Ultraviolet Photoelectron Spectra Research Articles

The anion photoelectron spectrum and diabatization of tetrazolyl.

The potential energy surface of tetrazolyl [cyclic (N4CH)] has a conical intersection seam between the two lowest-energy electronic states near the ground state minimum geometry. This work treats that molecule. The potential energy surfaces used in this study are based on a least-squares fitting procedure that includes abinitio energies, energy gradients, and derivative couplings described using polynomials up to fourth-order and abinitio data obtained from multireference configuration interaction wave functions. A five-electronic-state description was generated with a root mean square absolute energy error of 9.6cm-1, compared to 326.8cm-1 when only second-order terms were used. The time-independent multimode vibronic coupling in the KDC approximation was used to simulate and analyze the anion ultraviolet photoelectron spectrum of tetrazolide.

Helical Organic and Inorganic Polymers.

Despite being a staple of synthetic plastics and biomolecules, helical polymers are scarcely studied with Gaussian-basis-set ab initio electron-correlated methods on an equal footing with molecules. This article introduces an ab initio second-order many-body Green's function [MBGF(2)] method with nondiagonal, frequency-dependent Dyson self-energy for infinite helical polymers using screw-axis-symmetry-adapted Gaussian-spherical-harmonics basis functions. Together with the Gaussian-basis-set density-functional theory for energies, analytical atomic forces, translational-period force, and helical-angle force, it can compute correlated energy, quasiparticle energy bands, structures, and vibrational frequencies of an infinite helical polymer, which smoothly converge at the corresponding oligomer results. These methods can handle incommensurable structures, which have an infinite translational period and are hard to characterize by any other method, just as efficiently as commensurable structures. We apply them to polyethylene (2/1 helix), polyacetylene (Peierls' system) and polytetrafluoroethylene (13/6 helix) to establish the quantitative accuracy of MBGF(2)/cc-pVDZ in simulating their (angle-resolved) ultraviolet photoelectron spectra and of B3LYP/cc-pVDZ or 6-31G** in reproducing their structures, infrared and Raman band positions, phonon dispersions, and (coherent and incoherent) inelastic neutron scattering spectra. We then predict the same properties for infinitely catenated chains of nitrogen or oxygen and discuss their possible metastable existence under ambient conditions. They include planar zigzag polyazene (N2)x (Peierls' system), 11/3-helical isotactic polyazane (NH)x, 9/4-helical isotactic polyfluoroazane (NF)x, and 7/2-helical polyoxane (O)x as potential high-energy-density materials.

Interactions between PTCDI-C8 and Si(100) Surface

PTCDI-C8 molecules are vapor-deposited onto reconstructed Si(100)—(2 × 1) surface under ultra-high vacuum. X-ray photoelectron spectra reveal a bond formation between oxygen atoms of the molecules’ carboxylic groups and Si dangling bonds of the substrate. Following PTCDI—C8 film growth, ultraviolet photoelectron spectra show a drop in the HOMO level with respect to the Fermi level from 1.8 eV to 2.0 eV and a monotonic work function increase from 2.5 eV up to 3.3 eV. For a film thickness of 6.0 nm, a difference of 1.5 eV between the HOMO level of the film and the valence band maximum of the substrate is accomplished.

Unexpected Photocatalytic Degeneration of NAD + for Inducing Apoptosis of Hypoxia Cancer Cells

Unexpected Photocatalytic Degeneration of NAD ⁺ for Inducing Apoptosis of Hypoxia Cancer Cells

UV Photoelectron Spectroscopy of Aqueous Solutions.

ConspectusKnowledge of the electronic structure of an aqueous solution is a prerequisite to understanding its chemical and biological reactivity and its response to light. One of the most direct ways of determining electronic structure is to use photoelectron spectroscopy to measure electron binding energies. Initially, photoelectron spectroscopy was restricted to the gas or solid phases due to the requirement for high vacuum to minimize inelastic scattering of the emitted electrons. The introduction of liquid-jets and their combination with intense X-ray sources at synchrotrons in the late 1990s expanded the scope of photoelectron spectroscopy to include liquids. Liquid-jet photoelectron spectroscopy is now an active research field involving a growing number of research groups. A limitation of X-ray photoelectron spectroscopy of aqueous solutions is the requirement to use solutes with reasonably high concentrations in order to obtain photoelectron spectra with adequate signal-to-noise after subtracting the spectrum of water. This has excluded most studies of organic molecules, which tend to be only weakly soluble. A solution to this problem is to use resonance-enhanced photoelectron spectroscopy with ultraviolet (UV) light pulses (hν ≲ 6 eV). However, the development of UV liquid-jet photoelectron spectroscopy has been hampered by a lack of quantitative understanding of inelastic scattering of low kinetic energy electrons (≲5 eV) and the impact on spectral lineshapes and positions.In this Account, we describe the key steps involved in the measurement of UV photoelectron spectra of aqueous solutions: photoionization/detachment, electron transport of low kinetic energy electrons through the conduction band, transmission through the water-vacuum interface, and transport through the spectrometer. We also explain the steps we take to record accurate UV photoelectron spectra of liquids with excellent signal-to-noise. We then describe how we have combined Monte Carlo simulations of electron scattering and spectral inversion with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution to develop an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. The huge potential of our experimental and spectral retrieval methods is illustrated using three examples. The first is a measurement of the vertical detachment energy of the green fluorescent protein chromophore, a sparingly soluble organic anion whose electronic structure underpins its fluorescence and photooxidation properties. The second is a measurement of the vertical ionization energy of liquid water, which has been the subject of discussion since the first X-ray photoelectron spectroscopy measurement in 1997. The third is a UV photoelectron spectroscopy study of the vertical ionization energy of aqueous phenol which demonstrates the possibility of retrieving true photoelectron spectra from measurements with contributions from components with different concentration profiles.

Contributions of piezoelectricity and triboelectricity to a hydroxyapatite/PVDF–HFP fiber-film nanogenerator

Poly(vinylidene fluoride) (PVDF)-based piezoelectric nanogenerators (PENGs), which are prepared by electrospinning, are widely used in self-powered field owing to their exceptional generation performance. The generation performance of fiber PENG is usually attributed to piezoelectric effect, but the effect of triboelectricity generated among the fibers on the generation performance is not negligible. On the other hand, quantifying triboelectricity and piezoelectricity and revealing the mechanism of triboelectric–piezoelectric synergy to enhance the generation performance are urgent issues of concern. In this study, hydroxyapatite (HAP) was added into PVDF–hexafluoropropylene (HFP) as a filler to prepare nanofiber films by electrospinning. Compared with a pure PVDF–HFP nanofiber film, the generation performance of a 25 wt% HAP/PVDF–HFP nanofiber film improved from 0.2 to 1.1 V. The HAP enhancement effect was verified by HAP-content experiment and work-function measurement. The contributions of triboelectricity and piezoelectricity were 62.5% and 37.5%, respectively, calculated using the thermal-treatment experimental results. Finally, the ultraviolet photoelectron-spectrum method and model analysis revealed the synergistic effect of triboelectricity and piezoelectricity. This work employed the thermal-treatment method to quantify the contributions of triboelectricity and piezoelectricity while providing an insight into the design of PVDF fiber PENG to further enhances the generation performance.

Direct observation of carrier migration in heterojunctions to discuss the p–n and direct Z-scheme heterojunctions

Type II p–n heterojunction and direct Z-scheme heterojunction are identical staggered band alignments, but were reported ambiguously in many composite photocatalysts because their carriers migrate in opposite directions. In this research, metal oxides CuO, NiO and Co3O4-based heterojunctions with Na0.9Mg0.45Ti3.55O8 (NMTO) were synthesized via a simple hydrothermal method. The CuO/NMTO heterojunction was demonstrated as a direct Z-scheme heterojunction, whereas the NiO/NMTO and Co3O4/NMTO heterojunctions showed type II p–n band alignment, distinguished by the direct observation of carrier migration under light illumination, and confirmed by the x-ray photoelectron spectroscopy, Mott–Schottky measurements, ultraviolet photoelectron spectra and capture experiments. These all heterojunctions enjoyed better photocatalytic performance to degrade methylene blue and antibiotics (Enrofloxacin, Metronidazole and tetracycline) than the pure NMTO, attributed to their effective separation of the photoinduced electron–hole pairs owing to the staggered band alignment. Prominently, the NiO/NMTO and Co3O4/NMTO p–n heterojunctions exhibited superior degradation ability to the CuO/NMTO Z-scheme heterojunction. The initial relative Fermi position of two semiconductors is the prerequisite to determine whether the p–n heterojunction or direct Z-scheme heterojunction is built because the electrons diffuse from one semiconductor with a higher Fermi level to another with a lower Fermi level while the holes diffuse reversely until a united Fermi level when they combine. The built-in electric field at the heterojunction interface is determined by the difference in the initial Fermi levels or work functions of two semiconductors, regulating the separation ability of photogenerated electrons and holes to affect the photocatalytic performance. Thus, the high difference in the initial Fermi levels of semiconductors is crucial in the development of heterojunctions with staggered band alignment to obtain high performance in photocatalytic reactions.

Accurate Vertical Ionization Energy of Water and Retrieval of True Ultraviolet Photoelectron Spectra of Aqueous Solutions.

Ultraviolet (UV) photoelectron spectroscopy provides a direct way of measuring valence electronic structure; however, its application to aqueous solutions has been hampered by a lack of quantitative understanding of how inelastic scattering of low-energy (<5 eV) electrons in liquid water distorts the measured electron kinetic energy distributions. Here, we present an efficient and widely applicable method for retrieving true UV photoelectron spectra of aqueous solutions. Our method combines Monte Carlo simulations of electron scattering and spectral inversion, with molecular dynamics simulations of depth profiles of organic solutes in aqueous solution. Its application is demonstrated for both liquid water, and aqueous solutions of phenol and phenolate, which are ubiquitous biologically relevant structural motifs.

Preparation and characterization of novel Niln2S4/UiO-66 photocatalysts for the efficient degradation of antibiotics in water

Photocatalysis is considered an economical, environmentally friendly, and effective technology for removing pollutants. The construction of Z-Scheme heterojunctions has been identified as one of the feasible solutions capable of enhancing the photocatalytic activity. Herein, a series of visible light responsive photocatalysts (NiIn2S4/UiO-66 composites) with excellent activity and stability were prepared by using a solvothermal process. It is found that 20 mg L−1 of tetracycline (TC) could be almost completely degraded under visible light irradiation within 1 h, when the mass ratio of NiIn2S4 to UiO-66 is 0.5:1 (NISU-0.5) and the solution pH = 11. In addition, after six cycles, the degradation rate of tetracycline photocatalyzed by NISU-0.5 still reach up to 90%. Ultraviolet photoelectron spectra (UPS), X-ray photoelectron spectra (XPS) and electron spin resonance measurements (ESR) confirm the formation of the Z-Scheme heterostructure between NiIn2S4 and UiO-66. The synergistic effect between built-in electric field, energy band bending and coulomb interactions in interface of Z-Scheme heterojunction is conducive to restrain the recombination of photogenerated electrons and holes, which greatly improve the photocatalytic activity. In conclusion, this study offers a new thought for design and synthesis of Z-Scheme heterojunctions and provides a cost-effective strategy for solving environmental pollution and energy problems in the future.

Functionalized-MXene-nanosheet-doped tin oxide enhances the electrical properties in perovskite solar cells

An appropriate electron transport layer (ETL) with better energy alignment and enhanced charge transfer, thereby helping efficient extraction and transport of photogenerated carriers, is essential to achieve the creation of high-performance devices. In this work, we use functionalized MXene modified with fluoroalkylsilane and dodecyltrimethoxysilane molecules, denoted as SnO2-MF and SnO2-MH, as nanosheet dopants in the SnO2 ETL. From density functional theory (DFT) calculations and ultraviolet photoelectron spectra (UPS) spectra, we see that better band alignment is achieved for the SnO2-MH ETL. Meanwhile, functionalized MXene nanosheets represent high electrical conductivity and mobility and could form zero Schottky barrier heterojunction with SnO2, effectively and rapidly enhancing carrier transfer. Finally, the suitable surface energy achieved by functionalized MXene additives can enlarge the grain size of perovskite thin films. Consequently, a significant improvement of power conversion efficiency (PCE) from 20.98% to 23.66% (24.12% for the champion device with a fill factor [FF] over 0.84) can be achieved for devices based on the SnO2-MH ETL, which also possess improved moisture resistance and operational stability.

Organic cation – polystyrene sulfonate polyelectrolytes as hole transporting interfacial layers in perovskite solar cells

Interfacial layers are important tools that can be used to control the band structure and performance of perovskite solar cells, however, relatively few p-type materials are known compared to n-type interfacial materials. In this work, we demonstrate that anionic polystyrene sulfonate (PSS) polyelectrolytes, with organic cations as counter ions, are able to function as p-type interfacial layers in methylammonium lead triiodide (MAPbI3) perovskite solar cells. These polyelectrolytes show interfacial dipole formation resulting in a significant increase in the work function of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and decrease in the Fermi energy of the CH3NH3PbI3 layer (band bending) as evident from x-ray photoelectron spectra (XPS) and ultraviolet photoelectron spectra (UPS) analysis. These effects result in superior electron blocking and hole extracting characteristics at the anode. We show that open circuit voltage, power conversion efficiency and other device parameters can be increased compared to the devices without interfacial layers using these convenient polyelectrolytes.

Electronic and Optical Properties of Polythiophene Molecules and Derivatives

The electronic and optical properties of polythiophene (PT) for polymer light-emitting diodes (PLEDs) were calculated using density functional theory (DFT) and time-dependent DFT. We calculated the electronic and optical properties of thiophene and PT polymers with degrees of polymerization (DP) from 2 to 30 monomers (T1–T30) and their derivatives. The associated highest occupied molecular orbital (HOMO) energy, lowest unoccupied molecular orbital (LUMO) energy, band gaps, electron orbitals, and molecular structures were determined. As the DP increased, the LUMO energy gradually decreased, and the HOMO energy gradually increased. The band gap of PT approached 2 eV as the DP of the PT polymer increased from 1 to 30. The calculations and exchange–correlation functional were verified against values in the literature and experimental data from cyclic voltammetry (redox potential) and ultraviolet-visible, photoluminescence, and ultraviolet photoelectron spectra. The color of PT PLEDs can be adjusted by controlling the DP of the polymer and the substituents.

Resolving the Tribo-catalytic reaction mechanism for biochar regulated Zinc Oxide and its application in protein transformation

The utilization of mechanical energy to control water pollutants under dark conditions is currently a point of study focus. Herein, biochar -zinc oxide (BC-ZnO) composites with various structures were synthesized by co-pyrolysis of cotton and ZnO at different temperature and used for tribo-catalytic reaction. The introduction of BC can improve charge transmission and separation efficiency. Ultraviolet photoelectron spectra (UPS) and density functional theory (DFT) calculation prove the addition of BC can reduce work function of ZnO, and enhance its electron-donating ability. Specially, suitable adsorption amount is the key factor to improve the tribo-catalytic performance. When the pyrolysis temperature is 600 °C, BC-ZnO has the best degradation efficiency, which can degrade 90% Rhodamine B (RhB) in 75 min, while ZnO can degrade only 38%. On this basis, using bovine serum albumin (BSA) as a model, the effect of tribo-catalytic reaction on controlling proteins in water was studied by fluorescence excitation-emission matrix spectroscopy (3D EEM) and infrared microscope, and the transformation of proteins was further analyzed. This study provides a new strategy to improve the tribo-catalytic performance of ZnO, and explores its application prospects of biological wastewater control.

Correlating X-ray absorption spectra and ultraviolet photoelectron spectra to understand magnetic and transport properties of charge-ordered perovskite manganites

X-ray absorption spectra and ultraviolet photoelectron spectra have been used to compare the electronic structures of (La0.6Pr0.4)0.65Ca0.35MnO3 and (La0.4Pr0.6)0.65Ca0.35MnO3 systems to understand their magnetic and transport properties. Structural analysis showed that increased Pr-substitution triggers expansion in apical Mn-O bond lengths, which eventually causes narrowing of the eg band width. (La0.6Pr0.4)0.65Ca0.35MnO3 exhibits ferromagnetic ordering below 215 K and shows semiconductor to metal transition around 212 K, while (La0.4Pr0.6)0.65Ca0.35MnO3 exhibit complex magnetic behaviour and also show local semiconductor to metal transition around 95 K before re-entering into semiconducting region again below 80 K. A slight change in the slope of resistivity around 220 K and four orders increase in resistivity indicates presence of charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3. Moreover, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits ~ 100% magneto-resistance. X-ray absorption studies of Mn-L2,3 edge and O-K edge reveal the onset of Mn3+-Mn4+ charge-ordering in (La0.4Pr0.6)0.65Ca0.35MnO3 at room temperature. O-K edge spectra exhibits an increased spectral intensity of O2p-Mn3d(eg) states near Fermi-level for (La0.4Pr0.6)0.65Ca0.35MnO3, which reflects the number of unoccupied eg states. Valence band ultraviolet photoemission spectroscopy study revealed that Mn3d(eg) states of (La0.6Pr0.4)0.65Ca0.35MnO3 extends into conduction band by crossing-over the Fermi-level. On the other hand, (La0.4Pr0.6)0.65Ca0.35MnO3 exhibits no electronic state at Fermi-level explaining stronger semiconducting nature of x = 0.6 system than x = 0.4 near room temperature.

Identification of an ultrafast internal conversion pathway of pyrazine by time-resolved vacuum ultraviolet photoelectron spectrum simulations.

The internal conversion from the optically bright S2 (1B2u, ππ*) state to the dark S1 (1B3u, nπ*) state in pyrazine is a standard benchmark for experimental and theoretical studies on ultrafast radiationless decay. Since 2008, a few theoretical groups have suggested significant contributions of other dark states S3 (1Au, nπ*) and S4 (1B2g, nπ*) to the decay of S2. We have previously reported the results of nuclear wave packet simulations [Kanno et al., Phys. Chem. Chem. Phys. 17, 2012 (2015)] and photoelectron spectrum calculations [Mignolet et al., Chem. Phys. 515, 704 (2018)] that support the conventional two-state picture. In this article, the two different approaches, i.e., wave packet simulation and photoelectron spectrum calculation, are combined: We computed the time-resolved vacuum ultraviolet photoelectron spectrum and photoelectron angular distribution for the ionization of the wave packet transferred from S2 to S1. The present results reproduce almost all the characteristic features of the corresponding experimental time-resolved spectrum [Horio et al., J. Chem. Phys. 145, 044306 (2016)], such as a rapid change from a three-band to two-band structure. This further supports the existence and character of the widely accepted pathway (S2 → S1) of ultrafast internal conversion in pyrazine.

Temperature-induced first-order electronic topological transition in β -Ag2Se

β-Ag2Se is a promising material for room temperature thermoelectric applications and magneto-resistive sensors. However, no attention was paid earlier to the hysteresis in the temperature dependence of resistivity [ρ(T)]. Here, we show that a broad hysteresis above 35 K is observed not only in ρ(T), but also in other electronic properties such as Hall coefficient [RH(T)], Seebeck coefficient, thermal conductivity, and ultraviolet photoelectron spectra (UPS). We also show that the hysteresis is not associated with a structural transition. The ρ(T) and RH(T) show that β-Ag2Se is semiconducting above 300 K, but metallicity is retained below 300 K. While electronic states are absent in the energy range from the Fermi level (EF) to 0.4 eV below the EF at 300 K, a distinct Fermi edge is observed in the UPS at 15 K suggesting that the β-Ag2Se undergoes an electronic topological transition from a high-temperature semiconducting state to a low-temperature metallic state. Our study reveals that a constant and moderately high thermoelectric figure of merit in the range 300–395 K is observed due to the broad semiconductor to metal transition in β-Ag2Se.

Controllable construction of hierarchically CdIn2S4/CNFs/Co4S3 nanofiber networks towards photocatalytic hydrogen evolution

The efficient charge separation and adequate active sites are crucial factors for hydrogen (H2) evolution from solar-driven water splitting. Herein, a novel one-dimensional-two-dimensional (1D-2D) CdIn2S4/carbon nanofibers (CNFs)/Co4S3 tandem Schottky heterojunction was synthesized by in-situ electrospinning combined with a hydrothermal method. The CNFs with in-situ embedded Co4S3 nano-grains provided an excellent 1D substrate with plentiful active sites, which benefited the growth of 2D ultrathin CdIn2S4 nanosheets to construct the tandem Schottky heterojunction. It is noteworthy that the spatial charge separation and directional transportation originating from the rectification effect of the Schottky barrier remarkably prolong the charge carrier lifespan. The optimal composite shows the H2 production activity at a rate of 25.87 mmoL·g−1·h−1 and superior photostability. Ultraviolet photoelectron spectra and UV–vis diffuse reflectance spectra revealed the information of the band structure and built-in electric fields in the heterojunction. Moreover, photoelectrochemical measurements and in-situ irradiated X-ray photoelectron spectra verified the efficient carrier separation and electron-transfer path in the heterojunction. This work inaugurates a new avenue in designing the CNFs-based heterojunction for high-efficiency photocatalysis.

Controlling the Pd Metal Contact Polarity to Trigonal Tellurium by Atomic Hydrogen‐Removal of the Native Tellurium Oxide

AbstractPalladium (Pd) electrodes have enabled superior performance in Te‐based devices. Theory predicts strong Fermi level (EF) pinning near the Te valence band, which likely explains the predominant p‐type conduction in Te transistors. The effects of the native TeOx on the contact polarity has not been explored. In this work, X‐ray and ultraviolet photoelectron spectra (XPS and UPS, respectively) reveal the native surface oxide de‐pins the EF between Pd and trigonal Te (t‐Te) and can reduce contact resistance of hole and electron contacts to Te for back end of line‐compatible complementary logic circuits. Atomic hydrogen reduces the native t‐Te oxide below the XPS detection limit, after which, the EF resides near the t‐Te conduction band edge. Therefore, an n‐type band alignment is formed between Pd and t‐Te via an atomic hydrogen treatment. Furthermore, UPS shows the EF is pinned near the t‐Te conduction band edge. XPS indicates the formation of a PdTex intermetallic at the Pdt‐Te interface also affects the electrostatics of the interface. The concentration of PdTex is 40% higher when TeOx is removed from the t‐Te surface before Pd metallization, likely the root cause of the low (pinned) work function when the native oxide is absent.

Impact of gamma-ray irradiation on the electronic structures of PCBM and P3HT organic semiconductor films

Gamma-ray irradiation alters the material properties of organic semiconductors, especially the electronic structure. These changes due to ionization can be useful in dosimetry. Although there are some device-based reports, the effect of gamma-ray irradiation on the electronic structure of organic semiconductors is still unclear. In this study, we investigated the electronic structures of representative organic semiconductor films, namely, n-type [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and p-type poly(3-hexylthiophene-2,5-diyl) (P3HT), after gamma-ray irradiation generated by a Cs-137 source. The X-ray and ultraviolet photoelectron spectra of the PCBM and P3HT films were measured for various gamma-ray doses. In both PCBM and P3HT, chemical interaction with atmospheric oxygen, assisted by high-energy photons, led to significant oxidation. However, the degree of oxidation of PCBM was considerably higher than that of P3HT. The oxidation also affects the valence electronic structures. The possible chemical structures of the oxidized PCBM and P3HT are estimated using density functional theory calculations.

Improved Photovoltaic Characteristics of Inverted Polymer Solar Cells With Indium-Doped ZnO at Low-Temperature Annealing as Electron-Transport Layer

For inverted solar cells, UV-socking is usually necessary to improve the photovoltaic performance of the devices. In this article, inverted polymer solar cells with pure and indium-doped ZnO as an electron transport layer were fabricated, and their properties were investigated. We found that the In-doped ZnO-based device has a high power conversion efficiency of 5.99%, and a nearly 40% improvement in comparison with the pure ZnO-based device without the UV treatment. Those investigations of X-ray diffraction, Photoluminescence, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectra show that the doping of indium into the lattice of ZnO can decrease defect states and increase the work function, leading to more efficient electron extraction, and consequently, an enhancement of photovoltaic performance of the device.