Ultraviolet Photoelectron Spectroscopy Data Research Articles

Bi2WO6 Incorporation of g‐C3N4 to Enhance the Photocatalytic N2 Reduction Reaction and Antibiotic Pollutants Removal

Synthesis of ammonia from photocatalytic N2 reduction is challenging due to the fast recombination of electron–hole pairs and the low selectivity of N2 on catalysts. This can be addressed by creating heterojunctions to separate the photogenerated carriers adequately. In this regard, BW/g‐C3N4 is synthesized and the weight percentage of g‐C3N4 is varied. The best photocatalytic activity for N2 reduction reaction (N2RR) is achieved with a ratio of BW/gC3N4 in 3.5:2 ratio, deemed to be the optimized heterojunction. N2‐temperature programmed desorption analysis shows outstanding chemisorption of N2 adsorbed on the BW/g‐C3N4 surface compared to pristine g‐C3N4 and BW. Additionally, forming a heterojunction enhances the charge transfer process and well‐separated electron–hole pairs, significantly boosting the water oxidation process on the catalytic surface. Photoelectrochemical analysis reveals that BW/g‐C3N4 exhibits the shortest hole relaxation lifetime and higher current density than its pristine counterparts. The robust contact between g‐C3N4 and BW reduces the work function of BW/g‐C3N4 based on ultraviolet photoelectron spectroscopy data. Ammonia production with the optimized BW/gC3N4‐3.5:2 is 5.3 and 2.1 times higher than pure g‐C3N4 and Bi2WO6, respectively. Meanwhile, BW/g‐C3N4 demonstrates excellent photocatalytic activity toward antibiotic pollutant degradation as well. After 150 min of visible light irradiation, the removal of 94% ciprofloxacin (CIP) is observed. Finally, a possible mechanism is proposed for photocatalytic N2RR and CIP degradation.

Effect of the Energy Offset on the Charge Dynamics in Nonfullerene Organic Solar Cells.

The energy offset, considered as the driving force for charge transfer between organic molecules, has significant effects on both charge separation and charge recombination in organic solar cells. Herein, we designed material systems with gradually shifting energy offsets, including both positive and negative values. Time-resolved spectroscopy was used to monitor the charge dynamics within the bulk heterojunction. It is striking to find that there is still charge transfer and charge generation when the energy offset reached -0.10 eV (ultraviolet photoelectron spectroscopy data). This work not only indicates the feasibility of the free carrier generation and the following charge separation under the condition of a negative offset but also elucidates the relationship between the charge transfer and the energy offset in the case of polymer chlorination.

Surfactant-Enriched ZnO Surface via Sol-Gel Process for the Efficient Inverted Polymer Solar Cell.

In this study, we demonstrate that the top surface is enriched by surfactants, tetraoctylammonium bromide, and cetylpyridinium bromide (CPB), in the sol-gel ZnO, being evidenced by the Br depth profile of electron spectroscopy for chemical analysis data. X-ray photoelectron spectroscopy results showed the formation of Zn-Br bonding due to the oxygen defects occupied by Br at the surfactant-enriched ZnO surface. The surfactant-enriched ZnO surface possessed a smoother surface and more hydrophobicity than the pristine ZnO from the experimental results of atomic force microscopy and contact angle, respectively. On the basis of ultraviolet photoelectron spectroscopy data, the work function slightly reduced due to the dipole built-up by the electrostatic force between Br- and N+ to enhance the electron extraction ability. The improved properties benefited the power conversion efficiency (PCE) of bulk-heterojunction polymer solar cells (PSCs) by spin-coating the active layer on the surfactant-enriched ZnO surface. The inverted PSCs with the surfactant-enriched ZnO surface showed the highest PCE of 9.55% for the CPB case, in comparison with the pristine ZnO surface (8.08% PCE). This study discloses that turning the ZnO surface is easily achieved by the addition of surfactants with different molecular structures in the sol-gel ZnO for high performance polymer solar cells.

An electron acceptor molecule in a nanomesh: F4TCNQ on h-BN/Rh(111)

The adsorption of molecules on surfaces affects the surface dipole and thus changes in the work function may be expected. The effect in change of work function is particularly strong if charge between substrate and adsorbate is involved. Here we report the deposition of a strong electron acceptor molecule, tetrafluorotetracyanoquinodimethane C12F4N4 (F4TCNQ) on a monolayer of hexagonal boron nitride nanomesh (h-BN on Rh(111)). The work function of the F4TCNQ/h-BN/Rh system increases upon increasing molecular coverage. The magnitude of the effect indicates electron transfer from the substrate to the F4TCNQ molecules. Density functional theory calculations confirm the work function shift and predict doubly-charged F4TCNQ2− in the nanomesh pores, where the h-BN is closest to the Rh substrate, and to have the largest binding energy there. The preferred adsorption in the pores is conjectured from a series of ultraviolet photoelectron spectroscopy data, where the σ bands in the pores are first attenuated. Scanning tunneling microscopy measurements indicate that F4TCNQ molecules on the nanomesh are mobile at room temperature, as “hopping” between neighboring pores is observed.

Electronic Structures and Optical Properties of Al, Cu and Ag in Bulk, Thin Film and Nanoparticle Forms

The electronic structures of aluminum, copper, and silver were investigated in bulk, thin-film and nanoparticle forms using first-principles band structure methods. The density of states (DOS) and partial density of states (PDOS) were also calculated. The calculations show the progression from continuous bands to subbands to discrete states as spatial confinement is imposed by going from bulk to thin film to particle forms. The choice of metals in this study allows for the comparison of the degree of localization of the d-electrons on inter-band transitions, which, in part, governs the optical properties of the metals. The associated optical properties described by the imaginary component of the dielectric function, ε2(ω), were also investigated. The interband contributions to ε2(ω) were calculated from the band structure, while the intraband contributions were calculated using the Drude theory for free electrons. Both contributions to ε2(ω)are needed to understand the optical properties of metals and to interpret their reflectance spectra. The interband transitions need to be considered to explain reflectivity at energies lower than the plasma frequencies, but not all interband transitions result in reflectance peaks since they are significantly weaker compared to the intraband contributions at lower energies. We have studied the dependence of the DOS, PDOS and ε2(ω) on the choice of exchange potentials by comparing results obtained using the generalized gradient approximation (GGA) and the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional. In copper, where ultraviolet photoelectron spectroscopy data are available, the HSE potential leads to better agreement with the experiment than when using the GGA potential, and HSE reproduces correctly the variation of binding energy of the d electrons as spatial confinement is imposed. The corresponding dependence of exchange potential for silver exists, but is less severe.

Electronic structures and optical properties of Al, Cu, and Ag in zero, two, and three dimensional structures

The electronic structures and associated optical properties of aluminum, copper, and silver were investigated in bulk, thin-film and nanoparticle forms using first-principles band structure methods. The calculations show the progression from continuous bands to subbands to discrete states as spatial confinement is imposed in one and three dimensions. The associated optical properties described by the imaginary component of the dielectric function, ε2(ω), were also investigated. The interband contributions to ε2(ω) were calculated from the band structure, while the intraband contributions were calculated using the Drude theory for free electrons. Both contributions to ε2(ω) are needed to understand the optical properties of metals and to interpret their reflectance spectra. The interband transitions need to be considered to explain reflectivity at energies lower than the plasma frequencies, but not all interband transitions result in reflectance peaks since they are significantly weaker compared to the intraband contributions at lower energies. We have studied the dependence of the density of states on the choice of exchange potentials. In copper, where ultraviolet photoelectron spectroscopy data are available, using a hybrid functional, Heyd-Scuseria-Ernzerhof, leads to better agreement with the experiment than when using the generalized gradient approximation potential, and it reproduces correctly the variation of binding energy of the d electrons going from bulk to thin film. The corresponding dependence exchange potential for silver is less severe.

Non-volatile memory devices with redox-active diruthenium molecular compound

Reduction-oxidation (redox) active molecules hold potential for memory devices due to their many unique properties. We report the use of a novel diruthenium-based redox molecule incorporated into a non-volatile Flash-based memory device architecture. The memory capacitor device structure consists of a Pd/Al2O3/molecule/SiO2/Si structure. The bulky ruthenium redox molecule is attached to the surface by using a ‘click’ reaction and the monolayer structure is characterized by x-ray photoelectron spectroscopy to verify the Ru attachment and molecular density. The ‘click’ reaction is particularly advantageous for memory applications because of (1) ease of chemical design and synthesis, and (2) provides an additional spatial barrier between the oxide/silicon to the diruthenium molecule. Ultraviolet photoelectron spectroscopy data identified the energy of the electronic levels of the surface before and after surface modification. The molecular memory devices display an unsaturated charge storage window attributed to the intrinsic properties of the redox-active molecule. Our findings demonstrate the strengths and challenges with integrating molecular layers within solid-state devices, which will influence the future design of molecular memory devices.

Seleno groups control the energy-level alignment between conjugated organic molecules and metals

The charge injection from metallic electrodes into hole transporting layers of organic devices often suffers from deviations from vacuum-level alignment at the interface. Even for weakly interacting cases, Pauli repulsion causes an interface dipole between the metal and conjugated organic molecules (COMs) (so called "push-back" or "cushion" effect), which leads notoriously to an increase of the hole injection barrier. On the other hand, for chalcogenol self assembled monolayers (SAMs) on metal surfaces, chemisorption via the formation of chalcogen-metal bonds is commonly observed. In these cases, the energy-level alignment is governed by chalcogen-derived interface states in the vicinity of the metal Fermi-level. In this work, we present X-ray and ultraviolet photoelectron spectroscopy data that demonstrate that the interfacial energy-level alignment mechanism found for chalcogenol SAMs also applies to seleno-functionalized COMs. This can be exploited to mitigate the push-back effect at metal contacts, notably also when COMs with low ionization energies are employed, permitting exceedingly low hole injection barriers, as shown here for the interfaces of tetraseleno-tetracene with Au(111), Ag(111), and Cu(111).

Vibronic Coupling in the Ground State of Oligoacene Cations: The Performance of Range-Separated Hybrid Density Functionals

Oligoacenes such as naphthalene, anthracene, tetracene, and pentacene are among the best hole-transport organic semiconductors. An important parameter in the determination of the hole mobility is the coupling between the charge carrier and the vibrational modes. Here, we have evaluated the hole–vibration coupling constants in the radical-cation ground state of these molecules by means of the range-separated LC-ωPBE and ωB97 density functionals, with non-empirical optimization of the range-separation parameter ω. Our results indicate that both ω-tuned functionals yield similar relaxation energies and coupling constants. A comparison of the simulated vibrational structures of the first ionization band to the gas-phase ultraviolet photoelectron spectroscopy data underlines that the hole–vibration coupling constants derived by means of the non-empirically tuned LC-ωPBE and ωB97 functionals are in excellent agreement with experiment and superior to those derived from B3LYP calculations.

Geometric and Electronic Structure of Templated C60 on Diindenoperylene Thin Films

The structural order of C60 thin films is shown to be significantly improved by inserting a templating layer of diindenoperylene (DIP) between the SiO2 substrate and C60. In contrast to growth on an amorphous substrate like SiO2, C60 grown on DIP exhibits alignment of fcc-domains with the (111) plane parallel to the substrate and a significant increase of the coherent in-plane island size by a factor of ∼4. Modification of the structural quality of the DIP bottom layer leads to a change in structural order in the C60 top layer. In addition, ultraviolet photoelectron spectroscopy data from templated and nontemplated C60 films are discussed. In contrast to other anisotropic organic molecules, for C60 the spectral broadening and density of states of the highest occupied molecular orbital region do not depend significantly on the structural order in the C60 film, which can be rationalized by the isotropic shape of the C60 molecule.

Fermi level shift and doping efficiency inp-doped small molecule organic semiconductors: A photoelectron spectroscopy and theoretical study

We study the mechanism of molecular doping of the organic small molecule N,N,N${}^{\ensuremath{'}}$,N${}^{\ensuremath{'}}$-tetrakis(4-methoxyphenyl)-benzidine (MeO-TPD) doped with the fluorinated fullerene ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$ or the acceptor molecule 2,2${}^{\ensuremath{'}}$-(perfluoronaphthalene-2,6-diylidene)dimalononitrile (F6-TCNNQ). Varying the doping concentration, photoemission spectroscopy measurements show a comparable Fermi level shift for both dopants. The doping efficiency, defined as the ratio of free charge carriers (holes) to acceptors, is estimated from the depletion layer thickness in metal/intrinsic/$p$-doped structures. For low concentrations, we observe rather high doping efficiencies of up to 36$%$ for ${\mathrm{C}}_{60}{\mathrm{F}}_{36}$, whereas for both dopants the doping efficiency strongly decreases with increasing doping concentration down to less than 10$%$. By numerically solving the charge neutrality equation using a classical semiconductor physics approach and comparing the results to the ultraviolet photoelectron spectroscopy data, we show that for very low concentrations doping is hindered by deep intragap states. In particular, the calculations can statistically explain the strong decrease of the doping efficiency for high doping concentrations.

Optimizing organic photovoltaics using tailored heterojunctions: A photoinduced absorption study of oligothiophenes with low band gaps

A power conversion efficiency of 3.4% with an open-circuit voltage of $1\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ was recently demonstrated in a thin film solar cell utilizing fullerene ${\mathrm{C}}_{60}$ as acceptor and a new acceptor-substituted oligothiophene with an optical gap of $1.77\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ as donor [K. Schulze et al., Adv. Mater. (Weinheim, Ger.) 18, 2872 (2006)]. This prompted us to systematically study the energy- and electron transfer processes at the oligothiophene:fullerene heterojunction for a homologous series of these oligothiophenes. Cyclic voltammetry and ultraviolet photoelectron spectroscopy data show that the heterojunction is modified due to tuning of the highest occupied molecular orbital energy for different oligothiophene chain lengths, while the lowest unoccupied molecular orbital energy remains essentially fixed due to the presence of electron-withdrawing end groups (dicyanovinyl) attached to the oligothiophene. Use of photoinduced absorption (PA) allows the study of the electron transfer process at the heterojunction to ${\mathrm{C}}_{60}$. Quantum-chemical calculations performed at the density functional theory and/or time-dependent density functional theory level and cation absorption spectra of diluted $\mathrm{DCV}n\mathrm{T}$ provide an unambiguous identification of the transitions observed in the PA spectra. Upon increasing the effective energy gap of the donor-acceptor pair by increasing the ionization energy of the donor, photoinduced electron transfer is eventually replaced with energy transfer, which alters the photovoltaic operation conditions. The optimum open-circuit voltage of a solar cell is thus a trade-off between efficient charge separation at the interface and maximized effective gap. It appears that the open-circuit voltages of $1.0--1.1\phantom{\rule{0.3em}{0ex}}\mathrm{V}$ in our solar cell devices have reached an optimum since higher voltages result in a loss in charge separation efficiency.

Surfactant-assisted growth of CdS thin films for photovoltaic applications

A common nonionic surfactant, Triton X-100, was used to modify the chemical bath deposition of CdS “buffer” layers on Cu(In,Ga)Se2 (CIGS) thin films. Addition of the surfactant to the CdS deposition bath allowed increased wetting of Cu(In,Ga)Se2 substrates and an increase in the uniformity of films, especially on model hydrophobic substrates. X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy data demonstrate that films produced with the surfactant have the same chemical and electronic properties as films grown without it. In CdS∕Cu(In,Ga)Se2 devices, it was found that Triton X-100 allowed the use of CdS layers that were three to four times thinner than those used normally in high efficiency CIGS-based devices and eliminated the large drops in open-circuit voltage that usually accompany very thin buffer layers. For these thin CdS layers and relative to devices made without the surfactant, average absolute cell efficiencies were increased from 10.5% to 14.8% or by a relative 41%. Visual inspection of the CdS depositions reveals one possible mechanism of the surfactant’s effects: Bubbles that form and adhere to the CIGS surface during the chemical bath deposition are almost completely eliminated with the addition of the TX-100. Thus, junction nonuniformities, pinholes, and thin areas in the CdS layer caused by poor wetting of the substrate surface are sharply reduced, leading to large increases in the open-circuit voltage in devices produced with the surfactant.

Electronic properties calculation ofMgOthin films adsorbed on semi-infiniteAg(001)

We report a theoretical investigation of the electronic structure of ultrathin $\mathrm{MgO}$ films on $\mathrm{Ag}(001)$. Such a study has been motivated by recent experiments in which a peculiar behavior was observed when the film thickness is less than a few layers, suggesting that the interface properties may play a crucial role. We tried to verify such a statement by a comparative analysis between the clean surfaces of $\mathrm{Ag}$ and $\mathrm{MgO}$, and the 1-, 2-, and 3-monolayer (ML) adsorbed systems. To perform these calculations we applied the embedding method {proposed by Inglesfield [J. E. Inglesfield, J. Phys. C 14, 3795 (1981)]} which allows us to consider a semi-infinite substrate and is therefore one of the state-of-the-art methods for the theoretical treatment of surfaces. We find evidences of a weak interaction between the silver substrate and the $\mathrm{MgO}$ overlayers and we show how, as the film thickness exceeds the $2\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$ width, the electronic properties converge on those of a clean $\mathrm{MgO}$ surface. In the last part of the work we also suggest a possible interpretation of the ultraviolet photoelectron spectroscopy data obtained on these systems.

Adsorption of Ethyne on Cu(110): Experimental and Theoretical Study

High-resolution electron energy loss spectroscopy and angle-resolved ultraviolet photoelectron spectroscopy data indicate that ethyne adopts a low symmetry (most likely C1) adsorption geometry on Cu(110). Detailed ab initio Hartree−Fock cluster calculations identify a minimum on the potential energy surface for ethyne in a C1 adsorption geometry. This structure also provides the best agreement between the experimental and calculated vibrational frequencies of the geometries investigated. In addition, the calculations show that the internal structure of the ethyne molecule is relatively insensitive to the adsorption site and that the adsorbed molecule is essentially sp2 hybridized.

Substituted Aluminum and Zinc Quinolates with Blue-Shifted Absorbance/Luminescence Bands: Synthesis and Spectroscopic, Photoluminescence, and Electroluminescence Characterization

Two novel lumophores based on aluminum and zinc metallo-8-hydroxyquinolates have been prepared as electroluminescent materials, and their absorbance, photoluminescence, and electroluminescence properties compared with unsubstituted versions of these same complexes. 8-Hydroxy-5-piperidinylquinolinesulfonamide (1) was synthesized in order to add an electron-withdrawing substituent at the 5-position in 8-hydroxyquinoline, increasing the solubility of the corresponding metal quinolate complexes in nonpolar solvents, and producing a blue-shift in the emission wavelength maximum, relative to complexes formed from the unsubstituted compound. The aluminum complex (Al(QS)3) and the zinc complex (Zn(QS)2) of 1 were compared with the aluminum and zinc complexes of unsubstituted 8-hydroxyquinoline (AlQ3 and ZnQ2), both as solutions and as pure thin films, or as poly(N-vinylcarbazole) (PVK) thin films doped with the metal quinolates. Ultraviolet photoelectron spectroscopy data are presented to assist in estimating the energies of the highest occupied molecular orbitals (HOMO) of AlQ3, ZnQ2, Al(QS)3, and Zn(QS)2. Electroluminescence data shows that ITO/Al(QS)3−PVK/aluminum and ITO/Zn(QS)2−PVK/aluminum devices exhibit good diode-like electrical behavior. Electroluminescence spectra mimic the photoluminescence spectra for all complexes.

Electronic structure of poly(ether—ether—ketone), PEEK

The electronic structure of poly(ether—ether—ketone) (PEEK) is investigated by combining X-ray and ultraviolet photoelectron spectroscopy data with the results of theoretical calculations. XPS core level spectra are related to the charge distribution on the chain, and shake-up satellites are analyzed in terms of electron interaction between the phenyl rings. The UPS spectra are interpreted from the theoretical densities of valence states calculated by the valence effective Hamiltonian (VEH) method. The results show that the electronic structure of PEEK is basically identical to that of a short-chain model molecule, indicating that the ground state of the polymer remains localized, with no long-range electronic interaction along the chain.

CarbonKVVAuger line shapes of graphite and stage-one cesium and lithium intercalated graphite

A symmetry-resolved one-electron density of states (DOS) for graphite is extracted from existent x-ray photoelectron and x-ray emission spectroscopy (XES) data and used in conjunction with empirical atomic matrix elements to construct a carbon $\mathrm{KVV}$ Auger line shape for graphite which is in substantial agreement with experiment. For the intercalated graphites, an additional peak is added to the graphite one-electron DOS near the Fermi energy. In the absence of available XES data for ${\mathrm{C}}_{8}$Cs and ${\mathrm{C}}_{6}$Li, the energy and intensity of this peak are determined by fitting the C $\mathrm{KVV}$ Auger spectra. The enhanced intensities found for this peak, relative to the ultraviolet photoelectron spectroscopy data, are interpreted as being due to the screening charge surrounding the C $1s$ core. The measure of this enhancement corresponds to 0.5 and 0.6 electrons (intercalant charge donation plus screening) on the core-ionized carbon atom for the case of ${\mathrm{C}}_{8}$Cs and ${\mathrm{C}}_{6}$Li, respectively. No evidence is found for the selection rule or large matrix-element effects postulated in previous work.

Particle-Size-Induced Valence Changes in Samarium Clusters

Samarium clusters exhibit mixed-valence behavior which is sensitive to particle size. X-ray photoemission and ultraviolet photoelectron spectroscopy data show samarium to be primarily divalent ($4{f}^{6}$) at small particle size. The trivalent state ($4{f}^{5}$) becomes progressively more abundant with increasing size, becoming the dominant state for the bulk metal. These results are interpreted in terms of band narrowing due to reduced surface coordination, which is more dominant than surface tension effects in establishing the valence of small samarium clusters.

Extraction of Surface Structural Information from Angle-Resolved Ultraviolet Photoemission Spectroscopy

The dependences of photoionization cross section on incident photon energy and electron emission on polar exit angle are used to determine the location of chemisorption sites. Calculations are made for the $c(2\ifmmode\times\else\texttimes\fi{}2)$ S-Ni(001) and CO-Ni(001) systems. Comparing with experiment, it is found that angle-resolved ultraviolet photoelectron spectroscopy data are very sensitive to the coordination number of the adsorption site and less sensitive to the overlayer-substrate interlayer spacing.