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Register a new user3d transition metal impurities in diamond: electronic properties and chemical trends
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Added by
Joao Francisco Justo Filho
Publication date
2013
fields
Physics
and research's language is
English
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First principles calculations have been used to investigate the trends on the properties of isolated 3d transition metal impurities (from Sc to Cu) in diamond. Those impurities have small formation energies in the substitutional or double semi-vacancy sites, and large energies in the interstitial one. Going from Sc to Cu, the 3d-related energy levels in the bandgap move from the top of the bandgap toward the valence band in all three sites. Trends in electronic properties and transition energies of the impurities, in the substitutional or interstitial sites, are well described by a simple microscopic model considering the electronic occupation of the 3d-related levels. On the other hand, for the impurities in the double semi-vacancy site, there is a weak interaction between the divacancy- and the 3d-related orbitals, resulting in in vacancy- and 3d-related levels in the materials bandgap.
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We use the self-interaction corrected local spin-density approximation to investigate the ground state valency configuration of transition metal (TM = Mn, Co) impurities in n- and p-type ZnO. We find that in pure Zn1-xTMxO, the localized TM2+ configuration is energetically favored over the itinerant d-electron configuration of the local spin density (LSD) picture. Our calculations indicate furthermore that the (+/0) donor level is situated in the ZnO gap. Consequently, for n-type conditions, with the Fermi energy eF close to the conduction band minimum, TM remains in the 2+ charge state, while for p-type conditions, with eF close to the valence band maximum, the 3+ charge state is energetically preferred. In the latter scenario, modeled here by co-doping with N, the additional delocalized d-electron charge transfers into the entire states at the top of the valence band, and hole carriers will only exist, if the N concentration exceeds the TM impurity concentration.
We present a systematic investigation of molecule-metal interactions for transition-metal phthalocyanines (TMPc, with TM = Fe, Co, Ni, Cu) adsorbed on Ag(100). Scanning tunneling spectroscopy and density functional theory provide insight into the charge transfer and hybridization mechanisms of TMPc as a function of increasing occupancy of the 3d metal states. We show that all four TMPc receive approximately one electron from the substrate. Charge transfer occurs from the substrate to the molecules, inducing a charge reorganization in FePc and CoPc, while adding one electron to ligand pi-orbitals in NiPc and CuPc. This has opposite consequences on the molecular magnetic moment: in FePc and CoPc the interaction with the substrate tends to reduce the TM spin, whereas in NiPc and CuPc an additional spin is induced on the aromatic Pc ligand, leaving the TM spin unperturbed. In CuPc, the presence of both TM and ligand spins leads to a triplet ground state arising from intramolecular exchange coupling between d and pi electrons. In FePc and CoPc the magnetic moment of C and N atoms is antiparallel to that of the TM. The different character and symmetry of the frontier orbitals in the TMPc series leads to varying degrees of hybridization and correlation effects, ranging from the mixed-valence (FePc, CoPc) to the Kondo regime (NiPc, CuPc). Coherent coupling between Kondo and inelastic excitations induces finite-bias Kondo resonances involving vibrational transitions in both NiPc and CuPc and triplet-singlet transitions in CuPc.
The self-interaction corrected local spin-density approximation is used to investigate the ground-state valency configuration of transition metal (TM=Mn, Co) impurities in p-type ZnO. Based on the total energy considerations, we find a stable localised TM$^{2+}$ configuration for a TM impurity in ZnO if no additional hole donors are present. Our calculations indicate that the (+/0) donor level is situated in the band gap, as a consequence of which the TM$^{3+}$ becomes more favourable in p-type ZnO, where the Fermi level is positioned at the top of the valence band. When co-doping with N, it emerges that the carrier-mediated ferromagnetism can be realized in the scenario where the N concentration exceeds the TM impurity concentration. If TM and N concentrations are equal, the shallow acceptor levels introduced by N are fully compensated by delocalised TM d-electrons.
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