimensional-like beams, extending to coherence length scales. The authors check the accuracy of the approximation by comparing the results to an exact numerical calculation based on a tight-binding Hamiltonian and to STM results on Mn dimers on a superconductor.

Electric Field Control of Magnetism of a Mn Dimer Supported on a Carbon-Doped h-BN Surface

Using density functional theory we show that the interaction between two Mn atoms can be tuned from anti-ferromagnetic (AFM) to ferromagnetic (FM) state by creating charge disproportion between the two on a 2D surface. The non-metallic planar heterostructures, the 2D surface, in our work is designed by doping carbon hexagon rings in a hexagonal boron nitride (h-BN) sheet. In addition, we show that an external electric field can be used to control the charge disproportion and hence the magnetism. In fact, our calculations demonstrate that the magnetic states of the dimer can be switched from AFM to FM or vice versa in an external electric field. The origin of this magnetic switching is explained using the charge transfer from (or to) the Mn dimer to (or from) the 2D material. The switching between anti-ferromagnetic to ferromagnetic states can be useful for future spintronic applications.

Relativistic first-principles theory of Yu-Shiba-Rusinov states applied to Mn adatoms and Mn dimers on Nb(110)

We present a fully relativistic first principles based theoretical approach for the calculation of the spectral properties of magnetic impurities on the surface of a superconducting substrate, providing a material specific framework for the investigation of the Yu--Shiba--Rusinov (YSR) states. By using a suitable orbital decomposition of the local densities of states we discuss in great details the formation of the YSR states for an Mn adatom and for two kinds of Mn dimers placed on the Nb(110) surface and compare our results to recent experimental findings. In case of the adatom we find that the spin-orbit coupling slightly shifts some of the YSR peaks and also the local spin-polarization on the Nb atoms have marginal effects to the their positions. Moreover, by scaling the exchange field on the Mn site we could explain the lack of the $d_{x^2-y^2}$-like YSR state in the spectrum. While our results for a close packed ferromagnetic dimer are in satisfactory agreement with the experimentally observed splitting of the YSR states, in case of an antiferromagnetic dimer we find that the spin-orbit coupling is not sufficiently large to explain the splitting of the YSR states seen in the experiment. Changing the relative orientation of the magnetic moments in this dimer induces splitting of the YSR states and also shifts their energy, leading even to the formation of a zero bias peak in case of the deepest YSR state.

Mn Dimer Can Be Described Accurately with Density Functional Calculations When Self-Interaction Correction Is Applied.

Qualitatively incorrect results are obtained for the Mn dimer in density functional theory calculations using the generalized gradient approximation (GGA), and similar results are obtained from local density and meta-GGA functionals. The coupling is predicted to be ferromagnetic rather than antiferromagnetic, and the bond between the atoms is predicted to be an order of magnitude too strong and approximately an Ångstrøm too short. Explicit, self-interaction correction (SIC) applied to a commonly used GGA energy functional, however, provides close agreement with both experimental data and high-level, multireference wave function calculations. These results show that the failure is not due to a strong correlation but rather the single electron self-interaction that is necessarily introduced in estimates of the classical Coulomb and exchange-correlation energy when only the total electron density is used as the input. The corrected functional depends explicitly on the orbital densities and can, therefore, avoid the introduction of a self-Coulomb interaction. The error arises because of an overstabilization of bonding d-states in the minority spin channel resulting from an overestimate of the d-electron self-interaction in the semilocal exchange-correlation functionals. Since the computational effort in the SIC calculations scales with the system size in the same way as for regular semilocal functional calculations, this approach provides a way to calculate properties of Mn nanoclusters as well as biomolecules and extended solids, where Mn dimers and larger cluster are present, while multireference wave function calculations can only be applied to small systems.

Spin-orbit coupling induced splitting of Yu-Shiba-Rusinov states in antiferromagnetic dimers

Magnetic atoms coupled to the Cooper pairs of a superconductor induce Yu-Shiba-Rusinov states (in short Shiba states). In the presence of sufficiently strong spin-orbit coupling, the bands formed by hybridization of the Shiba states in ensembles of such atoms can support low-dimensional topological superconductivity with Majorana bound states localized on the ensembles’ edges. Yet, the role of spin-orbit coupling for the hybridization of Shiba states in dimers of magnetic atoms, the building blocks for such systems, is largely unexplored. Here, we reveal the evolution of hybridized multi-orbital Shiba states from a single Mn adatom to artificially constructed ferromagnetically and antiferromagnetically coupled Mn dimers placed on a Nb(110) surface. Upon dimer formation, the atomic Shiba orbitals split for both types of magnetic alignment. Our theoretical calculations attribute the unexpected splitting in antiferromagnetic dimers to spin-orbit coupling and broken inversion symmetry at the surface. Our observations point out the relevance of previously unconsidered factors on the formation of Shiba bands and their topological classification.

A combined experimental and theoretical study on a single, unsupported oxo-bridged Mn(III,III) dimer coordinated to two iminobenzosemiquinone π-radical anions.

Ligand H2LAP comprises a non-innocent 2-aminophenol unit and an innocent bis(pyridin-2-ylmethyl)amine unit. The ligand, upon reaction with an equivalent amount of Mn(ClO4)2·6H2O in the presence of Et3N under air in MeOH, provided a mono(oxo)-bridged dinuclear Mn2 complex ({[(LISQ)MnIII-O-MnIII(LISQ)][(ClO4)]2}; 1). X-ray crystal structure analysis of complex 1 revealed that in the dicationic unit, the physical oxidation state of each Mn ion was +III and the 2-aminophenol unit of ligand H2LAP was in its one-electron oxidized iminobenzosemiquinone form. 1H-NMR measurement of complex 1 confirmed that the complex acquired a diamagnetic ground state (St = 0). Thus, antiferromagnetic couplings among the paramagnetic centers were realized. The UV-Vis-NIR spectrum of complex 1 was consisted of ligand-to-metal charge-transfer transitions in the visible region, while ligand-to-metal and metal-to-ligand charge-transfer transitions were noticed in the near-infrared region due to the presence of iminobenzosemiquinone radical units. The cyclic voltammogram of the complex showed three one-electron oxidation waves and two one-electron reduction waves. While the first two oxidation processes were metal-based, the two successive reductions were ligand-centered. DFT-based theoretical studies confirmed the assignment.

Sign-reversal electron magnetization in Mn-doped semiconductor structures

The diversity of various manganese types and its complexes in the Mn-doped ${\rm A^{III}B^V}$ semiconductor structures leads to a number of intriguing phenomena. Here we show that the interplay between the ordinary substitutional Mn acceptors and interstitial Mn donors as well as donor-acceptor dimers could result in a reversal of electron magnetization. In our all-optical scheme the impurity-to-band excitation via the Mn dimers results in direct orientation of the ionized Mn-donor $d$ shell. A photoexcited electron is then captured by the interstitial Mn and the electron spin becomes parallel to the optically oriented $d$ shell. That produces, in the low excitation regime, the spin-reversal electron magnetization. As the excitation intensity increases the capture by donors is saturated and the polarization of delocalized electrons restores the normal average spin in accordance with the selection rules. A possibility of the experimental observation of the electron spin reversal by means of polarized photoluminescence is discussed.

Designing bifuncitonal molecular devices with a metalloporphyrin dimer.

Many organic molecules have unique magnetic properties and can potentially serve as excellent molecular spin devices, which is worth exploring deeply. Here, the spin transport properties of Mn, Fe, Co and Cu porphyrin dimer devices are investigated based on the first principles method. The spin filtering efficiencies of these molecular devices are maintained at 100% within certain applied voltage ranges and magnetoresistance ratios are higher than 108% which increase as the voltage increases. To explain the excellent spin-filtering and giant magnetoresistance effects, analysis of spin electron densities and transmission spectra indicates that magnetic properties are mainly contributed by the metal atoms and their neighbouring N atoms. From the transmission pathway studies, spin electrons come mainly through the π-conjugated structure of the metal porphyrin ring. Interestingly, in the Cu porphyrin dimer device, magnetic moments of the Cu-N structure in the Cu porphyrin dimer device show spin behaviors different from those of Mn, Fe and Co porphyrin dimer devices.

N,N,O Pincer Ligand with a Deprotonatable Site That Promotes Redox‐Leveling, High Mn Oxidation States, and a Mn2O2 Dimer Competent for Catalytic Oxygen Evolution

The new complex [Mn(bipyalk)(H2O)(µ‐O)]2(OTf)2 {bipyalk = 2‐([2,2′‐bipyridin]‐6‐yl)propan‐2‐olate} catalyzes oxygen evolution with a TOF of 0.0055 s–1 when driven by the sacrificial oxidant KHSO5 in water. It is proposed on the basis of EPR experiments that the catalyst proceeds through a MnV(µ‐O)2MnV=O intermediate supported by the highly donating tertiary alkoxide moiety of the ligand. The Mn(IV,IV) dimer can also be formed electrochemically from its precursor, Mn(bipyalkH)Cl2. A related series of bis‐ligated monomers of the type [Mnn(bipyalkHx)(bipyalkHy)](PF6)2 is also reported, where the dication features MnII, MnIII, and MnIV oxidation states and corresponding protonation states. Electrochemical data on this series underscore the importance of proton loss at the alcohol/alkoxide moiety during oxidation to maintain low overpotentials required for catalysis.

Application of metal complexes as biomimetic catalysts on glycerol oxidation

Two biomimetic complexes were evaluated as catalysts in the H2O2 mediated oxidation of glycerol, namely a peroxidase mimetic Fe(III) protoporphyrin complex (hematin) and the superoxide-dismutase mimetic complex of Mn(III) with 1,3-bis(5-sulphonatesalycilidenamino) propane (MnL−). Catalysis was targeted to glyceraldehyde since antimicrobial power was proved for it. Glyceraldehyde evolved at a higher rate than the uncatalyzed reaction only with hematin acid treated solutions and kinetics were typical of a radical mechanism. Nonetheless, glycerol conversions were low. H2O2 bleached hematin and the immobilization on a porous matrix could not prevent this. Meanwhile, the catalatic activity of hematin was high but its peroxidatic activity was inhibited at pH > 8. Thus, the coordination of hematin compound I to H2O2 over glycerol may be the preferred route with the accumulation of peroxy radicals, able to degrade the porphyrinic ring -with probable iron releasing- but also contributing to glycerol oxidation. On the other hand, a prompt decay with time of the catalatic and peroxidatic activities of MnL− was observed, which was improved by the addition of dimethylsulfoxide (DMSO), dimethylformamide (DMF) or acetone to the basic buffer system. Finally, EPR spectroscopy of MnL− supported the hypothesis of the formation of an inactive bis-oxo-bridged Mn(IV)Mn(IV) dimer upon addition of H2O2.

Local environment dependence of Mn magnetism in bcc iron-manganese alloys: A first-principles study

Magnetic behavior of a $3d$ solute in a ferromagnetic lattice can be very sensitive to local environment, which is the case of manganese in bcc Fe. Body-centered cubic iron-manganese alloys are studied by means of density functional theory in order to elucidate properties of the lowest-energy magnetic states. Multiple magnetic minima are determined even for the simplest case of an isolated Mn and a Mn dimer in bcc iron. The magnetoenergetic landscape is analyzed within and beyond the collinear magnetic approximation. A direct correlation is identified between the local electronic charge and the local magnetic moment of a Mn solute, being either isolated or forming a small cluster. In particular, the presence of a vacancy near the Mn atom, inducing a local charge decrease, tends to favor the antiferromagnetic Fe-Mn interaction while the presence of an interstitial impurity with a strong electronic hybridization with Mn can favor a ferromagnetic Fe-Mn interaction. Energetic and magnetic properties of Fe-Mn alloys are systematically investigated for a large range of Mn concentrations. An unmixing tendency is clearly noted. A detailed magnetic analysis suggests the Mn-Mn magnetic interactions to be generally dominant over the Fe-Mn interactions, both exhibiting an antiferromagnetic tendency. The average magnetic moment of the Mn atoms in locally random alloys tends to be antiparallel (parallel) to lattice Fe moments for Mn concentrations smaller (larger) than approximately 6 at. %. The transition concentration is shown to be lowered if considering Mn clustering which is energetically favorable. The unsolved discrepancies between experimental and theoretical predictions on the critical concentration for the Mn magnetic behavior change in Fe-Mn solid solution are discussed in the light of the obtained results.