X-ray and UV photoelectron spectroscopy of Ag nanoclusters

The main purpose of the present work is to analyze a series of Ag nanoparticles (NPs) with different size or ligand functionalization by using X-ray photoelectron spectroscopy (XPS) and to identify the differences in the band-shape and energy peak position of photoemission spectra due to the particle dimension. A transmission electron microscopy characterization was performed, to verify the consistency of the results. Three types of samples were prepared starting from AgNO₃ water solution and adding different capping agents. In the first two cases, the formation of NPs was promoted by the reduction of silver ions Ag⁺¹ to metallic Ag⁰ through the addition of sodium borohydride, whereas in the last case, it was triggered by the exposure to UV light. Depending on the size of the NPs, a different physical behavior can be recognized. NPs with diameter of about 5 nm are characterized by the phenomenon of localized surface plasmon resonance (LSPR). The other type of samples having a diameter of about 1.5 nm presents discrete energy levels instead of electronic bands, and in this case, a typical fluorescence phenomenon can be observed. In the latter case, we can refer to such systems as nanoclusters. The XPS analyses were focused on the Ag 3D spectra looking for the possible shifts of the Ag doublet as a function of the particles size. The ultraviolet photoelectron spectroscopy with He II source was used for the investigation of possible changes in the valence band.     

Ultraviolet Photoelectron Spectra of Potassium Dosed Higher Fullerenes

Abstract

Ultraviolet photoelectron spectra of potassium dosed higher fullerenes are measured with a synchrotron radiation light source. Potassium dosing to higher fullerenes brings a new structure between the spectral onset of pristine fullerenes and the Fermi level. As the spectral edge of the new structure does not cross the Fermi level, potassium dosed higher fullerenes are not metallic but semiconductive. When the potassium is excessively dosed to the fullerenes, the lower binding energy structures above 5 eV become faint. In contrast to this phenomenon, four distinct structures appear between 5 and 14 eV.     

Electronic and adsorption properties of Ce‐Ag layers

Silver has rarely been considered as the catalyst for CO oxidation, although it has been recognized to be very active in several partial oxidation reactions such as ethylene epoxidation and formaldehyde synthesis. It is generally believed that a metal support interaction plays an important role in catalytic processes. Therefore in our study, we examined electronic and adsorption properties of cerium deposited onto a polycrystalline silver substrate. Layers of approximately one monolayer of cerium deposited on a clean silver substrate were examined in situ using surface‐sensitive techniques—by XPS, ultraviolet photoelectron spectroscopy (UPS) and low energy ion scattering (LEIS). CO molecular adsorption was observed by UPS and LEIS; experimental results exhibited CO adsorption on Ce atoms sites. Oxygen adsorption on deposited layers led to a strong oxidation; stoichiometry of oxidized layers was given by amount of adsorbed oxygen and by temperature. Copyright © 2011 John Wiley & Sons, Ltd.     

The electronic band structure analysis of OLED device by means of in situ LEIPS and UPS combined with GCIB

Low-energy inverse photoelectron spectroscopy (LEIPS) and ultraviolet photoelectron spectroscopy (UPS) incorporated into the multitechnique XPS system were used to probe the ionization potential and the electron affinity of organic materials, respectively. By utilizing gas cluster ion beam (GCIB), in situ analyses and depth profiling of LEIPS and UPS were also demonstrated. The band structures of the 10-nm-thick buckminsterfullerene (C₆₀) thin film on Au (100 nm)/indium tin oxide (100 nm)/glass substrate were successfully evaluated in depth direction.     

Work function and negative electron affinity of ultrathin barium fluoride films

Thin films of barium fluorides with different thicknesses were deposited on GaAs substrate by electron beam evaporation. The aim of the work was to identify the best growth conditions for the production of coatings with a low work function suitable for the anode of hybrid thermionic-photovoltaic (TIPV) devices. The chemical composition and work function φ of the films with different thicknesses were investigated by X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). The lowest value of φ = 2.1 eV was obtained for the film with a thickness of ~2 nm. In the valence band spectra of the films at low kinetic energy, near the cutoff, a characteristic peak of negative electron affinity was present. This effect contributed to a further reduction of the film's work function.     

Modification of Hole Transport Layers for Fabricating High Performance Non‐fullerene Polymer Solar Cells

Interfacial engineering is expected to be a feasible strategy to improve the charge transport properties of the hole transport layer (HTL), which is of crucial importance to boost the device performance of organic solar cells (OSCs). In this study, two types of alcohol soluble materials, 2,3,5,6‐tetrafluoro‐7,7,8,8‐tetracyanoquinodimethane (F₄‐TCNQ) and di‐tetrabutylammoniumcis–bis(isothiocyanato)bis (2,2’‐bipyridyl‐4,4’‐dicarboxylato) ruthenium(II) (N719) dye were selected as the dopant for HTL. The doping of F₄‐TCNQ and N719 dye in poly (ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) with and without integrating a graphene quantum‐dots (G‐QDs) layer has been explored in poly[[2,6′‐4‐8‐di(5‐ethylhexylthienyl)benzo[1,2‐b:3,3‐b]dithiophene][3‐fluoro‐2[(2‐ethylhexyl)carbonyl]thieno[3,4‐b]thio‐phenediyl:(2,2′‐((2Z,2′Z)‐(((4,4,9, 9‐tetrakis(4‐hexylphenyl)‐4,9‐dihydro‐s‐indaceno[1,2‐b:5,6‐b′]dithiophene‐2,7‐diyl)bis(4‐((2‐ethylhexyl)oxy)thiophene‐5,2‐diyl))bis(methanylylidene))bis(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐indene‐2,1‐diylidene))dimalononitrile (PTB7‐Th:IEICO‐4F) OSCs. The power conversion efficiency of the non‐fullerene OSCs has been increased to 10.12% from 8.84%. The influence of HTL modification on the nano‐morphological structures and photophysical properties is analyzed based on the comparative studies performed on the control and modified devices. The use of chemical doping and bilayer strategy optimizes the energy level alignment, nanomorphology, hole mobility, and work‐function of HTL, leading to considerable reduction of the leakage current and recombination losses. Our work demonstrates that the doping of HTL and the incorporation of G‐QDs layer to constitute a bilayer HTL is an promising strategy to fabricate high performance non‐fullerene polymer solar cells     

Theoretical study of new acceptor and donor molecules based on polycyclic aromatic hydrocarbons

Functionalized polycyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied theoretically with the help of B3LYP hybrid density functionals. Using the ΔSCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While σ-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.     

Studies of degradation behaviors of poly (3‐hexylthiophene) layers by X‐ray photoelectron spectroscopy

Degradation behaviors of poly(3‐hexylthiophene‐2,5‐diyl) (P3HT) layers on NiO in the presence of H₂O at ambient pressure and dark conditions were studied using X‐ray photoelectron spectroscopy (XPS). Upon H₂O exposure at 120 °C, partial oxidation of P3HT together with molecular water incorporation, but with the maintained local ring‐structure, were deduced by XPS. Valence band spectra of XPS evidenced that the partial oxidation of P3HT local structure could alter π‐conjugation systems of P3HT layers, forming additional electronic states close to its original highest occupied molecular orbital. For comparison, P3HT surface was also exposed to O₂, and no change in the S 2p and C 1s spectra was found by O₂ exposure at 120 °C, implying that H₂O plays a major role at the initial stage of P3HT oxidation. Copyright © 2014 John Wiley & Sons, Ltd.     

Probing the interfacial interaction between monolayer molybdenum disulfide and Au nanoclusters

Coupling of plasmonic metal nanostructures on two‐dimensional materials represents one promising approach to improve their optoelectronic device performance. In this article, we systematically investigated the interfacial interactions between Au nanoclusters and monolayer molybdenum disulfide (MoS₂) and the effect of Au decoration on the electrical transport and optical properties of MoS₂, through the combination of in situ MoS₂ field‐effect transistor device evaluation and in situ ultraviolet photoelectron spectroscopy and X‐ray photoelectron spectroscopy measurements. The in situ X‐ray photoelectron spectroscopy/ultraviolet photoelectron spectroscopy experiments revealed a weak interfacial coupling between Au nanoclusters and monolayer MoS₂. The absence of strong charge transfer between Au nanoclusters and MoS₂ was further confirmed by the photoluminescence and Raman measurements. It was also found that the electron charge‐carrier concentration in monolayer MoS₂ weakly depended on the coverage of Au nanoclusters. Copyright © 2017 John Wiley & Sons, Ltd.     

Growth and characterization of ultrathin carbon films on electrodeposited Cu and Ni

Ultrathin carbon films were grown on different types of metallic substrates. Free‐standing foils of Cu and Ni were prepared by electroforming, and a pure Ni film was obtained by galvanic displacement on a Si wafer. Commercial foil of Ni 99.95% was used as a reference substrate. Carbon films were grown on these substrates by chemical vapour deposition in a CH₄‐H₂ atmosphere. Obtained films were characterized by Raman spectroscopy, X‐ray photoelectron spectroscopy (XPS), Auger electron spectroscopy, and ultraviolet photoemission spectroscopy. The XPS at grazing collection angle was used to determine the thickness of carbon films. Depending on the deposition parameters, the films of graphene or graphite were obtained on the different substrates. The uniformity of graphene and its distribution over the sample area were investigated from Raman data, optical images, and XPS chemical maps. The presence of graphene or graphite in the films was determined from the Raman spectra and Auger peak of C KVV. For this purpose, the D parameter, which is a fingerprint of carbon allotropes, was determined from C KVV spectra acquired by using X‐rays and electron beam. A formation of an intermediate layer of metal hydroxide was revealed in the samples with graphene overlayer.