Graphitic Silicon Nitride: A Metal‐Free Ferromagnet with Charge and Spin Current Rectification

As a first example, herein we show that g‐Si₄N₃ is expected to act as a metal‐free ferromagnet featuring both charge and spin current rectification simultaneously. Such rectification is crucial for envisioning devices that contain both logic and memory functionality on a single chip. The spin coherent quantum‐transport calculations on g‐Si₄N₃ reveal that the chosen system is a unique molecular spin filter, the current‐voltage characteristics of which is asymmetric in nature, which can create a perfect background for synchronous charge and spin current rectification. To shed light on this highly unusual in‐silico observation, we have meticulously inspected the bias‐dependent modulation of the spin‐polarized eigenstates. The results indicate that, whereas only the localized 2p orbitals of the outer‐ring (OR) Si atoms participate in the transmission process in the positive bias, both OR Si and N atoms contribute in the reverse bias. Furthermore, we have evaluated the spin‐polarized electron‐transfer rate in the tunneling regime, and the results demonstrate that the transfer rates are unequal in the positive and negative bias range, leading to the possible realization of a simultaneous logic–memory device.     

Atomic‐Level Elucidation of the Initial Stages of Self‐Assembled Monolayer Metallization and Nanoparticle Formation

The development of high‐performance molecular electronics and nanotech applications requires deep understanding of atomic level structural, electronic, and magnetic properties of electrode/molecular interfaces. Recent electrochemical experiments on self‐assembled monolayers (SAMs) have identified highly practical means to generate nanoparticles and metal monolayers suspended above substrate surfaces through SAM metallizations. A rational basis why this process is even possible is not yet well‐understood. To clarify the initial stages of interface formation during SAM metallization, we used first‐principles spin‐polarized density functional theory (DFT) calculations to study Pd diffusion on top of 4‐mercaptopyridine (4MP) SAMs on Au(111). After distinguishing potential‐energy surfaces (PESs) for different spin configurations for transition metal atoms on the SAM, we find adatom diffusion is not possible over the clean 4MP–SAM surface. Pre‐adsorption of transition‐metal atoms, however, facilitates atomic diffusion that appears to explain multiple reports on experimentally observed island and monolayer formation on top of SAMs. Furthermore, these diffusions most likely occur by moving across low‐lying and intersecting PESs of different spin states, opening the possibility of magnetic control over these systems. Vertical diffusion processes were also investigated, and the electrolyte was found to play a key role in preventing metal permeation through the SAM to the substrate.     

Spin-Crossover Assisted Spin-Switching and Rectification Action in Half-Metallic Graphitic Carbon Nitride(g-C4N3)

Herein, we report on the potential multifunctional spintronic action of half-metallic graphitic carbon nitride (g-C₄N₃). We observed electrostatic spin-crossover action at an applied electric field of −0.77 V nm⁻¹, which eventually leads to spin-switching action and change in sign of bias dependent spin injection coefficient. The system also acts as a spin polarized charge current rectifier with rectification ratio of 65.41 in spin-up channel only. This electric field-controlled spin switching action and simultaneous existence of rectification action makes graphitic carbon nitride a perfect multifunctional spintronic system-an ideal material for quantum logic gate design. Results obtained have been substantiated through transmission spectra and transmission pathways analyses. An analysis of projected device density of states of the system and molecular projected self consistent Hamiltonian states analysis reveals that the electron flow of the system is mainly facilitated by 2p orbitals of C and N atoms.     

Computational Design of One‐Dimensional Ferromagnetic Semiconductors in Transition Metal Embedded Stannaspherene Nanowires

Developing low dimensional semiconductors with moderate band gaps, intrinsic ferromagnetism and large magnetic anisotropy energies (MAEs) is very desirable for high‐speed nano‐spintronic devices, which, however, still remains a big challenge. Here, via first principles calculations, a potential route to realize such materials is proposed based on a new class of one‐dimensional transition metal (TM) embedded stannaspherene (Sn₁₂^2–) nanowires [TM₂(Sn₁₂)]_∞ (TM = Ti‐Ni). Three semiconductors with robust ferromagnetism are achieved with TM = V, Cr and Fe, which all exhibit direct or quasi‐direct band gaps around 1.0 eV, rendering their great potentials for visible light optoelectronic applications. Interestingly, [Cr₂(Sn₁₂)]_∞ and [Fe₂(Sn₁₂)]_∞ are both identified as bipolar magnetic semiconductors (BMS) with valence and conduction band edges spin polarized in the opposite directions, which are promising for realizing switch of carriers’ spin orientation by electrical gating, while [V₂(Sn₁₂)]_∞ exhibits a half semiconductor (HSC) property with valence and conduction band edges spin polarized in the same direction and can be used for spin‐polarized carriers generation. Moreover, sizable MAEs are discovered in these nanowires, which are at least two orders of magnitude larger than those of Fe, Co and Ni bulks and also significantly larger than current ferromagnetic semiconductors.     

Curvature dependence of the metal catalyst atom interaction with carbon nanotubes walls

Interactions of the transition metal atoms with carbon nanotube walls are investigated using a tight-binding molecular dynamics method that allows for spin unrestricted geometry optimization. Comparison with the results for bonding on graphite indicates major differences in bonding sites, magnetic moments and the direction of charge transfer. The significant values of magnetic moments obtained for the metal atoms on nanotube walls is consistent with the recent experimental findings.     

Spin transport properties of single metallocene molecules attached to single-walled carbon nanotubes via nickel adatoms

The spin-dependent transport properties of single ferrocene, cobaltocene, and nickelocene molecules attached to the sidewall of a (4,4) armchair single-walled carbon nanotube via a Ni adatom are investigated by using a self-consistent ab initio approach that combines the non-equilibrium Green's function formalism with the spin density functional theory. Our calculations show that the Ni adatom not only binds strongly to the sidewall of the nanotube, but also maintains the spin degeneracy and affects little the transmission around the Fermi level. When the Ni adatom further binds to a metallocene molecule, its density of states is modulated by that of the molecule and electron scattering takes place in the nanotube. In particular, we find that for both cobaltocene and nickelocene the transport across the nanotube becomes spin-polarized. This demonstrates that metallocene molecules and carbon nanotubes can become a promising materials platform for applications in molecular spintronics.     

Exciton spin relaxation in colloidal CdSe quantum dots at room temperature

Size dependence of spin dynamics in colloidal CdSe quantum dots (QDs) are investigated with circularly polarized pump-probe transmission spectroscopy at room temperature. The excitation energy is tuned to resonance with the lowest exciton (1S(h)1S(e)) energy of the CdSe QDs. The exciton spin dynamics of CdSe QD with the diameter of 5.2 nm shows monoexponential decay with a typical time constant of about 1-3 ps depending on the excitation energy. For the cases of CdSe QDs with smaller size (with the diameter of 4.0 and 2.4 nm), the exciton spin relaxation shows biexponential decay, a fast component with time constant of several ps and a slow one with time constant of hundreds of ps to nanosecond time scale. The fast spin relaxation arises from the bright-dark transition, i.e., J = ±1 ? -/+2 transition. This process is dominated by the hole spin flips, while the electron spin conserves. The slow spin relaxation is attributed to the intralevel exciton transitions (J = ±1 ? -/+1 transition), which is relevant to the electron spin flip. Our results indicate that the exciton spin relaxation pathways in CdSe QD are controllable by monitoring the particle size, and polarized pump-probe spectroscopy is proved to be a sensitive method to probe the exciton transition among the fine structures.     

Spin uncoupling in ethylene activation by palladium and platinum atoms

Spin‐dependent effects in complex formation reactions of the ethylene molecule with palladium and platinum atoms were studied by electron correlation calculations with account of spin–orbit coupling. Simple correlation diagrams illustrating spin‐uncoupling mechanisms were obtained, showing that the low spin state of the transition‐metal atom or the transition‐metal atom complex is always more reactive than are the high spin states because of the involvement of the triplet excited molecule in the chemical activation. Spin–orbit coupling calculations of the reaction between a platinum atom and ethylene explain the high‐spin Pt(³D) reactivity as due to an effective spin flip at the stage of the weak triplet complex formation. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 581–596, 1999     

Spin polarized metastable helium de-excitation processes on metal surfaces

Spin polarized de-excitation of a metastable helium atom interacting with metal surfaces is treated within density functional theory. The method is based on a self-consistent calculation of the spin dependent electronic properties of the system, such as the surface density of states and the localized surface states, to compute the transition rate. On the high work function Ag(100) and Ag(111) surfaces, the helium 2s electron is delocalized in the metal and hence the transition rate is weakly spin dependent. The existence of a Shockley surface state in Ag(111) determines a neutralization rate that is about 59% larger than that from Ag(100). On a low work function metal, namely Na(100), the rate is of smaller magnitude than those on silver because the 2s triplet resonance is found to be more occupied. Consequently, emitted electrons can display a strong spin dependence also for a paramagnetic surface.     

π-Electron-Assisted Relaxation of Spin Excited States in Cobalt Phthalocyanine Molecules on Au(111) Surface

We present our investigation on the spin relaxation of cobalt phthalocyanine (CoPc) films on Au(111) (CoPc/Au(111)) surface using scanning tunneling microscopy and spectroscopy. The spin relaxation time derived from the linewidth of spin-flip inelastic electron tunneling spectroscopy is quantitatively analyzed according to the Korringa-like formula. We find that although this regime of the spin relaxation time calculation by just considering the exchange interaction between itinerant conduction electrons and localized d-shells (s-d exchange interaction) can successfully reproduce the experimental value of the adsorbed magnetic atom, it fails in our case of CoPc/Au(111). Instead, we can obtain the relaxation time that is in good agreement with the experimental result by considering the fact that the π electrons in CoPc molecules are spin polarized, where the spin polarized π electrons extended at the Pc macrocycle may also scatter the conduction electrons in addition to the localized d spins. Our analyses indicate that the scattering by the π electrons provides an efficient spin relaxation channel in addition to the s-d interaction and thus leads to much short relaxation time in such a kind of molecular system on a metal substrate.     

Low- and high-spin iron (II) complexes studied by effective crystal field method combined with molecular mechanics

A computational method targeted to Werner-type complexes is developed on the basis of quantum mechanical effective Hamiltonian crystal field (EHCF) methodology (previously proposed for describing electronic structure of transition metal complexes) combined with the Gillespie-Kepert version of molecular mechanics (MM). It is a special version of the hybrid quantum/MM approach. The MM part is responsible for representing the whole molecule, including ligand atoms and metal ion coordination sphere, but leaving out the effects of the d-shell. The quantum mechanical EHCF part is limited to the metal ion d-shell. The method reproduces with reasonable accuracy geometry and spin states of the Fe(II) complexes with monodentate and polydentate aromatic ligands with nitrogen donor atoms. In this setting a single set of MM parameters set is shown to be sufficient for handling all spin states of the complexes under consideration.     

Acetylene cyclotrimerization by early second-row transition metals in the gas phase. A theoretical study

The acetylene cyclotrimerization reaction mediated by the left-hand-side bare transition metal atoms Y, Zr, Nb, and Mo has been studied theoretically, employing DFT in its B3LYP formulation. The complete reaction mechanism has been analyzed, identifying intermediates and transition states. Both the ground spin state and at least one low-lying excited state have been considered to establish whether possible spin crossings between surfaces of different multiplicity can occur. Our results show that the overall reaction is highly favorable from a thermodynamic point of view and ground state transition states lie always below the energy limit represented by ground state reactants. After the activation of two acetylene molecules and formation of a bis-ligated complex, the reaction proceeds to give a metallacycle intermediate, as the alternative formation of a cyclobutadiene complex is energetically disfavored. All the examined reaction paths involve formation of a metallacycloheptatriene intermediate that in turn generates a metal-benzene adduct from which finally benzene is released. Similarities and differences in the behaviors of the considered four metal atoms have been examined.     

Spin-Phonon Coupling and Slow-Magnetic Relaxation in Pristine Ferrocenium

We report the spin dynamic properties of non-substituted ferrocenium complexes. Ferrocenium shows a field-induced single-molecule magnet behaviour in DMF solution while cobaltocene lacks slow spin relaxation neither in powder nor in solution. Multireference quantum mechanical calculations give a non-Aufbau orbital occupation for ferrocenium with small first excitation energy that agrees with the relatively large measured magnetic anisotropy for a transition metal S=1/2 system. The analysis of the spin relaxation shows an important participation of quantum tunnelling, Raman, direct and local-mode mechanisms which depend on temperature and the external field conditions. The calculation of spin-phonon coupling constants for the vibrational modes shows that the first vibrational mode, despite having a low spin-phonon constant, is the most efficient process for the spin relaxation at low temperatures. In such conditions, vibrational modes with higher spin-phonon coupling constants are not populated. Additionally, the vibrational energy of this first mode is in excellent agreement with the experimental fitted value obtained from the local-mode mechanism.     

Charge transfer and formation of reduced Ce3+ upon adsorption of metal atoms at the ceria (110) surface

The modification of cerium dioxide with nanoscale metal clusters is intensely researched for catalysis applications, with gold, silver, and copper having been particularly well studied. The interaction of the metal cluster with ceria is driven principally by a localised interaction between a small number of metal atoms (as small as one) and the surface and understanding the fundamentals of the interaction of metal atoms with ceria surfaces is therefore of great interest. Much attention has been focused on the interaction of metals with the (111) surface of ceria, since this is the most stable surface and can be grown as films, which are probed experimentally. However, nanostructures exposing other surfaces such as (110) show high activity for reactions including CO oxidation and require further study; these nanostructures could be modified by deposition of metal atoms or small clusters, but there is no information to date on the atomic level details of metal-ceria interactions involving the (110) surface. This paper presents the results of density functional theory (DFT) corrected for on-site Coulomb interactions (DFT+U) calculations of the adsorption of a number of different metal atoms at an extended ceria (110) surface; the metals are Au, Ag, Cu, Al, Ga, In, La, Ce, V, Cr, and Fe. Upon adsorption all metals are oxidised, transferring electron(s) to the surface, resulting in localised surface distortions. The precise details depend on the identity of the metal atom. Au, Ag, Cu each transfer one electron to the surface, reducing one Ce ion to Ce(3+), while of the trivalent metals, Al and La are fully oxidised, but Ga and In are only partially oxidised. Ce and the transition metals are also partially oxidised, with the number of reduced Ce ions possible in this surface no more than three per adsorbed metal atom. The predicted oxidation states of the adsorbed metal atoms should be testable in experiments on ceria nanostructures modified with metal atoms.     

Spectroscopic and computational investigations of stable radical anions of triosmium benzoheterocycle clusters

The variable temperature (1)H and (13)C NMR and EPR spectra of the stable radical anions [Os(3)(CO)(9)(micro(3)-eta(2)-L)(micro-H)] (LH=phenanthridine, 1; 5,6-benzoquinoline, 2), and [Os(3)(CO)(10)(micro(3)-eta(2)-L)(micro-H)] (LH=quinoxaline, 3) are reported. The radical anions 1(-), 2(-), and 3(-) can be prepared by both exhaustive electrolysis and partially by chemical reduction with cobaltocene and with sodium dispersion (only with sodium dispersion in the case of 3(-)). DFT calculations on 1-3 reveal that the LUMO for the electron-deficient compounds 1 and 2 involves significant contributions from both the heterocyclic ligand and the two metal atoms bridged by the ligand and the micro-hydride. The character of this orbital rationalizes the previously observed regioselective reactions of these complexes with nucleophiles. In contrast, the LUMO for the electron precise 3 involves only ligand-based orbitals. Partial chemical reduction of 1 and 2 requires an excess of either cobaltocene or sodium, and their (1)H and (13)C NMR spectra reveal selective line broadening of those proton resonances that are predicted by DFT calculations to bear the greatest amount of free spin density. The variable temperature behavior of the partially chemically reduced species of 1 and 2 indicates that electron transfer between the reduced/unreduced cluster pair and between the cobaltocene/cobaltocenium pair occurs on the NMR timescale. The radical anions of 1 and 2 prepared by exhaustive electrolysis show an EPR signal at room temperature, while the NMR signals are uniformly broadened. Compound 3 appears to be partially reduced by sodium at room temperature and shows uniformly broadened (1)H NMR resonances at room temperature that sharpen significantly at -80 degrees C. The temperature dependence of the spectra are discussed in terms of the effects of relative electron nuclear relaxation processes, chemical exchange, and the results of the DFT calculations.     

UV Resonance Raman Spectroscopic Identification of Titanium Atoms in the Framework of TS-1 Zeolite

Framework titanium atoms in titanium-substituted silicalite (TS-1) can be identified by UV resonance Raman spectroscopy since the associated Raman bands at 1125, 530, and 490 cm⁻¹ (see figure) are observed only when the charge transfer transition associated with the framework Ti atoms is excited by a UV laser. Thus, framework Ti atoms can be distinguished from nonframework Ti atoms and other defect sites. This method can be applicable to identifying transition metal atoms in the frameworks of other molecular sieves.     

Electronic and magnetic properties of substituted BN sheets: a density functional theory study

Using density functional calculations, we investigate the geometries, electronic structures and magnetic properties of hexagonal BN sheets with 3d transition metal (TM) and nonmetal atoms embedded in three types of vacancies: V(B), V(N), and V(B+N). We show that some embedded configurations, except TM atoms in V(N) vacancy, are stable in BN sheets and yield interesting phenomena. For instance, the band gaps and magnetic moments of BN sheets can be tuned depending on the embedded dopant species and vacancy type. In particular, embedment such as Cr in V(B+N), Co in V(B), and Ni in V(B) leads to half-metallic BN sheets interesting for spin filter applications. From the investigation of Mn-chain (C(Mn)) embedments, a regular 1D structure can be formed in BN sheets as an electron waveguide, a metal nanometre wire with a single atom thickness.     

Imaging of tautomerism in a single molecule

Fluorescence imaging is used to visualize directly the transfer of two inner hydrogen atoms in single porphycene molecules. This reaction leads to a chemically equivalent but differently oriented structure and hence results in a rotation of the transition dipole moments. By probing single immobilized molecules with an azimuthally polarized laser beam in the focal spot of a confocal microscope we observe ring-like emission patterns, possible only for a chromophore with two nearly orthogonal transition dipole moments. Numerical simulations of the observed emission patterns yield a value of 72 degrees for the angle between the S0-S1 transition moments in the two tautomeric forms.     

Mössbauer studies of Fe x Zn1−x (4-amino-1,2,4-triazole)3(NO3)2 complexes possessing the1 A 1⇄5 T 2 spin transition

Mössbauer studies were carried out for the mixed complexes Fe_xZn_1?x(ATr)₃(NO₃)₂ (0.2≤x≤1) having a polynuclear chain structure, for which we had earlier found a significant decrease in the¹A₁ ⁵T₂ spin transition temperature when iron was replaced with zinc. For these complexes, we have found for the first time a tendency toward an increase in the chemical shifts and quadrupole splitting of iron atoms in the low-spin state with the degree of their replacement by other metal atoms. A correlation between these Mössbauer spectral data and the spin transition temperature was found. The results of these studies are explained in terms of the model of steric strains in molecular fragments of the chain structure of the complexes appearing when iron atoms are replaced by zinc atoms.     

Anionogenic ferromagnets

Magnetism in molecules and solids is understood to originate from atoms in that part of the periodic table where a particular value of the angular momentum appears first (i.e., the 2p, 3d, and 4f series). In contrast to the many magnetic compounds containing transition metal or lanthanide atoms, ferromagnetism based on atoms from the 2p series is very rare. We report density functional calculations that show the existing compound rubidium sesquioxide is a ferromagnet with an estimated Curie temperature of 300 K, unprecedented in p-electron magnetism. The magnetic moment is carried by the anion. Rubidium sesquioxide is a conductor, but only for the minority spin electrons (a so-called "half-metal"). Half-metals play an important role in spintronics, that is, electronics that exploits the electron spin. Since the magnetic moment resides on a light element (oxygen), spin-orbit interactions are considerably reduced compared to other half-metals. Consequently spin relaxation is expected to be suppressed by up to 2 orders of magnitude in comparison with materials presently used in spintronics.