A Synchrotron Radiation Photoemission Study of SiGe(001)-2×1 Grown on Ge and Si Substrates: The Surface Electronic Structure for Various Ge Concentrations.

Beyond the macroscopic perspective, this study microscopically investigates Si1-xGex(001)-2×1 samples that were grown on the epi Ge(001) and epi Si(001) substrates via molecular-beam epitaxy, using the high-resolution synchrotron radiation photoelectron spectroscopy (SRPES) as a probe. The low-energy electron diffraction equipped in the SRPES chamber showed 2×1 double-domain reconstruction. Analyses of the Ge 3d core-level spectra acquired using different photon energies and emission angles consistently reveal the ordered spots to be in a Ge-Ge tilted configuration, which is similar to that in epi Ge(001)-2×1. It was further found that the subsurface layer was actually dominated by Ge, which supported the buckled configuration. The Si atoms were first found in the third surface layer. These Si atoms were further divided into two parts, one underneath the Ge-Ge dimer and one between the dimer row. The distinct energy positions of the Si 2p core-level spectrum were caused by stresses, not by charge alternations.

Optomechanical switching of adsorption configurations of polar organic molecules by UV radiation pressure.

Using photoemission spectroscopy (PES), we have systematically investigated the behavior of polar organic molecule, chloroaluminum phthalocyanine (ClAlPc), adsorbed in the Cl-down configuration on the Ag(111) substrate at low temperature - 195 °C under UV irradiation with a range of different photon fluxes. Judging from the evolution of photoemission spectral line shapes of molecular energy states, we discovered that the Cl atoms are so robustly anchored at Ag(111) that the impinging photons cannot flip the ClAlPc molecules, but instead they crouch them down due to radiation pressure; we observe that the phthalocyanine (Pc) lobes bend down to interact with Ag atoms on the substrate and induce charge transfer from them. As photon flux is increased, radiation pressure on the Pc plane initiates tunneling of the Cl atom through the molecular plane to turn the adsorption configuration of ClAlPc from Cl-down to an upheld Cl-up configuration, elucidating an optomechanical way of manipulating the dipole direction of polar molecules. Finally, work function measurements provide a distinct signature of the resulting upheld Cl-up configuration as it leads to a large increase in vacuum level (VL), ~ 0.4 eV higher than that of a typical flat-on Cl-up configuration driven by thermal annealing.

Microscopic Views of Atomic and Molecular Oxygen Bonding with epi Ge(001)-2 × 1 Studied by High-Resolution Synchrotron Radiation Photoemission.

In this paper, we investigate the embryonic stage of oxidation of an epi Ge(001)-2 × 1 by atomic oxygen and molecular O2 via synchrotron radiation photoemission. The topmost buckled surface with the up- and down-dimer atoms, and the first subsurface layer behaves distinctly from the bulk by exhibiting surface core-level shifts in the Ge 3d core-level spectrum. The O2 molecules become dissociated upon reaching the epi Ge(001)-2 × 1 surface. One of the O atoms removes the up-dimer atom and the other bonds with the underneath Ge atom in the subsurface layer. Atomic oxygen preferentially adsorbed on the epi Ge(001)-2 ×1 in between the up-dimer atoms and the underneath subsurface atoms, without affecting the down-dimer atoms. The electronic environment of the O-affiliated Ge up-dimer atoms becomes similar to that of the down-dimer atoms. They both exhibit an enrichment in charge, where the subsurface of the Ge layer is maintained in a charge-deficient state. The dipole moment that was originally generated in the buckled reconstruction no longer exists, thereby resulting in a decrease in the ionization potential. The down-dimer Ge atoms and the back-bonded subsurface atoms remain inert to atomic O and molecular O2, which might account for the low reliability in the Ge-related metal-oxide-semiconductor (MOS) devices.

Atomic Nature of the Growth Mechanism of Atomic Layer Deposited High-κ Y2O3 on GaAs(001)-4 × 6 Based on in Situ Synchrotron Radiation Photoelectron Spectroscopy.

Y2O3 was in situ deposited on a freshly grown molecular beam epitaxy GaAs(001)-4 × 6 surface by atomic layer deposition (ALD). In situ synchrotron radiation photoemission was used to study the mechanism of the tris(ethylcyclopentadienyl)yttrium [Y(CpEt)3] and H2O process. The exponential attenuation of Ga 3d photoelectrons confirmed the laminar growth of ALD-Y2O3. In the embryo stage of the first ALD half-cycle with only Y(CpEt)3, the precursors reside on the faulted As atoms and undergo a charge transfer to the bonded As atoms. The subsequent ALD half-cycle of H2O molecules removes the bonded As atoms, and the oxygen atoms bond with the underneath Ga atoms. The product of a line of Ga-O-Y bonds stabilizes the Y2O3 films on the GaAs substrate. The resulting coordinatively unsaturated Y-O pairs of Y2O3 open the next ALD series. The absence of Ga2O3, As2O3, and As2O5 states may play an important role in the attainment of low interfacial trap densities (D it) of <1012 cm-2 eV-1 in our established reports.

Successive Phase Transitions in Fe2+ Ladder Compounds Sr2Fe3Ch2O3 (Ch = S, Se).

Small single crystals of Sr2Fe3Ch2O3 (Ch = S, Se) have been synthesized by flux methods, and bulk materials have been obtained by solid state reactions. Both compounds are isostructural to the compound Sr2Co3S2O3 (space group Pbam), which contains a novel hybrid spin ladder: a combination of a 2-leg rectangular ladder and a necklace ladder. The 2-leg ladder acts as a well-defined magnetic entity, while intimate magnetic coupling to the necklace ladder induces three successive phase transitions in the range of 40-120 K in each composition (Ch = S or Se), as revealed by Mössbauer spectroscopy, thermodynamics, and magnetometry. The complex magnetic behaviors can be explained by the unique spin-lattice topology.

Canted Antiferromagnetism on Rectangular Layers of Fe2+ in Polymorphic CaFeSeO.

From stoichiometric amounts of CaO, Fe, and Se, pure powders and single crystals of quaternary [Formula: see text] can be obtained by solid-state reaction and self-flux growth, respectively. The as-synthesized compound exhibits a polymorphic crystal structure, where the two modifications have different stacking sequences of [Formula: see text] layers. The two polymorphs have similar unit cells but different crystal symmetries (Cmc21 and Pnma), of which the former is non-centrosymmetric. Fe is divalent (d6) and high-spin, as proven by X-ray spectroscopy, Mössbauer spectroscopy, and powder neutron diffraction data. The latter two, in combination with magnetic susceptibility and specific heat data, reveal a long-range antiferromagnetic spin order (TN = 160 K) with a minor spin canting. CaFeSeO is an electronic insulator, as confirmed by resistivity measurements and density functional theory calculations. The latter also suggest a relatively small energy difference between the two polymorphs, explaining their intimate intergrowth.

Influence of Fe substitution on the Jahn-Teller distortion and orbital anisotropy in orthorhombic Y(Mn1-xFex)O3 epitaxial films.

Multiferroic YMn1-xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn-Teller effect of Mn(3+) on Fe substitution and also the local octahedral-site distortion of Fe(3+) in an environment of Jahn-Teller-active Mn(3+) ions in YMn1-xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn(3+) ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y(2)-r(2) orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe(3+) ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe(3+) ion inside the YMnO3 lattice was similar to that of the Mn(3+) ion, whereas the Jahn-Teller distortion and GdFeO3-type distortion of the Mn(3+) ion were decreased by the spherical high-spin Fe(3+) ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn-Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1-xFexO3 thin films.

[Cs6 Cl][Fe24 Se26 ]: A Host-Guest Compound with Unique Fe-Se Topology.

The novel host-guest compound [Cs6Cl][Fe24Se26] (I4/mmm; a=11.0991(9), c=22.143(2) Å) was obtained by reacting Cs2Se,CsCl, Fe, and Se in closed ampoules. This is the first member of a family of compounds with unique Fe-Se topology, which consists of edge-sharing, extended fused cubane [Fe8Se6Se8/3] blocks that host a guest complex ion, [Cs6Cl](5+). Thus Fe is tetrahedrally coordinated and divalent with strong exchange couplings, which results in an ordered antiferromagnetic state below TN =221 K. At low temperatures, a distribution of hyperfine fields in the Mössbauer spectra suggests a structural distortion or a complex spin structure. With its strong Fe-Se covalency, the compound is close to electronic itinerancy and is, therefore, prone to exhibit tunable properties.

Graphene Anodes and Cathodes: Tuning the Work Function of Graphene by Nearly 2 eV with an Aqueous Intercalation Process.

To expand the applications of graphene in optoelectronics and microelectronics, simple and effective doping processes need to be developed. In this paper, we demonstrate an aqueous process that can simultaneously transfer chemical vapor deposition grown graphene from Cu to other substrates and produce stacked graphene/dopant intercalation films with tunable work functions, which differs significantly from conventional doping methods using vacuum evaporation or spin-coating processes. The work function of graphene layers can be tuned from 3.25 to 5.10 eV, which practically covers the wide range of the anode and cathode applications. Doped graphene films in intercalation structures also exhibit excellent transparency and low resistance. The polymer-based solar cells with either low work function graphene as cathodes or high work function graphene as anodes are demonstrated.

Synthesis and Characterization of Ba[CoSO]: Magnetic Complexity in the Presence of Chalcogen Ordering.

Barium thio-oxocobaltate(II), Ba[CoS2/2 O2/2 ], was synthesized by the reaction of equimolar amounts of BaO, Co, and S in closed silica ampoules. The title compound (Cmcm, a=3.98808(3), b=12.75518(9), c=6.10697(4) Å) is isostructural to Ba[ZnSO]. The use of soft X-ray absorption spectroscopy confirmed that cobalt is in the oxidation state +2 and tetrahedrally coordinated. Its coordination consists of two sulfur and two oxygen atoms in an ordered fashion. High-temperature magnetic susceptibility data indicate strong low-dimensional spin-spin interactions, which are suggested to be closely related to the layer-type crystal structure and perhaps the ordered distribution of sulfur and oxygen. Antiferromagnetic ordering below TN =222 K is observed as an anomaly in the specific heat, coinciding with a significant lowering of the magnetic susceptibility. Density functional theory calculations within a generalized-gradient approximation (GGA)+U approach identify an antiferromagnetic ground state within the square-like two-dimensional layers of Co, and antiferromagnetic correlations for nearest and next nearest neighbors along bonds mediated by oxygen or sulfur. However, this magnetic state is subject to frustration by relatively strong interlayer couplings.

Using lithium carbonate-based electron injection structures in high-performance inverted organic light-emitting diodes.

A lithium carbonate-based bi-layered electron injection layer was introduced into inverted organic light-emitting diodes (OLEDs) to reduce operation voltages and achieve carrier balance. Ultraviolet photoemission spectroscopy was used to confirm the existence of an interfacial dipole between the organic and lithium carbonate layers, which is a dominating factor related to the device performance. The respective maximum efficiencies of 15.9%, 16.9%, and 8.4% were achieved for blue, green, and red phosphorescent inverted OLEDs with identical architectures, indicating that carrier balance was easily obtained. Moreover, adoption of this sophisticated electron injection layer design resulted in respective turn on voltages of only 3.4 V, 3.2 V, and 3.2 V. Furthermore, the inverted OLEDs equipped with silicon dioxide nanoparticle based light-extraction films achieved an approximately 1.3 fold efficiency improvement over pristine devices due to the low refractive index of the silicon dioxide nanoparticles along with an effective scattering function. The blue, green, and red inverted OLEDs with the nanocomposite layer achieved respective peak efficiencies of 20.9%, 21.3%, and 10.1%.

Ba3 v2 s4 o3 : a mott insulating frustrated quasi-one-dimensional s=1 magnet.

Through a solid-state reaction, a practically phase pure powder of Ba3 V2 S4 O3 was obtained. The crystal structure was confirmed by X-ray single-crystal and synchrotron X-ray powder diffraction (P63 , a=10.1620(2), c=5.93212(1) Å). X-ray absorption spectroscopy, in conjunction with multiplet calculations, clearly describes the vanadium in charge-disproportionated V(III) S6 and V(V) SO3 coordinations. The compound is shown to be a strongly correlated Mott insulator, which contradicts previous predictions. Magnetic and specific heat measurements suggest dominant antiferromagnetic spin interactions concomitant with a weak residual ferromagnetic component, and that intrinsic geometric frustration prevents long-range order from evolving.

Oxygen-driven competition between low-dimensional structures of Sr3CoMO6 and Sr3CoMO7-δ with M = Ru, Ir.

We have realized a reversible structure transformation of one-dimensional 1D K4CdCl6-type Sr3CoMO6 with the Co(2+)/M(4+) cation ordering into the two-dimensional 2D double layered Ruddlesden-Popper structure Sr3CoMO7-δ with a random distribution of Co and M (with M = Ru, Ir) upon increasing the partial oxygen pressure. The combined soft and hard X-ray absorption spectroscopy studies show that under transformation, Co and M cations were oxidized to Co(3+) and M(5+). During oxidation, high-spin Co(2+) in Sr3CoMO6 first transforms into high-spin Co(3+) in oxygen-deficient Sr3CoMO7-δ, and then further transforms into low-spin Co(3+) in fully oxidized Sr3CoMO7 upon further increasing the partial pressure of oxygen. The 1D Sr3CoMO6 compound is magnetically ordered at low temperatures with the magnetic moments lying along the c-axis. Their alignment is parallel for Sr3CoRuO6 and antiparallel for Sr3CoIrO6. The 2D compounds reveal a spin-glass-like behavior related to the random distribution of magnetic cations in the structure.

Tuning gap states at organic-metal interfaces via quantum size effects.

Organic-metal interfaces are key elements in organic-based electronics. The energy-level alignment between the metal Fermi level and the molecular orbital levels determines the injection barriers for the charge carriers at the interfaces, which are crucial for the performance of organic electronic devices. Dipole formation at the interfaces has been regarded as the main factor that affects the energy-level alignment. Several models have been proposed for the mechanism of dipole formation in the context of the interface between organic molecules and a bulk metal crystal surface, at which surface states were mostly used to probe the interfacial properties. Here we report that when the bulk metal crystal is replaced by a uniform metal thin film, the resulting two-dimensional quantum-well states will be able to not only probe but also modify the interfacial electronic structures, such as gap states, that have no counterpart at the organic-bulk crystal interface.

High-efficiency small-molecule-based organic light emitting devices with solution processes and oxadiazole-based electron transport materials.

We demonstrate high-efficiency small-molecule-based white phosphorescent organic light emitting diodes (PHOLEDs) by single-active-layer solution-based processes with the current efficiency of 17.3 cdA(-1) and maximum luminous efficiency of 8.86 lmW(-1) at a current density of 1 mA cm(-2). The small-molecule based emitting layers are codoped with blue and orange phosphorescent dyes. We show that the presence of CsF/Al at cathodes not only improves electron transport in oxadiazole-containing electron transport layers (ETLs), but also facilitates electron injection through the reacted oxadiazole moiety to reduce interface resistance, which results in the enhancement of current efficiency. By selecting oxadiazole-based materials as ETLs with proper electron injection layer (EIL)/cathode structures, the brightness and efficiency of white PHOLEDs are significantly improved.

Atom-to-atom interactions for atomic layer deposition of trimethylaluminum on Ga-rich GaAs(001)-4 × 6 and As-rich GaAs(001)-2 × 4 surfaces: a synchrotron radiation photoemission study.

High-resolution synchrotron radiation photoemission was employed to study the effects of atomic-layer-deposited trimethylaluminum (TMA) and water on Ga-rich GaAs(001)-4 × 6 and As-rich GaAs(001)-2 × 4 surfaces. No high charge states were found in either As 3d or Ga 3d core-level spectra before and after the deposition of the precursors. TMA adsorption does not disrupt the GaAs surface structure. For the (4 × 6) surface, the TMA precursor existed in both chemisorbed and physisorbed forms. In the former, TMA has lost a methyl group and is bonded to the As of the As-Ga dimer. Upon water purge, the dimethylaluminum-As group was etched off, allowing the now exposed Ga to bond with oxygen. Water also changed the physisorbed TMA into the As-O-Al(CH3)2 configuration. This configuration was also found in 1 cycle of TMA and water exposure of the (2 × 4) surface, but with a greater strength, accounting for the high interface defect density in the mid-gap region.

Bond cutting in K-doped tris(8-hydroxyquinoline) aluminium.

A series of Al 2p, K 2p, O 1s and N 1s core-level spectra have been used to characterize the interaction between potassium (K) and tris(8-hydroxyquinoline) aluminium (Alq(3)) molecules in the K-doped Alq(3) layer. All core-level spectra were tuned to be very surface sensitive in selecting various photon energies provided by the wide-range beamline at the National Synchrotron Radiation Research Center, Taiwan. A critical K concentration (x = 2.4) exists in the K-doped Alq(3) layer, below which the K-doped atoms generate a strained environment near the O and N atoms within 8-quinolinoline ligands. This creates new O 1s and N 1s components on the lower binding-energy side. Above the critical K coverage, the K-doped atoms attach the O atoms in the Al-O-C bonds next to the phenoxide ring and replace Al-O-C bonds by forming K-O-C bonds. An Alq(3) molecule is disassembled into Alq(2) and Kq by bond cutting and bond formation. The Alq(2) molecule can be further dissociated into Alq, or even Al, through subsequent formations of Kq.

Bond cutting in Cs-doped tris(8-hydroxyquinoline) aluminium.

A series of Cs 4d and Al 2p spectra associated with valence-band and cut-off spectra have been used to characterize the interaction between caesium and tris(8-hydroxyquinoline) aluminium (Alq(3)) molecules in a Cs-doped Alq(3) layer. The Cs 4d and Al 2p spectra were tuned to be very surface sensitive by selecting a photon energy of 120 eV at the National Synchrotron Radiation Research Center, Taiwan. A critical Cs concentration exists, above which a new Al 2p signal appears next to the Al 2p peak of Alq(3) in the lower binding-energy side. The Al 2p signal was analyzed and assigned as being contributed from a mixture of Alq(2), Alq and Al. Experimental data supported the observation that bond cutting of Alq(3) by the doped Cs atoms occurred at high Cs doping concentration.