A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Twophoton excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene. DOI: 10.1103/PhysRevLett.97.166601 PACS numbers: 72.80.Le, 71.20.Rv, 71.35.ÿy Charge transport in organic molecular crystals is notably different from inorganic semiconductors. Despite a large number of investigations, understanding the fundamental processes governing charge transport remains a central issue for organic-based devices. In particular, the origin of band gap states and their effect on electronic and optical characteristics are both controversial and important questions [1,2]. Rubrene (C 42 H 28 ) in single crystal form has emerged as an important aromatic molecular solid, showing the highest field effect transistor (FET) mobility to date among organics (20 cm 2 =V s) [3]. In addition, several studies suggest that bandlike transport is operative [3][4][5][6]. Electronic properties in rubrene are, however, highly sensitive to both atmospheric environment as well as material treatment. As is true for inorganic semiconductors, device applications demand the understanding and control of such environmental factors, but the situation in rubrene is far from clear to date. Oxidation of organic crystals often results in modification of transport characteristics [7][8][9][10][11], and recent studies indicate that the presence of ambient oxygen also affects the conductivity of crystalline rubrene [12,13]. Furthermore, chemical analysis of naturally oxidized crystalline rubrene shows that the relative concentration of rubrene peroxide (C 42 H 28 O 2 ) can be as much as 1% at a depth of 50 nm [14], a length scale greater than a typical FET channel depth. Such a high impurity concentration will likely have a significant effect on charge transport, particularly if the impurity forms a band gap state.In this Letter, we demonstrate the formation of a band gap state in oxidized rubrene crystals and address the impact of the state on charge transport and exciton recombination. A spectral signature consistent with the molecular exciton model is established and investigated under different ambient conditions. A well-defined photoluminescence (PL) band at an energy lower than the highest exciton recombination transition energy develops in the spectra of oxidized rubrene. The PL band reflects an additional radiative recombination path through the oxygenrelated state in the band gap. The PL depth profile indicates that the oxygen-related band is emitted only from a surface region. Correlation between the ...
We present results of electronic structure and magnetization measurements of Co:ZnO and Co:ZnO codoped with Al thin-film samples fabricated by solution-based methods together with ab initio electronic structure calculations. Electronic structure measurements indicate that the Co states lie close to the valence-band edge with pinning of the Fermi level primarily due to native defects yielding a heavily n-doped material. The findings in the electronic structure measurements are corroborated by results from theoretical calculations. We find that it is necessary to go beyond the local-density approximation to achieve agreement with experiments. Moreover, the theoretical calculations indicate a tendency for the formation of Co clusters, giving rise to an antiferromagnetic exchange interaction between the Co atoms. The magnetization measurements are well in line with the theoretical predictions, showing a dominating superparamagnetic behavior arising from small antiferromagnetic clusters containing uncompensated spins.
We report on investigations of the electronic structure and magnetic properties of ZnO doped with 15 at. % Co and postgrowth annealed at temperatures ranging between 250 and 800°C. In particular, we demonstrate how the presence of Co 3+ , indicative of secondary phases, is manifested in spectroscopy. Through resonant photoemmision spectroscopy we have found that x-ray diffraction in some cases underestimates or does not reveal the presence of secondary phases, possibly due to unrelaxed structures or structural arrangements with sizes below the detection limit. The magnetic properties are in most cases understood by assuming small antiferromagnetic clusters but can also show a behavior indicative of ferromagnetic interactions.
We present an investigation of the magnetic behavior of epitaxial MnAs films grown on GaAs(100). We address the dependence of the magnetic moment, ferromagnetic transition temperature (Tc) and magnetocrystalline anisotropy constants on epitaxial conditions. From thorough structural and magnetic investigations, our findings indicate a more complex relationship between strain and magnetic properties in MnAs films than a simple stretch/compression of the unit cell axes. While a small increase is seen in the anisotropy constants the enhancement of the magnetic moment at saturation is significant. X-ray magnetic circular dichroism results show a behavior of the spinand orbital-moment which is consistent with a structural transition at Tc. In particular, we find that the ratio of the orbital to spin moment shows a marked increase in the coexistence region of the ferromagnetic α-and paramagnetic β-phases, a result that is well in accord with the observed increase of the c/a-ratio in the same temperature region. The ab initio density functional calculations reveal that the magnetic properties are more sensitive towards change in in-plane axis as compared to a change of the out-of-plane axis, which is explained by the analysis of band structures. The effects of electron correlation in MnAs using ab initio dynamical mean field theory are also presented.
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