Beagle: a near-edge X-ray absorption fine-structure spectroscopy data processing solution for beamline experiments at Pohang Accelerator Laboratory.

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy is a powerful tool for identifying chemical bonding states at synchrotron radiation facilities. Advances in new materials require researchers in both academia and industry to measure tens to hundreds of samples during the available beam time on a synchrotron beamline, which is typically allocated to users. Automated measurement methods, along with analysis software, have been developed for beamlines. Automated measurements facilitate high-throughput experiments and accumulate vast amounts of measured spectral data. The analysis software supports various functions for analyzing the experimental data; however, these analysis methods are complicated, and learning them can be time-consuming. To process large amounts of spectral data, a new analysis software, dedicated to NEXAFS spectroscopy, that is easy to use and can provide results in a short time is desired. Herein, the development of Beagle is described, software calculating molecular orientation from NEXAFS spectroscopy data that can report results in a short time comparable with that required to measure one sample at the beamline. It was designed to progress in a single sequence from data loading to the printing of the results with a `click of a button'. The functions of the software include recognizing the dataset, correcting the background, normalizing the plot, calculating the electron yield and determining the molecular orientation. The analysis results can be saved as {\tt{.txt}} files (spectral data), {\tt{.pdf}} files (graphic images) and Origin files (spectral data and graphic images).

Surface-to-Bulk Redox Coupling through Thermally Driven Li Redistribution in Li- and Mn-Rich Layered Cathode Materials.

Li- and Mn-rich (LMR) layered cathode materials have demonstrated impressive capacity and specific energy density thanks to their intertwined redox centers including transition metal cations and oxygen anions. Although tremendous efforts have been devoted to the investigation of the electrochemically driven redox evolution in LMR cathode at ambient temperature, their behavior under a mildly elevated temperature (up to ∼100 °C), with or without electrochemical driving force, remains largely unexplored. Here we show a systematic study of the thermally driven surface-to-bulk redox coupling effect in charged Li1.2Ni0.15Co0.1Mn0.55O2. We for the first time observed a charge transfer between the bulk oxygen anions and the surface transition metal cations under ∼100 °C, which is attributed to the thermally driven redistribution of Li ions. This finding highlights the nonequilibrium state and dynamic nature of the LMR material at deeply delithiated state upon a mild temperature perturbation.

Enhanced Hot Electron Flow and Catalytic Synergy by Engineering Core-Shell Structures on Au-Pd Nanocatalysts.

The synergistic catalytic performances of bimetallic catalysts are often attributed to the reaction mechanism associated with the alloying process of the catalytic metals. Chemically induced hot electron flux is strongly correlated with catalytic activity, and the interference between two metals at the atomic level can have a huge impact on the hot electron generation on the bimetallic catalysts. In this study, we investigate the correlation between catalytic synergy and hot electron chemistry driven by the electron coupling effect using a model system of Au-Pd bimetallic nanoparticles. We show that the bimetallic nanocatalysts exhibit enhanced catalytic activity under the hydrogen oxidation reaction compared with that of monometallic Pd nanocatalysts. Analysis of the hot electron flux generated in each system revealed the formation of Au/PdOx interfaces, resulting in high reactivity on the bimetallic catalyst. In further experiments with engineering the Au@Pd core-shell structures, we reveal that the hot electron flux, when the topmost surface Pd atoms were less affected by inner Au, due to the concrete shell, was smaller than the alloyed one. The alloyed bimetallic catalyst forming the metal-oxide interfaces has a more direct effect on the hot electron chemistry, as well as on the catalytic reactivity. The great significance of this study is in the confirmation that the change in the hot electron formation rate with the metal-oxide interfaces can be observed by shell engineering of nanocatalysts.

Direct methane protonic ceramic fuel cells with self-assembled Ni-Rh bimetallic catalyst.

Direct methane protonic ceramic fuel cells are promising electrochemical devices that address the technical and economic challenges of conventional ceramic fuel cells. However, Ni, a catalyst of protonic ceramic fuel cells exhibits sluggish reaction kinetics for CH4 conversion and a low tolerance against carbon-coking, limiting its wider applications. Herein, we introduce a self-assembled Ni-Rh bimetallic catalyst that exhibits a significantly high CH4 conversion and carbon-coking tolerance. It enables direct methane protonic ceramic fuel cells to operate with a high maximum power density of ~0.50 W·cm-2 at 500 °C, surpassing all other previously reported values from direct methane protonic ceramic fuel cells and even solid oxide fuel cells. Moreover, it allows stable operation with a degradation rate of 0.02%·h-1 at 500 °C over 500 h, which is ~20-fold lower than that of conventional protonic ceramic fuel cells (0.4%·h-1). High-resolution in-situ surface characterization techniques reveal that high-water interaction on the Ni-Rh surface facilitates the carbon cleaning process, enabling sustainable long-term operation.

Adsorption behavior of furan at Ge(100) surface.

The adsorption behavior of furan on the Ge(100) surface was studied using a combination of high-resolution photoemission spectroscopy (HRPES) and density functional theory (DFT) calculations. We identified the two adsorption species produced by the [4 + 2] cycloaddition and deoxygenation reactions of furan with the Ge(100) surface in a ratio of approximately 76:24 at the surveyed coverages, via an analysis of the binding energies and relative area proportions of all the peaks in the C 1s and O 1s core-level spectra. The DFT simulation results revealed that the [4 + 2] cycloaddition and deoxygenation adducts are thermodynamically preferred by the reaction of furan with the Ge(100) surface compared with others, which is consistent with the HRPES results. The findings will further our understanding of the surface reactions of five-membered heterocyclic molecules.

Selective dealloying of chemically disordered Pt-Ni bimetallic nanoparticles for the oxygen reduction reaction.

Changes in electronic and compositional structures of Pt-Ni electrocatalysts with 44% of Ni fraction with repeated chemical dealloying have been studied. By comparing the Pt-enriched surfaces formed using hydroquinone and sulfuric acid as a leaching agent, we found that hydroquinone generated Pt-enriched surfaces exhibit the highest oxygen reduction reaction (ORR) activity after repeating the treatment twice. In particular, it was found that while sulfuric acid causes an uncontrollable dissolution of Ni clusters, the unique selectivity of hydroquinone allows the preferential dissolution of Ni atoms alloyed with Pt. Despite its wide usage in the field, the results show that traditional acid leaching is unsuitable for Pt-Ni alloys with a high Ni content and an incomplete alloying level. We finally proved that the unique and lasting selectivity of hydroquinone enables an incompletely alloyed Pt-Ni catalyst to obtain a highly ORR active Pt shell region without an extensive loss of Ni.

Property Modulation of Graphene Oxide Incorporated with TiO2 for Dye-Sensitized Solar Cells.

Graphene oxide (GO) nano-powder is synthesized by the modified Hummer's method, and further thin films are deposited by using the water solution of GO through spin-coating. These films are thermally reduced along with the synthesized GO nano-powder at 50 to 200 °C in a high vacuum. Microstructural, electrical, and optical properties are expectedly controlled by thermal reduction. The electronic properties of GO are investigated by X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure. The reduction is confirmed by Raman spectroscopy. The work function and band gap of GO are tuned with the thermal reduction. The changes in properties of GO are not linear, and anomalous changes are observed for the reduction around 150 °C. Pristine and reduced GO nano-powder is incorporated into TiO2 paste to be the photoanode for dye-sensitized solar cells (DSSCs). It is observed that the performance of the fabricated cells is significantly enhanced for the GO reduced at 150 °C, and the cell exhibited a significant increment of ∼23% for the power conversion efficiency in comparison to DSSC based on an unmodified TiO2 photoanode.

Effect of Threading Dislocations on the Electronic Structure of La-Doped BaSnO3 Thin Films.

In spite of great application potential as transparent n-type oxides with high electrical mobility at room temperature, threading dislocations (TDs) often found in the (Ba,La)SnO3 (BLSO) films can limit their intrinsic properties so that their role in the physical properties of BLSO films need to be properly understood. The electrical properties and electronic structure of BLSO films grown on SrTiO3 (001) (STO) and BaSnO3 (001) (BSO) substrates are comparatively studied to investigate the effect of the TDs. In the BLSO/STO films with TD density of ~1.32 × 1011 cm-2, n-type carrier density ne and electron mobility are significantly reduced, as compared with the BLSO/BSO films with nearly no TDs. This indicates that TDs play the role of scattering-centers as well as acceptor-centers to reduce n-type carriers. Moreover, in the BLSO/STO films, both binding energies of an Sn 3d core level and a valence band maximum are reduced, being qualitatively consistent with the Fermi level shift with the reduced n-type carriers. However, the reduced binding energies of the Sn 3d core level and the valence band maximum are clearly different as 0.39 and 0.19 eV, respectively, suggesting that the band gap renormalization preexisting in proportion to ne is further suppressed to restore the band gap in the BLSO/STO films with the TDs.

Giant Orbital Anisotropy with Strong Spin-Orbit Coupling Established at the Pseudomorphic Interface of the Co/Pd Superlattice.

Orbital anisotropy at interfaces in magnetic heterostructures has been key to pioneering spin-orbit-related phenomena. However, modulating the interface's electronic structure to make it abnormally asymmetric has been challenging because of lack of appropriate methods. Here, the authors report that low-energy proton irradiation achieves a strong level of inversion asymmetry and unusual strain at interfaces in [Co/Pd] superlattices through nondestructive, selective removal of oxygen from Co3 O4 /Pd superlattices during irradiation. Structural investigations corroborate that progressive reduction of Co3 O4 into Co establishes pseudomorphic growth with sharp interfaces and atypically large tensile stress. The normal component of orbital to spin magnetic moment at the interface is the largest among those observed in layered Co systems, which is associated with giant orbital anisotropy theoretically confirmed, and resulting very large interfacial magnetic anisotropy is observed. All results attribute not only to giant orbital anisotropy but to enhanced interfacial spin-orbit coupling owing to the pseudomorphic nature at the interface. They are strongly supported by the observation of reversal of polarity of temperature-dependent Anomalous Hall signal, a signature of Berry phase. This work suggests that establishing both giant orbital anisotropy and strong spin-orbit coupling at the interface is key to exploring spintronic devices with new functionalities.

Selective reactions and adsorption geometries of a multifunctional molecule: cis-2-butene-1,4-diol on Si(100)-2 x 1.

The adsorption geometry of cis-2-butene-1,4-diol (BEDO, HOCH(2)CH=CHCH(2)OH) on Si(100)-2 x 1 was studied using scanning tunneling microscopy (STM), high resolution X-ray photoemission spectroscopy (XPS), and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. Bias-voltage-dependent STM images exhibited features characteristic of two distinct BEDO adsorption geometries. One feature was a bright protrusion located on the center of a single dimer, indicating an on-top configuration. The low bias-voltage STM image clearly showed dark features indicative of the formation of Si-H bonds on adjacent dimers in the same dimer row. The other feature was a bright protrusion bridged on end between two adjacent dimers in the same dimer row, indicating an end-bridge configuration. Accompanying this feature, two dark features attributed to Si-H bonds were observed on opposite positions to the bridged protrusion. From the XPS results, the Si 2p core level spectra revealed that the dimer atoms are involved in the formation of Si-O and Si-H bonds. On the other hand, carbon K-edge NEXAFS spectra showed that the C=C bond does not participate in the adsorption reaction and remains as an unreacted group. Collectively, the experimental results indicate that the adsorption of BEDO on Si(100)-2 x 1 occurs through the formation of two Si-O bonds via nucleophilic reaction between the two OH groups of BEDO and two Si-Si dimers. Importantly, the maintenance of the C=C bond means that the C=C functional group can be utilized as a new reaction site for further surface chemical reactions.

Direct Evidence of Electronic Interaction at the Atomic-Layer-Deposited MoS2 Monolayer/SiO2 Interface.

The electronic structure of an atomic-layer-deposited MoS2 monolayer on SiO2 was investigated using X-ray absorption spectroscopy (XAS) and synchrotron X-ray photoelectron spectroscopy (XPS). The angle-dependent evolution of the XAS spectra and the photon-energy-dependent evolution of the XPS spectra were analyzed in detail using an ab initio electronic structure simulation. Although similar to the theoretical spectra of an ideal free-standing MoS2 ML, the experimental spectra exhibit features that are distinct from those of an ideal ML, which can be interpreted as a consequence of S-O van der Waals (vdW) interactions. The strong consensus among the experimental and theoretical spectra suggests that the vdW interactions between MoS2 and adjacent SiO2 layers can influence the electronic structure of the system, manifesting a substantial electronic interaction at the MoS2-SiO2 interface.

Exceptionally Uniform and Scalable Multilayer MoS2 Phototransistor Array Based on Large-Scale MoS2 Grown by RF Sputtering, Electron Beam Irradiation, and Sulfurization.

Two-dimensional molybdenum disulfide (MoS2) has emerged as a promising material for optoelectronic applications because of its superior electrical and optical properties. However, the difficulty in synthesizing large-scale MoS2 films has been recognized as a bottleneck in uniform and reproducible device fabrication and performance. Here, we proposed a radio-frequency magnetron sputter system, and post-treatments of electron beam irradiation and sulfurization to obtain large-scale continuous and high-quality multilayer MoS2 films. Large-area uniformity was confirmed by no deviation of electrical performance in fabricated MoS2 thin-film transistors (TFTs) with an average on/off ratio of 103 and a transconductance of 0.67 nS. Especially, the photoresponsivity of our MoS2 TFT reached 3.7 A W-1, which is a dramatic improvement over that of a previously reported multilayer MoS2 TFT (0.1 A W-1) because of the photogating effect induced by the formation of trap states in the band gap. Finally, we organized a 4 × 4 MoS2 phototransistor array with high photosensitivity, linearity, and uniformity for light detection, which demonstrates the great potential of 2D MoS2 for future-oriented optoelectronic devices.

Preferential Orientation of Tetrahedral Silicon-Based Hosts in Phosphorescent Organic Light-Emitting Diodes.

Vacuum-processed diphenylbis(3-(pyridine-2-yl)phenyl)silane (2PTPS), diphenylbis(3-(pyridine-3-yl)phenyl)silane (3PTPS), and diphenylbis(3-(pyridine-4-yl)phenyl)silane (4PTPS) have been used as electron-transporting host materials combined with tris(4-carbazoyl-9-ylphenyl)amine (TCTA) as the hole-transporting host, which induce balanced charge carrier transport for high-efficiency phosphorescent organic light-emitting diodes. The 4PTPS-based organic light-emitting diodes with tris[2-phenylpyridinato-C 2,N]iridium(III) [Ir(ppy)3] dopant showed highest current efficiency and external quantum efficiency of 53.54 cd/A and 15.61%, compared to 2PTPS (40.75 cd/A, 11.84%) and 3PTPS (29.35 cd/A, 8.54%). These results were attributed to the well-aligned structure with preferential horizontal orientation of the emitting material layer by the diffraction intensity distribution as a function of azimuthal angle in two-dimensional grazing incidence X-ray diffraction analysis. The molecular orientation of TCTA:4PTPS material with a narrow azimuthal intensity distribution had better priority to the horizontal direction than the other TCTA:2PTPS and TCTA:3PTPS materials, which is related to the charge transport as well as the device efficiency. We found that the preferential horizontal orientation of the co-host material with a balanced charge carrier was not affected by Ir(ppy)3 dopant with a homoleptic structure and bis-[2-(4,6-difluorophenyl)pyridinato-N,C 2](picolinato)iridium [Firpic] dopant with a heteroleptic structure in the co-host/dopant system.

Chemical Doping Effects in Multilayer MoS2 and Its Application in Complementary Inverter.

Multilayer MoS2 has been gaining interest as a new semiconducting material for flexible displays, memory devices, chemical/biosensors, and photodetectors. However, conventional multilayer MoS2 devices have exhibited limited performances due to the Schottky barrier and defects. Here, we demonstrate poly(diketopyrrolopyrrole-terthiophene) (PDPP3T) doping effects in multilayer MoS2, which results in improved electrical characteristics (∼4.6× higher on-current compared to the baseline and a high current on/off ratio of 106). Synchrotron-based study using X-ray photoelectron spectroscopy and grazing incidence wide-angle X-ray diffraction provides mechanisms that align the edge-on crystallites (97.5%) of the PDPP3T as well as a larger interaction with MoS2 that leads to dipole and charge transfer effects (at annealing temperature of 300 °C), which support the observed enhancement of the electrical characteristics. Furthermore, we demonstrate a complementary metal-oxide-semiconductor inverter that uses a p-type MoSe2 and a PDPP3T-doped MoS2 as charging and discharging channels, respectively.

Balancing Hole and Electron Conduction in Ambipolar Split-Gate Thin-Film Transistors.

Complementary organic electronics is a key enabling technology for the development of new applications including smart ubiquitous sensors, wearable electronics, and healthcare devices. High-performance, high-functionality and reliable complementary circuits require n- and p-type thin-film transistors with balanced characteristics. Recent advancements in ambipolar organic transistors in terms of semiconductor and device engineering demonstrate the great potential of this route but, unfortunately, the actual development of ambipolar organic complementary electronics is currently hampered by the uneven electron (n-type) and hole (p-type) conduction in ambipolar organic transistors. Here we show ambipolar organic thin-film transistors with balanced n-type and p-type operation. By manipulating air exposure and vacuum annealing conditions, we show that well-balanced electron and hole transport properties can be easily obtained. The method is used to control hole and electron conductions in split-gate transistors based on a solution-processed donor-acceptor semiconducting polymer. Complementary logic inverters with balanced charging and discharging characteristics are demonstrated. These findings may open up new opportunities for the rational design of complementary electronics based on ambipolar organic transistors.

Structural Analyses of Phase Stability in Amorphous and Partially Crystallized Ge-Rich GeTe Films Prepared by Atomic Layer Deposition.

The local bonding structures of GexTe1-x (x = 0.5, 0.6, and 0.7) films prepared through atomic layer deposition (ALD) with Ge(N(Si(CH3)3)2)2 and ((CH3)3Si)2Te precursors were investigated using Ge K-edge X-ray absorption spectroscopy (XAS). The results of the X-ray absorption fine structure analyses show that for all of the compositions, the as-grown films were amorphous with a tetrahedral Ge coordination of a mixture of Ge-Te and Ge-Ge bonds but without any signature of Ge-GeTe decomposition. The compositional evolution in the valence band electronic structures probed through X-ray photoelectron spectroscopy suggests a substantial chemical influence of additional Ge on the nonstoichiometric GeTe. This implies that the ALD process can stabilize Ge-abundant bonding networks like -Te-Ge-Ge-Te- in amorphous GeTe. Meanwhile, the XAS results on the Ge-rich films that had undergone post-deposition annealing at 350 °C show that the parts of the crystalline Ge-rich GeTe became separated into Ge crystallites and rhombohedral GeTe in accordance with the bulk phase diagram, whereas the disordered GeTe domains still remained, consistent with the observations of transmission electron microscopy and Raman spectroscopy. Therefore, amorphousness in GeTe may be essential for the nonsegregated Ge-rich phases and the low growth temperature of the ALD enables the achievement of the structurally metastable phases.

Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NtBu)(NEt2)3, Ta(NtBu)(NEt2)2Cp, and H2O.

The growth characteristics of Ta2O5 thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt2)3 (TBTDET) and Ta(NtBu)(NEt2)2Cp (TBDETCp) as Ta-precursors, where tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta2O5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta2O5 films were 0.77 Å cycle-1 at 250 °C and 0.67 Å cycle-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt2) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta2O5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta2O5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta2O5 films was 2.1-2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.

Hydrogenated monolayer graphene with reversible and tunable wide band gap and its field-effect transistor.

Graphene is currently at the forefront of cutting-edge science and technology due to exceptional electronic, optical, mechanical, and thermal properties. However, the absence of a sizeable band gap in graphene has been a major obstacle for application. To open and control a band gap in functionalized graphene, several gapping strategies have been developed. In particular, hydrogen plasma treatment has triggered a great scientific interest, because it has been known to be an efficient way to modify the surface of single-layered graphene and to apply for standard wafer-scale fabrication. Here we show a monolayer chemical-vapour-deposited graphene hydrogenated by indirect hydrogen plasma without structural defect and we demonstrate that a band gap can be tuned as wide as 3.9 eV by varying hydrogen coverage. We also show a hydrogenated graphene field-effect transistor, showing that on/off ratio changes over three orders of magnitude at room temperature.

Reconfigurable Complementary Logic Circuits with Ambipolar Organic Transistors.

Ambipolar organic electronics offer great potential for simple and low-cost fabrication of complementary logic circuits on large-area and mechanically flexible substrates. Ambipolar transistors are ideal candidates for the simple and low-cost development of complementary logic circuits since they can operate as n-type and p-type transistors. Nevertheless, the experimental demonstration of ambipolar organic complementary circuits is limited to inverters. The control of the transistor polarity is crucial for proper circuit operation. Novel gating techniques enable to control the transistor polarity but result in dramatically reduced performances. Here we show high-performance non-planar ambipolar organic transistors with electrical control of the polarity and orders of magnitude higher performances with respect to state-of-art split-gate ambipolar transistors. Electrically reconfigurable complementary logic gates based on ambipolar organic transistors are experimentally demonstrated, thus opening up new opportunities for ambipolar organic complementary electronics.

Mussel-Inspired Anchoring of Polymer Loops That Provide Superior Surface Lubrication and Antifouling Properties.

We describe robustly anchored triblock copolymers that adopt loop conformations on surfaces and endow them with unprecedented lubricating and antifouling properties. The triblocks have two end blocks with catechol-anchoring groups and a looping poly(ethylene oxide) (PEO) midblock. The loops mediate strong steric repulsion between two mica surfaces. When sheared at constant speeds of ∼2.5 µm/s, the surfaces exhibit an extremely low friction coefficient of ∼0.002-0.004 without any signs of damage up to pressures of ∼2-3 MPa that are close to most biological bearing systems. Moreover, the polymer loops enhance inhibition of cell adhesion and proliferation compared to polymers in the random coil or brush conformations. These results demonstrate that strongly anchored polymer loops are effective for high lubrication and low cell adhesion and represent a promising candidate for the development of specialized high-performance biomedical coatings.