Ultraviolet Photoemission Spectra Research Articles

Identifying the secondary electron cutoff in ultraviolet photoemission spectra for work function measurements of non-ideal surfaces

Absolute values of work functions can be determined in ultraviolet photoemission spectroscopy (UPS) by measuring the minimum kinetic energy of secondary electrons generated by a known photon energy. However, some samples can produce spectra that are difficult to interpret due to additional intensity below the true secondary electron cutoff. Disordered absorbates on elemental metals add small intensity below the onset for the transition metal surfaces studied, which can be attributed to energy losses after photoelectrons are generated. In contrast, spectra from WO3−x films can produce multiple onsets with comparable intensity which do not fit this model. False onsets (in the context of work function measurements) can be minimized by optimizing experimental detection parameters including limiting analyzer acceptance angles and pass energy. True work functions can be identified by examining the onsets as the sample bias is varied.

Room temperature spontaneous surface condensation of boronic acids observed by scanning tunneling microscopy

Herein, we discovered that the surface-confined condensation of boronic acid can happen spontaneously at room temperature, by comparing the kinetics of condensation of boronic acids with and without the negative sample bias, we found that the negative sample bias indeed accelerates the self-condensation reaction of boronic acid. Combining with in-situ STM images and ultraviolet photoemission spectrum (UPS) analysis, a reversible adsorption mechanism model was proposed and reasonably explains the reversible electric-field-induced phase transformation.

Electronic properties of carbon nanotubes as detected by photoemission and inverse photoemission

The relation between morphology and energy level alignment in carbon nanotubes (CNT) is a crucial information for the optimization of applications in nanoelectronics, optics, mechanics and (bio)chemistry. Here we present a study of the relation between the electronic properties and the morphology of single wall CNT (SWCNT), aligned multi wall CNT (MWCNT) and unaligned MWCNT. The CNT were synthesized via catalytic chemical vapor deposition in ultra-high vacuum conditions. Combined ultraviolet photoemission and inverse photoemission (IPES) spectra reveal a high sensitivity to the nanotube morphology. In the case of unaligned SWCNT the distinctive unoccupied Van Hove singularities (vHs) features are observed in the high resolution IPES spectra. Those features are assigned to semiconducting and metallic SWCNT states, according to calculated vHs DOS. The two MWCNT samples are similar in the diameter of the tube (about 15 nm) and present similar filled and empty electronic states, although the measured features in the aligned MWCNT are more defined. Noteworthy, interlayer states are also revealed. Their intensities are directly related to the MWCNT alignment. Focussing and geometrical effects associated to the MWCNT alignment are discussed to account the spectral differences.

Study of band structure, transport and magnetic properties of BiFeO3–TbMnO3 composite

Charge transfer across the interface of two materials in a composite can create reconstruction of bands near the interface which in turn brings multiple changes in physical properties of the materials. Thus, investigation of band structure experimentally is of immense importance in studying composite materials to understand their physical properties. Here, we have studied magnetoelectric multiferroic composite of two types of multiferroic (types I and II) consisting of BiFeO3 and TbMnO3 for enhanced magnetic and transport properties. The band structure was investigated with the help of UV–visible absorption spectrum, the valence band X-ray photoemission spectra (XPS), and ultraviolet photoemission spectra. The band structure thus obtained can successfully explain the magnetic and transport properties of the composite. The insulating behavior of the system is understood from the reconstruction of the energy bands at the interface and subsequent decrease in the band gap which happens due to lattice mismatch of the two materials. The large coercivity and the increase in the magnetization value are understood to be due to superexchange interaction between different Mn ions (Mn2+, Mn3+, and Mn4+). From the composition study of EDXA and core-level XPS, oxygen vacancy was found which in turn creates the mixed valence state of Mn to maintain the charge neutrality.

Study of structural, dielectric, optical properties and electronic structure of Cr-doped LaInO3 perovskite nanoparticles

This work reports the change in the structural, dielectric and optical behaviour of Cr-doped LaInO3 nanoparticles synthesized via sol-gel method. X-ray diffraction patterns confirm that systems crystallize in orthorhombic Pnma space group. Raman spectroscopy measurements exhibit blue-shift in Raman modes with Cr-doping due to the compressive stress introduced by smaller Cr3+ cations. Trivalent oxidation state of Cr was confirmed from Cr(2p) core-level X-ray photoemission spectroscopy measurements, while valence band spectra suggest the absence of electronic states near Fermi level. High-resolution Ultraviolet photoemission spectra near Fermi level shows the electronic density of states arises sharply near Fermi level. Room temperature dielectric constant decreases with increasing Cr-concentration, which shows that conductivity has been increased after Cr-doping. Ultraviolet–visible spectroscopy measurements provide band gap of 4.8eV for LaInO3 and Cr-doped LaInO3nanoparticles.Moreover, Cr-doped LaInO3systems exhibit an intermediate band gap of 3eV, which might have been created from Cr(3d) states.

Structural, magnetic, magneto-transport properties, and electronic structure study of charge-ordered (La0.4Pr0.6)0.65Ca0.35MnO3

Structural, magnetic and magneto-transport properties of nanocrystalline (La0.4Pr0.6)0.65Ca0.35MnO3 have been investigated along with their electronic structures. Temperature dependent magnetization measurement shows different magnetic phases in different temperature regions. (La0.4Pr0.6)0.65Ca0.35MnO3 completely transforms into a ferromagnet below 50 K. The temperature dependent resistivity measurement of (La0.4Pr0.6)0.65Ca0.35MnO3 system displays semiconducting behavior up to 45 K under zero magnetic field. Application of 1 T magnetic field enforces semiconductor to metal transition around 65 K during cooling and around 115 K during warming. This shows that (La0.4Pr0.6)0.65Ca0.35MnO3 still behaves as itinerant ferromagnet under applied magnetic field. X-ray photoemission spectra of Mn2p and Mn3s core-level confirm dual valence states of Mn (Mn3+ and Mn4+), which is responsible for the magnetic behavior of the (La0.4Pr0.6)0.65Ca0.35MnO3 system. High-resolution ultraviolet photoemission spectra near Fermi-edge confirms the presence of finite electronic states at Fermi-level, which explains the observed low-temperature metallic behavior of the (La0.4Pr0.6)0.65Ca0.35MnO3 system.

Investigation of Energy Levels and Crystal Structures of Cesium Lead Halides and Their Application in Full‐Color Light‐Emitting Diodes

Inorganic CsPbX3 perovskites with compositions including CsPbBrxCl3−x, CsPbBr3, and CsPbBrxI3−x are synthesized, and their properties are investigated. Tauc plots calculated from the UV–vis spectra of the materials show that the bandgaps of CsPbBrxCl3−x, CsPbBr3, and CsPbBrxI3−x are 2.7, 2.35, and 1.8 eV, respectively. The as‐prepared CsPbX3 nanodots have a cubic structure and their crystal sizes are around 5–10 nm. The diffraction peak intensity of the (110) plane is increased by adding Cl anion and reduced by adding I anion. By contrast, the peak intensity of the (200) plane is reduced by the introduction of Cl− ions and increased by the introduction of I− ions, suggesting that the nature of the halide anions affects the crystal orientation of CsPbX3 quantum dots. The highest occupied molecular orbital/lowest unoccupied molecular orbital levels of CsPbBrxCl3−x, CsPbBr3, and CsPbBrxI3−x calculated from ultraviolet photoemission spectra and UV–vis spectra are 6.5/3.8, 6.5/4.15, and 6.1/4.3 eV, respectively. The maximum luminance values measured for CsPbBrxCl3−x, CsPbBr3, and CsPbBrxI3−x‐based light‐emitting diodes (LEDs) are 15.2, 51.7, and 21.7 cd m−2, respectively. This research provides an overview of the energy levels and crystal structures of CsPbX3 quantum dots for the design of inorganic perovskite‐based LEDs with high luminance and power efficiencies.

First-Principles Photoemission Spectroscopy of DNA and RNA Nucleobases from Koopmans-Compliant Functionals.

The need to interpret ultraviolet photoemission data strongly motivates the refinement of first-principles techniques that are able to accurately predict spectral properties. In this work, we employ Koopmans-compliant functionals, constructed to enforce piecewise linearity in approximate density functionals, to calculate the structural and electronic properties of DNA and RNA nucleobases. Our results show that not only ionization potentials and electron affinities are accurately predicted with mean absolute errors of <0.1 eV, but also that calculated photoemission spectra are in excellent agreement with experimental ultraviolet photoemission spectra. In particular, the role and contribution of different tautomers to the photoemission spectra are highlighted and discussed in detail. The structural properties of nucleobases are also investigated, showing an improved description with respect to local and semilocal density-functional theory. Methodologically, our results further consolidate the role of Koopmans-compliant functionals in providing, through orbital-density-dependent potentials, accurate electronic and spectral properties.

Electronic Structure of TmPdIn

Electronic structure of a ternary TmPdIn compound, which crystallizes in the hexagonal ZrNiAl-type structure, was studied by X-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Density of states in the valence band was calculated by means of the augmented plane wave/local orbital method based on density functional theory. The results showed that the valence band is formed mainly of Tm 4f and Pd 4d states. In the ultraviolet photoemission spectra one can distinguish Pd 4d maximum and Tm 4f multiplet peaks, which are displaced with respect to those of pure Tm.

Valence Band Edge Shifts and Charge-transfer Dynamics in Li-Doped NiO Based p-type DSSCs

ABSTRACT The objective of this work is to gain deeper insight into behavior of p-type DSSCs, paying special attention to photoelectric properties of Li-doped NiO photoelectrode, and studying their effect on interfacial photoelectrochemical processes under illumination and device performance. Here we find that Li-doped NiO photoelectrodes show higher charge carrier densities, down-shift of valence band and narrowing trap energy distribution. Furthermore, the performance enhancement of p-type DSSCs is observed for the Li-doped NiO photoelectrodes with respect to the unmodified ones. These improvement is found to arise from more rapid hole transport combined with reduced recombination, contributing to improved hole collection. With regard to interfacial carriers transfer process, electrochemical impedance spectroscopy (EIS), intensity-modulated photovoltage spectroscopy (IMPS) and ultra-violet photoemission spectra (UPS) provide evidence that the decreased recombination of Li-doped NiO DSSCs is attributed to the downward shift of valence band, which increases energy barrier of interface recombination. Another reason results in narrowing trap energy distribution of Li-doped NiO photoelectrode, which can weaken energy overlap with the LUMO level of dye, and subsequently the recombination path of hole back transfer to dye is hindered. These findings suggest that it will be important to carefully optimize band edge alignment and trap states distribution of p-type oxide semiconductors film to further improve the efficiency of p-DSSCs.

Hot electron cooling in graphite: Supercollision versus hot phonon decay

Disorder-assisted electron-phonon scattering processes (supercollision processes) have been reported to dominate the cooling of hot carriers in graphene. Here, we determine to what extent this type of relaxation mechanism governs the hot carrier dynamics in the parent compound graphite. Electron temperature transients derived from time- and angle-resolved extreme ultraviolet photoemission spectra are analyzed based on a three-temperature model which considers electron gas, optical phonons, and acoustic phonons as coupled subsystems. In the probed fluence regime of $0.035--1.4\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$, we find no indications for supercollision processes being involved in the cooling of the hot carriers. The data are, by contrast, compatible with a hot phonon assisted mechanism involving anharmonic coupling between optical phonons and acoustic phonons, a process which has previously been suggested for graphite. We attribute the striking difference to the reported findings for (substrate-supported) graphene to the low defect density of highly ordered pyrolitic graphite.

Magnetic interactions and electronic structure of Ni–Mn–In

The electronic structure and magnetic properties of a magnetic shape memory alloy Ni–Mn–In have been studied using spin polarized fully relativistic Korringa–Kohn–Rostoker (SPRKKR) method. The total energy calculations with different starting magnetic spin configurations show that the ground state of Ni2Mn1.4In0.6 is ferromagnetic. The spin and orbital magnetic moments of Ni2Mn1.4In0.6 and Ni2MnIn are in good agreement with the magnetization measurements. The exchange coupling parameters of the different sublattice interactions exhibit a strong competition between ferromagnetic and antiferromagnetic configurations, due to the substitution of excess Mn atoms at the In site in Ni2Mn1.4In0.6. The Curie temperature of Ni2MnIn, calculated under a mean field approximation, is found to be in relatively good agreement with the experimental values. While Ni2MnIn does not undergo martensite transition, it is shown that a Jahn–Teller splitting of the Ni 3d eg states plays an important role in driving the martensite transformation in Ni2Mn1.4In0.6. We find that both the calculated ultra-violet photoemission spectra and the inverse photoemission spectra are in good agreement with the existing experimental data.

Toward a Visible Light-Driven Photocatalyst: The Effect of Midgap-States-Induced Energy Gap of Undoped TiO2 Nanoparticles

TiO2 is one of the most promising candidate materials for clean-energy generation and environmental remediation. However, the larger-than 3.1 eV bandgap of perfectly crystalline TiO2 confines its application to the ultraviolet (UV) range. In this study, the electronic and the optical properties of undoped mixed-phase TiO2 nanoparticles were investigated using UV and inverse photoemission, low intensity X-ray photoelectron (XP), and diffused reflectance spectroscopy methods. The facile solution-phase synthesized nanoparticles exhibited a midgap-states-induced energy gap of only ∼2.2 eV. The diffused reflectance spectrum showed sub-bandgap absorption due to the existence of a large Urbach tail at 2.2 eV. The UV photoemission spectrum evidenced the presence of midgap states. The 2.2 eV energy gap enables the nanoparticles to be photoactive in the visible energy range. The gas-phase CO2 photoreduction test with water vapor under visible light illumination was studied in the presence of the synthesized TiO2 na...

Inverse photoemission and photoemission spectroscopic studies on sputter-annealed Ni–Mn–Sn and Ni–Mn–In surfaces

The electronic structure of nearly stoichiometric Ni–Mn–Sn and Ni–Mn–In surface is investigated by inverse photoemission and photoemission spectroscopy. Comparison of the experimental and calculated inverse photoemission spectra shows that the dominant feature is related to Mn 3d-like states. The overall shape and peak position of the theoretically obtained spectra show good agreement with the experimental ultraviolet photoemission valence band spectra. The changes in the composition dependent ultraviolet photoemission spectra reveal the change in degree of Ni 3d and Mn 3d band hybridization. Both inverse photoemission and ultraviolet photoemission study show a rigid band shift between Ni2MnIn and Ni2MnSn because of band filling, due to increase in the number of 5p electrons from In to Sn. Mn 2p and 3s core-level reveal unambiguous existence of exchange splitting in both the materials.

Fluoropolymer-assisted graphene electrode for organic light-emitting diodes

Organic light-emitting diodes (OLEDs) were fabricated on a graphene electrode, with synthesized graphene being transferred and simultaneously doped with supporting polymers. Poly[methyl methacrylate] (PMMA) and fluoropolymer (CYTOP) layers were used as the supporting polymers. The sheet resistance of CYTOP-assisted graphene (CYTOP-G) with 4 layers of graphene is 200Ω/sq., which is lower than that of PMMA-assisted graphene (PMMA-G, 330Ω/sq.) The transmittance value of PMMA-G and CYTOP-G with 4 graphene layers is higher than 85%. CYTOP-G is shown to exhibit a higher tolerance to UV–O3 treatment and thermal annealing than PMMA-G. Work function of CYTOP-G is 4.7eV, which is higher than that of PMMA-G (4.3eV). X-ray photoemission and Raman spectroscopy data indicate that CYTOP-G has numerous C-F bonds on the surface exhibiting p-type semiconductor properties, owing to the high electronegativity of fluorine. The turn-on voltage of an OLED based on CYTOP-G with 4 graphene layers is 4.2V, which is lower than that of indium tin oxide (ITO)-based one (4.5eV). Furthermore, the luminance ratio of graphene-based OLEDs to ITO-based OLEDs was calculated to be 104% for CYTOP-G, and 97% for PMMA-G. According to the ultraviolet photoemission spectra, the hole injection barrier in CYTOP-G is lower by about 0.5eV than the hole injection barrier in PMMA-G. These results are very encouraging to the prospect of replacing ITO electrodes with graphene ones in OLED applications.

A Minimal Cluster Model of Valence Electrons in Adatom-Assisted Adsorbed Molecules: NCH3/Cu(110) and OCH3/Cu(110)

In this study, we found that the local density of states and ionization energy spectrum of the valence electrons of methylnitrene (NCH3) adsorbed on Cu(110) can be calculated from molecular orbital calculations of a simple artificial isolated NCH3–Cu2 molecular cluster in which the two Cu atoms form bonds to the N atom. Such a NCH3–Cu2 cluster represents the basic structural unit of a NCH3 molecule adsorbing on a Cu double-added-row structure. This finite NCH3–Cu2 cluster structure is not optimized as a single system but is extracted directly from an optimized surface structure obtained by density functional theory with periodic boundary conditions. With this approach, we obtained excellent agreement between the measured ultraviolet photoemission spectra (UPS) and the theoretical calculation results. To further examine this minimal cluster concept, we analyzed methoxy (OCH3) adsorption on Cu(110) and found a OCH3–Cu3 cluster structure. On the basis of this structure, we calculated UPS and also obtained substantial agreement with the experimental UPS. These results may indicate that, when substrate adatoms bridge the adsorption of a molecule and a surface, a small cluster consisting of the adsorbate and the neighboring bonding substrate adatoms suffices to describe the electronic structure of the valence electrons of the adsorbates.

Electron-injecting properties of Rb2CO3-doped Alq3 thin films in organic light-emitting diodes

Rubidium carbonate (Rb2CO3)-doped tris(8-quinolinolato)aluminum (III) (Alq3) thin films have been investigated as electron-injecting materials for organic light-emitting diodes (OLEDs). Electron-only devices consisting of glass/tin-doped indium oxide (ITO)/Rb2CO3-doped Alq3 (10 nm)/aluminum (Al) showed an electron-ohmic contact property between the electrode and the organic layer at the doping concentration of 10% and higher. The electron-injecting ability of these contacts was largely enhanced by the n-doping effect of Rb2CO3 into the Alq3 layer. The ultraviolet photoemission spectra revealed that when the doping concentration was increased, the n-doping effect reduced the carrier-injecting barrier height by lowering the work function at the Rb2CO3-doped Alq3 interfaces. Also, the x-ray photoemission spectra showed that as the doping concentration was increased at the interfaces, Alq3 molecules decomposed in a chemical reaction with Rb2CO3. The OLED device, having the glass/ITO/molybdenum oxide (MoOx, 25%)-doped N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB, 5 nm)/NPB (63 nm)/Alq3 (32 nm)/Rb2CO3-doped Alq3 (10%, 10 nm)/Al (100 nm) structure, showed the best performance at the optimal doping concentration of Rb2CO3-doped Alq3, both the maximum luminance of 114 400 cd/m2 at the bias voltage of 9.8 V and the power efficiency of 2.7 lm/W at the luminance of 1000 cd/m2 were obtained.

Conductive zinc oxide thin film coatings by combustion chemical vapour deposition at atmospheric pressure

We have established a combustion chemical vapour deposition (C-CVD) system for the deposition of zinc oxide (ZnO) at atmospheric pressure. This C-CVD process has the advantage of a short exposure of the substrates to the flame. It is also potentially applicable as an inline coating system.Fundamental studies were performed on undoped ZnO. The specific resistivity of these layers strongly depends on the film thickness and decreases with increasing thickness. As the lowest resistivities, values of about 2.0·10−1Ωcm are achieved. Ultra-violet photoemission spectra show the valence band structure of the deposited ZnO. The work function and valence band edge were determined. UV–vis spectra were taken to investigate the transmission of the coated glass samples. From these spectra the band gap energy was obtained. Raman spectroscopy as well as infrared spectroscopy confirmed the presence of ordered ZnO crystallites. The X-ray diffraction verified this result and illustrates the hexagonal structure. In the mid-infrared range precursor deposits were detected for low substrate temperatures.

Electronic band structure and photoemission states of La0.67Pb0.33Mn1−xFexO3

The calculation was carried out in manganese perovskite La0.67Pb0.33Mn1−xFexO3 for x=0, 0.083 (1/12) and 0.33 based on the first-principles Density Functional Theory with general gradient approximation GGA+U using WIEN2k package. The P3¯c1 crystal structure was taken from the X-ray diffraction data for the perovskite. The original unit cell was multiplied twice for x=0.083 in all direction and twice in c for x=0 and x=0.33 in order to place proper amount of Fe. Final crystal structure with 120 atoms was used for calculation x=0.083. For Mn and Fe d electrons the exact exchange energy was utilized. As a results half-metallic density of state was obtained with top of valance band for spin down 1.5eV below EF and spin up filled up to Fermi level by d electrons of Mn, Fe and oxygen p electrons. Comparison of total DOS with ultraviolet photoemission spectra shows similar features up to −10eV and especially the decrease in DOS from −2eV to EF.

Optoelectronic Characteristics of Organic Light-Emitting Diodes with a Rb2CO3-Mixed C60 Layer as an Electron Ohmic-Contact

A rubidium carbonate (Rb2CO3)-mixed fullerene (C60) layer was used as the electron ohmic-contact layer and the effect of Rb2CO3 mixing concentration on the optoelectronic properties of organic light-emitting diodes (OLEDs) was investigated. The performance of an electron-only device, with a glass/ITO/Rb2CO3-mixed C60 (10 nm)/Al structure, was strongly dependent on both the thickness and the mixing concentration of the Rb2CO3 layer in the Rb2CO3-mixed C60 thin films. The ultraviolet photoemission spectra of the Rb2CO3-mixed fullerene (C60) layer showed the formation of new Fermi level crossing states, below the Fermi level caused by a chemical interaction between Rb species and C60, and which led the n-mixing effect to the contact. The emergence of new FLCSs contributed to the formation of an electron ohmic-contact between the cathode and the organic layer by increasing the number of electron carriers. The OLED device with Rb2CO3-mixed C60 as an electron ohmic contact layer, and composed of a glass/ITO/MoO3-mixed NPB (25%, 5 nm)/NPB (63 nm)/Alq3 (39 nm)/Rb2CO3-mixed C60 (75%, 3 nm)/Al (150 nm) structure, showed better optoelectronic properties than the OLED fabricated with C60 (5 nm)/LiF (1 nm) as the electron ohmic-contact layer.