Triplet Valence States And Atomic-scale P-n Junction Characterization In Manganese Perovskite Oxide

Perovskite oxides with a general formula ABO3 (A = 12-coordinated ions and B = 6-coordinated ions) provide magical structural models for superconducting and colossal magnetoresistance . The well-known example for mixed valence creation was the formation of the CMR manganites with mixed valence of Mn3+-Mn4+ by the partial substitution of alkaline-earth metals for rare earth elements. The mixed valence of Mn ion dramatically affects the electron-electron and electron-lattice interaction according to double-exchange mechanism and thus the preparation of mixed valance oxides becomes essential. High-temperature solid state reactions, charge disproportionation and flactuation were considered to be useful, but the obtained mixed valence compounds contain only two oxidation states in a stable phase. Therefore, finding more complicated oxidation states of manganese in perovskite oxides has been a great challenge in the search for new materials with nano-scale and atomic-scale functions through specific preparations.Hydrothermal synthesis has been successfully applied to the most important crystalline materials with the advantages of producing perfect crystals and stabilizing unusual oxidation states. In current study, we conducted the hydrothermal synthesis of the manganese perovskites by the partial substitution of Ca2+ and K+ for La3+ in A-sites. We prepared a family of manganese perovskites, La1-x-yCaxKyMnO3 (x = 0.74-0.18, y = 0.01-0.14) containing La3+, Ca2+, and K+ under strong alkali conditions, where the formation of MnO2 could be avoided and Mn species with multivalent states were expected. The composition of the family analysed by inductively coupled plasma (ICP) and the energy dispersive X-ray spectroscopy (EDX) contains La, Ca and K ions. Its single crystal X-ray diffraction structural determination indicated a cubic perovskite structure. The analysis of powder X-ray diffraction,the high-resolution transmission electron microscopy (HRTEM) and select area electron diffraction(SAED) shows the complicated modulated structures in material.The Mn average valence in La1-x-yCaxKyMnO3 was measured by oxidation-reduction titration (Iodometry). The experimental average valence of 3.36 is much close to its theoretical value of 3.38. The identification of Mn5+ in our sample was made by its characteristic near infrared (IR) absorption at 625 nm due to the 3T1(t22)→1A1(t22) excitation according to Noginov, et al. Mn K-edge X-ray absorption near-edge spectroscopic (XANES) spectra for La1-x-yCaxKyMnO3, different from the known manganese perovskite oxides, evidenced an additional peak centred at 6564.3 eV, the energy of the band edge of 1s to 4p transitions for Mn5+. The same peak at ca 6565 eV was found in the K-edge XANES spectrum of the synthetic apatite, Ba5(PO4)2.5(MnO4)0.5Cl, where all of the Mn ions have +5 oxidation state, and it has been taken as a reference of Mn5+ in tetrahedral coordination. In our case, a slight decrease in the K-edge absorption energy is expected since the octahedral Mn5+ has less effectively positive charges than the tetrahedral Mn5+. We thus measured I-V curves on our single crystal by conductive atomic force microscopy (C-AFM). C-AFM is more useful to detect electric properties of small sized samples (in our case 20-30μm"cubic"crystals) than tradition method. It is very important that ohm contanct is ensured during measurement .We tested the work function of our sample. The work function of our sample is 4.9 eV, much close to that of the reference gold (5.1 eV), avoiding the Schottky effect.We repeatedly measured I-V curves on our single crystals and all measurements gave a ideal rectifying characteristics of p-n junctions on a single crystal. This ideal I-V characteristic of p-n junctions in our single crystals reveals the feature of atomic-scale p-n junctions. As we know, the two valence state manganese perovskites did not show any rectifying effect, the finding of p-n junction characteristic in our single crystal rectifier implies the contribution of three oxidation states of manganese. The I-V curve of the triplet valence manganese perovskite oxide is the same as that of the molecular rectifier theoretically calculated by Aviram and Ratner. All of the phenomena, reflected from the single crystal structure and its I-V measurement, lead to the conclusion of the existence of the atomic-scale p-n junctions in our crystal. In addition, our crystals show high thermal stability, which gave the same I-V curves after being treated at 1473 K for 10 h, showing possible high-temperature applications.We have proposed the model of atomic-scale p-n junction. In the triplet mixed valence perovskite oxide, the structural linkages such as Mn3+-O-Mn4+, Mn3+-O-Mn4+-O-Mn5+ and even Mn3+-O-Mn5+ are expected according to its structure and composition. Among them, the most possible array is Mn3+-O-Mn4+-O-Mn5+, which gives rise to the architecture of an atomic-scale p-n junction, similar to a macroscopic p-n junction in a semiconductor. In the atomic-scale p-n junction, Mn3+ (t2g3eg1) and Mn5+ (t2g2eg0) in the octahedral site symmetry may serve as a donor and an acceptor, respectively through the localized Mn4+ (t2g3eg0). A built-in field may be set up by eg electron tunnelling from Mn3+ (t2g3eg1, donor) to Mn5+ (t2g2eg0, acceptor) over an energy barrier. The above I-V measurements on the single crystals strongly support the arrangement of atomic-scale p-n junction. When a forward bias is applied to a p-n junction, the potential barrier is reduced if the p-side is made positive. It is fairly obvious that the number of electrons flowing from p-side to n-side is not affected in either case, but the flow of electrons from n-side to p-side is seriously affected. So we can see qualitatively that the total current flowing at a positive voltage will differ from the current flowing at a negative voltage. Our model may provide a basis for further theoretical study on atomic-scale p-n junctions.Conventionally it is possible to turn a crystal such as pure silicon into a moderately good semiconductor by adding impurity phosphorus for an n-type semiconductor or gallium for a p-type semiconductor. Useful applications start to happen only when a single semiconductor crystal contains both n-type and p-type regions, forming a macroscopic p-n junction. Moreover, since the silicon with a bandgap of 1.12 eV can effectively use only the wavelengths in the range of ca 0.4-1.1μm, more complex photovoltaic devices with multi-junctions can use more of the spectrum and should be operated at high efficiencies than single-junction devices. In our case, as the junctions may be connected in series, the switching voltage of each junction must match; hence the device switching voltage will be determined by the junction numbers. Although some of the individual molecules of the type donor-spacer-acceptor between two electrodes would behave as molecular rectifiers under an electrical voltage bias, our current study provides the potential applications of atomic-scale p-n junctions operated on a macroscopic crystal. The ready operation on macroscopic crystals with microscopically electric features guides the future trend for applications and fundamental study.Our results present clearly the unique features of the atomic-scale p-n junctions based on the triplet valence states of manganese in the perovskite oxide. The atomic-scale p-n junctions of the single crystals naturally formed from hydrothermal systems may serve as a variety of electric devices. Moreover, they may lead to entirely new atomic-scale devices with potential applications to the atomic-effective processes on the photoelectric conversion and the quantum information. This study has experimentally revealed fundamentally extraordinary phenomena of the triplet valence states for manganese, opening up space for the synthesis of other elements with complicated mixed oxidation states.