Low-dimensional systems host many exotic physical properties. Using extended resonant soft x-ray scattering supported with theoretical calculations, we observe concomitantly a Wigner-like crystal in a stripe-like QHKL = (001) superlattice with surprisingly long coherence lengths and new soft x-ray correlated plasmons in WS2. A new electronic transition is found at 125 K upon cooling, which is a transition from two-dimensional ordering to three-dimensional ordering, a result of p–d hybridization anisotropy and electronic correlations. Our result shows the importance of the interplay of long-range electron–electron correlation and short-range electronic screening in determining electronics and optical properties of transition metal dichalcogenides.

We report strong ferromagnetism of quasiparticle doped holes both within the ab- plane and along the c- axis of Cu-O planes in low-dimensional Au/ d- La 1.8 Ba 0.2 CuO 4 /LaAlO 3 (001) heterostructures ( d = 4, 8 and 12 unit-cells) using resonant soft X-ray and magnetic scattering together with X-ray magnetic circular dichroism. Interestingly, ferromagnetism is stronger at a hole doped peak and at an upper Hubbard band of O with spin-polarization degree as high as 40%, revealing strong ferromagnetism of Mottness. For in- ab -plane spin-polarizations, the spin of doped holes in O2 p –Cu3 d –O2 p is a triplet state yielding strong ferromagnetism. For out-of- ab -plane spin-polarization, while the spins of doped holes in both O2 p –O2 p and Cu3 d –Cu3 d are triplet states, the spin of doped holes in Cu3 d –O2 p is a singlet state yielding ferrimagnetism. A ferromagnetic-(002) Bragg-peak of the doped holes is observed and enhanced as a function of d revealing strong ferromagnetism coupling between Cu-O layers along the c -axis.

Electronic correlations play important roles in driving exotic phenomena in condensed matter physics. They determine low-energy properties through high-energy bands well-beyond optics. Great effort has been made to understand low-energy excitations such as low-energy excitons in transition metal dichalcogenides (TMDCs), however their high-energy bands and interlayer correlation remain mysteries. Herewith, by measuring temperature- and polarization-dependent complex dielectric and loss functions of bulk molybdenum disulphide from near-infrared to soft X-ray, supported with theoretical calculations, we discover unconventional soft X-ray correlated-plasmons with low-loss, and electronic transitions that reduce dimensionality and increase correlations, accompanied with significantly modified low-energy excitons. At room temperature, interlayer electronic correlations, together with the intralayer correlations in the c -axis, are surprisingly strong, yielding a three-dimensional-like system. Upon cooling, wide-range spectral-weight transfer occurs across a few tens of eV and in-plane p–d hybridizations become enhanced, revealing strong Coulomb correlations and electronic anisotropy, yielding a two-dimensional- like system. Our result shows the importance of strong electronic, interlayer and intralayer correlations in determining electronic structure and opens up applications of utilizing TMDCs on plasmonic nanolithrography.

The energy level alignment of a blended mixture of organic semiconductors is often depicted as having a common vacuum level. However, this is not a universal phenomenon among the vast number of organic semiconductors that currently exist, as in many cases the energy levels align via the Fermi level. In this report, the energy level alignments of the mixtures; poly(9-vinylcarbazole) (PVK) and 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13) and poly(3-hexylthiophene-2,5-diyl) (P3HT) and SPPO13, with varying SPPO13 concentrations, are measured. It was found that the blended systems exhibit two different vacuum levels with the dipole between the PVK and SPPO13 increasing with the SPPO13 concentration, whilst the P3HT and SPPO13 vacuum levels only experience a small change. This is attributed to the decreasing electronic screening with increasing SPPO13 concentration. These new observations have an important implication in our understanding of interfacial behaviour for blended systems commonly used in various organic electronic devices.

Inorganic perovskites have recently attracted much attention as promising new nanocrystalline materials that have interesting fundamental phenomena and great potential in several applications. Herein, we reveal unusual structural and electronic changes in nanocrystalline cesium lead bromide (CsPbBr3) as a function of temperature using high-resolution spectroscopic ellipsometry, high-resolution transmission electron microscopy and terahertz spectroscopy measurements supported by first-principles calculations. New dual phases of crystalline and electronic structures are observed due to the nanocrystalline nature of the material. Interestingly, a change in the electronic structure occurs below 150 K, and the rate at which the nanocrystal transitions from the tetragonal to orthorhombic phase is found to be nonlinear with temperature. Our results show the importance of the charge and lattice interplay in determining the dual phases and fundamental properties of nanocrystalline materials.

Electronic correlation is believed to play an important role in exotic phenomena such as insulator-metal transition, colossal magneto resistance and high temperature superconductivity in correlated electron systems. Recently, it has been shown that electronic correlation may also be responsible for the formation of unconventional plasmons. Herewith, using a combination of angle-dependent spectroscopic ellipsometry, angle resolved photoemission spectroscopy and Hall measurements all as a function of temperature supported by first-principles calculations, the existence of low-loss high-energy correlated plasmons accompanied by spectral weight transfer, a fingerprint of electronic correlation, in topological insulator (Bi$_{0.8}$Sb$_{0.2}$)$_2$Se$_3$ is revealed. Upon cooling, the density of free charge carriers in the surface states decreases whereas those in the bulk states increase, and that the newly-discovered correlated plasmons are key to explaining this phenomenon. Our result shows the importance of electronic correlation in determining new correlated plasmons and opens a new path in engineering plasmonic-based topologically-insulating devices.

The interactions between delocalized and localized charges play important roles in correlated electron systems. Here, using a combination of transport measurements, spectroscopic ellipsometry (SE), and X‐ray absorption spectroscopy (XAS) supported by theoretical calculations, we reveal the important role of interfacial localized charges and their screening effects in determining the mobility of (La 0.3 Sr 0.7 )(Al 0.65 Ta 0.35 )O 3 /SrTiO 3 (LSAT/SrTiO 3 ) interfaces. When the LSAT layer thickness reaches the critical value of 5 uc, the insulating interface abruptly becomes conducting, accompanied by the appearance of a new midgap state. This midgap state emerges at ≈1 eV below the Ti t 2g band and shows a strong character of Ti 3d xy – O 2p hybridization. Increasing the LSAT layer from 5 to 18 uc, the number of localized charges increases, resulting in an enhanced screening effect and higher mobile electron mobility. This observation contradicts the traditional semiconductor interface where the localized charges always suppress the carrier mobility. These results demonstrate a new strategy to probe localized charges and mobile electrons in correlated electronic systems and highlight the important role of screening effects from localized charges in improving the mobile electron mobility at complex oxide interfaces.

Perovskite compounds show promise for charge generation in solar cells because of their high density of excitons when exposed to light. New experiments reveal the origin of this behavior, which could point the way to more efficient optoelectronic devices.

We present in-depth analysis of an n/p heterojunction that consists of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT(CN)(6)) (n-type) and Poly(9-vinylcarbazole) (PVK) (p-type) via X-ray Photoelectron Spectroscopy (XPS) and Ultra-violet Photoelectron Spectroscopy (UPS) measurement. The p-type layer is doped with 2 wt% of 1,1-bis-(4-bis(4-tolyl)-aminophenyl) cyclohexene (TAPC). The energy difference (triangle E) at the hetero-junction, magnitude of band bending (V-b) and the vacuum level shift at the interface is modified when PVK is doped with 2 wt% TAPC. The presence of V-b at the HAT(CN)(6)/PVK (PVK:TAPC) interface makes it easier to reach a triangle E approximate to 0 energy offset in order to facilitate charge generation at the interface. Via a Fowler-Nordheim (FN) tunneling curve, it is found that the electron extraction from PVK to HAT(CN)(6) at the interface could occur via the tunneling process. This finding provides new insights into novel solutions for high efficiency tandem OLEDs. (C) 2017 Elsevier B.V. All rights reserved.

The determination of the Gaussian density of states (DOS) width and its influence on charge injection/extraction across an interface are of critical importance for the accurate physical understanding of the optoelectronic properties of organic electronics. A new technique is developed here, whereby the DOS width is extracted by numerical fitting of experimental band-bending observed across an interface. This is done by exploiting the fact that the band-bending can be influenced by the disordered nature of energetic sites that are involved in charge transfer across an interface. This technique is applied on a single component and blended mixture of materials. This approach is successfully tested on poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), poly(N-vinylcarbazole) (PVK), 2,7-bis(diphenylphosphoryl)-9,9'-spirobifluorene (SPPO13) and poly(3-hexylthiophene-2,5-diyl) (P3HT) in a number of interfaces. The DOS width of PEDOT:PSS, PVK, SPPO13 and P3HT were calculated to be 105 meV, 144 meV, 178 meV and 80 meV respectively. The impact of DOS width on the interface between two materials is investigated. The Fermi level pinning model used to describe the hole barrier between an inorganic electrode and an organic semiconductor fails when applied to the interface between two organic semiconductors. Their energetic barriers are very highly dependent on the DOS of the materials.

By inserting lithium fluoride (LiF) between solution-processed MoO3 with optimal thickness on top of super yellow poly-(p-phenylenevinylene) (SY-PPV), the efficiency of the SY-PPV fluorescent-based devices can be significantly improved by more than two-fold. Despite the increased driving voltage, the device showed a current and a luminance efficiency up to 22.8 cd A(-1) and 14.3 lm W-1 respectively, which is a more than a two-fold increase in efficiency compared to the control device using LiF/Al at a brightness of 1000 cdm(-2). Ultraviolet photoelectron spectroscopy (UPS) is used to analyze the energy alignment between SY-PPV and the solution processed MoO3 and MoO3/LiF/Al interfaces. We found that the solution processed MoO3 using diluted sodium hydroxide has relatively low ionization energy (IA), electron affinity (EA) and work function decreasing with increasing thickness of MoO3. However, the optical bandgap increases with increasing spin-speed. A large energetic barrier is always present between the SY-PPY and deep lying valence band of MoO3. This is supported by suppression of hole current in hole dominating devices. The ability of thin MoO3 (similar to 2 nm) acting as a hole blocking layer while allowing electrons to be transported across the layer and a large upward vacuum shift appeared to be the origin of efficiency enhancement of SY-PPV light-emitting diode when MoO3/LiF/Al is used.

The interfacial properties between electrodes and the various organic layers that comprise an organic electronic device are of direct relevance in understanding charge injection, extraction and generation. The energy levels and energy-bending of three interfaces; indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), ITO/poly(N-vinylcarbazole) (PVK) and PEDOT:PSS/PVK were measured using ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS). By decoupling the vacuum shift and energy-bending, the energy-bending at these interfaces can be simulated using an electrostatic model that takes into account the energetic disorder of the polymers. The model is further extended to include blended mixtures of semiconductors at differing concentrations and it was found that a very good agreement exists between the experiment and theory for all interfaces. This suggests that the electrostatic model can be used to describe energy-bending at the interface between any organic semiconductors. Further investigation into the effect of the Gaussian density of states width on energy-bending is warranted.

Understanding the mechanism of resistive switching in a memory device is fundamental in order to improve device performance. The mechanism of current switching in a basic organic write-once read-many (WORM) memory device is investigated by determining the energy level alignments of indium tin oxide (ITO), poly(9-vinylcarbazole) (PVK) and aluminum (Al) using x-ray and ultraviolet photoelectron spectroscopy, current-voltage characterization and Auger depth profiling. The current switching mechanism was determined to be controlled by the interface between the ITO and the PVK. The electric field applied across the device causes the ITO from the uneven surface of the anode to form metallic filaments through the PVK, causing a shorting effect within the device leading to increased conduction. This was found to be independent of the PVK thickness, although the switch-on voltage was non-linearly dependent on the thickness. The formation of these filaments also caused the destruction of the interfacial dipole at the PVK-AI interface.

We report on the development of a carbon dioxide gas sensor from the room-temperature reduction of graphene oxide via hydrogen plasma. The hydrogen plasma contains radicals and atoms which give dissociation energies for oxygen functional groups, which is capable of reducing the graphene oxide. The sample morphology, degree of reduction, chemical bonding and gas sensing capability were systematically studied. The effective removal of oxygen functional groups at the edges and both basal planes while restoring C=C bonds has been observed by AFM, XPS and Raman analysis. The C/O ratio increased from 0.81 to 7.9 and the resistance decreased significantly from 33kΩ to 1.6kΩ after the reduction process. The fabricated rGO-F20 sensor shows the highest CO2 gas sensing response of 71% and 15% at 1500ppm in N2 (37% RH) and air environment (68% RH), respectively. In addition, the sensor shows a good repeatability performance with the sensing and recovery time of about 4min when exposed to 750 and 769ppm CO2 concentration. The repeatability performance was measured in air environment at 68% RH without external assisted recovery. This simple, room-temperature reduction process and sensing capability, low cost fabrication process of a graphene sensor could lead to the implementation of a practical indoor air quality monitor.

The work function of indium tin oxide (ITO) was increased by treating ITO with dichlorobenzene with UV light. Carbon contamination of the Cl-ITO was measured using X-ray Photoelectron Spectroscopy (XPS) and argon ion sputtering was used to remove the carbon from the surface. It was found that the carbon contamination from residual dichlorobenzene significantly lowered the work function of the ITO and after argon ion sputtering the work function increased to 5.8 eV. It was found that chlorination of ITO occurs after more than 6 min of UV exposure. Further sputtering of ITO resulted in the removal of the functionalized chlorine, the introduction of argon ion contaminants on the ITO decreases its work function. (C) 2014 Elsevier B.V. All rights reserved.

Poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) PEDOT : PSS is extensively used as a hole injection layer (HIL) in solution-processed organic light emitting diodes (OLEDs). The high work function of a HIL is crucial in improving OLED efficiency. The work function of PEDOT : PSS is usually around 5.1-5.3 eV. By adding perfluorinated ionomer (PFI), the work function of PEDOT : PSS has been reported to reach as high as 5.95 eV. We investigated the effects of PFI-modified PEDOT : PSS in a single-layer solution-processed blue phosphorescent OLED (PHOLED). We observed that high concentrations of a PFI in PEDOT : PSS has detrimental effects on the device efficiency due to the low conductivity of the PFI. Using this approach, blue PHOLEDs with efficiencies of 9.4 lm W-1 (18.2 cdA(-1)) and 7.9 lm W-1 (20.4 cdA(-1)) at 100 cdm(-2) and 1000 cdm(-2), respectively, were demonstrated.

By use of high intensity XUV radiation from the FLASH free-electron laser at DESY, we have created highly excited exotic states of matter in solid-density aluminum samples. The XUV intensity is sufficiently high to excite an inner-shell electron from a large fraction of the atoms in the focal region. We show that soft-x-ray emission spectroscopy measurements reveal the electronic temperature and density of this highly excited system immediately after the excitation pulse, with detailed calculations of the electronic structure, based on finite-temperature density functional theory, in good agreement with the experimental results.

We present calculations of the free-free XUV opacity of warm, solid-density aluminum at photon energies between the plasma frequency at 15 eV and the L-edge at 73 eV, using both density functional theory combined with molecular dynamics and a semi-analytical model in the RPA framework with the inclusion of local field corrections. As the temperature is increased from room temperature to 10 eV, with the ion and electron temperatures equal, we calculate an increase in the opacity in the range over which the degree of ionization is constant. The effect is less pronounced if only the electron temperature is allowed to increase. The physical significance of these increases is discussed in terms of intense XUV-laser matter interactions on both femtosecond and picosecond time-scales.