Ultraviolet Photoemission Spectroscopy Research Articles

Individualized p-Doped Carbon Nanohorns.

A facile approach to individualize spherically aggregated pristine carbon nanohorns (pr-CNHs) was established. Specifically, we found that treatment of pr-CNHs with chlorosulfonic acid generates positively charged polarized species, which disintegrate toward individualized carbon nanohorns (in-CNHs). Interestingly, the isolated in-CNHs were revealed to be p-doped owing to the adsorption of chlorosulfonate units. The findings were confirmed by data derived from high-resolution transmission electron microscopy imaging, Raman and ultraviolet photoemission spectroscopy, and additionally supported by theoretical calculations and thermogravimetry.

Individualized p‐Doped Carbon Nanohorns

AbstractA facile approach to individualize spherically aggregated pristine carbon nanohorns (pr‐CNHs) was established. Specifically, we found that treatment of pr‐CNHs with chlorosulfonic acid generates positively charged polarized species, which disintegrate toward individualized carbon nanohorns (in‐CNHs). Interestingly, the isolated in‐CNHs were revealed to be p‐doped owing to the adsorption of chlorosulfonate units. The findings were confirmed by data derived from high‐resolution transmission electron microscopy imaging, Raman and ultraviolet photoemission spectroscopy, and additionally supported by theoretical calculations and thermogravimetry.

Anode engineering of highly efficient polymer solar cells using treated ITO

ITO substrates were treated with organic solvent cleaning(OSC), SC1 treatment[V(NH4OH):V(H2O2): V(H2O)=1:1:5], O2 plasma and UV ozone, respectively. Combined investigations of atom force microscopy(AFM), water contact angle measurements, ultraviolet photoemission spectroscopy(UPS) and X-ray photoemission spectroscopy(XPS) demonstrated that UV ozone treatment could give rise to the smoothest surface, the most hydrophilic property and the highest work function(WF) of ITO due to the removal of hydrophobic C―O impurity from the ITO surface and the enrichments of more oxygen on the ITO surface. When PEDOT:PSS film[(poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate)] was deposited on the ITO substrates treated with UV ozone, it showed a lower root-mean- square roughness in AFM images, a higher transmission in UV-Vis transmission spectra and a higher WF in UPS spectra than the PEDOT:PSS films deposited on the ITO substrates treated by other three methods. As a result, the power conversion efficiency of polymer solar cells(PSCs) based on PTB7:PC71BM as an active layer and ITO treated by UV ozone as an anode can reach 8.48% because of the simultaneously improved short circuit current, open circuit voltage and fill factor compared to the PSCs with ITO treated with other three methods.

Valence and Conduction Band Densities of States of Metal Halide Perovskites: A Combined Experimental-Theoretical Study.

We report valence and conduction band densities of states measured via ultraviolet and inverse photoemission spectroscopies on three metal halide perovskites, specifically methylammonium lead iodide and bromide and cesium lead bromide (MAPbI3, MAPbBr3, CsPbBr3), grown at two different institutions on different substrates. These are compared with theoretical densities of states (DOS) calculated via density functional theory. The qualitative agreement achieved between experiment and theory leads to the identification of valence and conduction band spectral features, and allows a precise determination of the position of the band edges, ionization energy and electron affinity of the materials. The comparison reveals an unusually low DOS at the valence band maximum (VBM) of these compounds, which confirms and generalizes previous predictions of strong band dispersion and low DOS at the MAPbI3 VBM. This low DOS calls for special attention when using electron spectroscopy to determine the frontier electronic states of lead halide perovskites.

Electronic structure of hydrothermally synthesized single crystal U0.22Th0.78O2

Single crystals of ThO2, UO2, and their solid solutions, UxTh1–xO2, have been obtained through various hydrothermal growth conditions. This technique offers the better of two other growth processes: (i) single crystal purity as by photochemical growth of nanocrystals; and (ii) large/bulk sizes as obtained by the arc melt method. The band gap of the UxTh1–xO2 single crystal solid solution, along with the luminescence transition, have been characterized. The occupied and unoccupied structures are determined using ultraviolet and inverse photoemission spectroscopy and the electronic band gap was measured to be 3–4 eV. The strain of incorporating U into the ThO2 is analyzed through Vegard's law. In this crystal there are defect and impurity sites, likely arising from the kinetic growth process, giving rise to a similar yet slightly different optical gap evident with cathodoluminescence spectroscopy. There is a major luminescence feature spanning the range from 3.18 to 4.96 eV (250–390 nm) with a maximum at 4.09 eV (303 nm), corresponding with the measured electronic band gap. In this paper, the electronic properties of a solid solution U0.22Th0.78O2 are measured and interpreted compared to the pure actinide oxides, ThO2 and UO2.

Nanoscale scanning electron microscopy based graphitization in tetrahedral amorphous carbon thin films

Electron beam lithography is a powerful maskless tool to fabricate structures on the nanometer scale. Here, we show that low-keV-electron beams enable a direct patterning of tetrahedral amorphous carbon (ta-C) thin films by inducing a local graphitization without the need for any resist or development process step. Irradiation with 4 keV electrons leads to a local decrease of the ta-C film’s electrical resistance and an increase of both the sp2/sp3-ratio and the material’s work function. We investigate the impact of electron exposure on ta-C by a variety of microscopy as well as spectroscopy methods including scanning tunneling microscope-based current-distance spectroscopy, conductive atomic force microscopy, spatially resolved ultraviolet and x-ray photo emission spectroscopy, and μ-Raman spectroscopy. The electron exposure has been performed under ultrahigh vacuum conditions to prevent from electron-induced deposition of contaminants which may obstruct the application of surface-sensitive analysis techniques to the modified ta-C films.

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.

Large Work Function Modulation of Monolayer MoS2 by Ambient Gases

Although two-dimensional monolayer transition-metal dichalcogenides reveal numerous unique features that are inaccessible in bulk materials, their intrinsic properties are often obscured by environmental effects. Among them, work function, which is the energy required to extract an electron from a material to vacuum, is one critical parameter in electronic/optoelectronic devices. Here, we report a large work function modulation in MoS2 via ambient gases. The work function was measured by an in situ Kelvin probe technique and further confirmed by ultraviolet photoemission spectroscopy and theoretical calculations. A measured work function of 4.04 eV in vacuum was converted to 4.47 eV with O2 exposure, which is comparable with a large variation in graphene. The homojunction diode by partially passivating a transistor reveals an ideal junction with an ideality factor of almost one and perfect electrical reversibility. The estimated depletion width obtained from photocurrent mapping was ∼200 nm, which is much narrower than bulk semiconductors.

Low-Temperature, Chemically Grown Titanium Oxide Thin Films with a High Hole Tunneling Rate for Si Solar Cells

In this paper, we propose a chemically grown titanium oxide (TiO2) on Si to form a heterojunction for photovoltaic devices. The chemically grown TiO2 does not block hole transport. Ultraviolet photoemission spectroscopy was used to study the band alignment. A substantial band offset at the TiO2/Si interface was observed. X-ray photoemission spectroscopy (XPS) revealed that the chemically grown TiO2 is oxygen-deficient and contains numerous gap states. A multiple-trap-assisted tunneling (TAT) model was used to explain the high hole injection rate. According to this model, the tunneling rate can be 105 orders of magnitude higher for holes passing through TiO2 than for flow through SiO2. With 24-nm-thick TiO2, a Si solar cell achieves a 33.2 mA/cm2 photocurrent on a planar substrate, with a 9.4% power conversion efficiency. Plan-view scanning electron microscopy images indicate that a moth-eye-like structure formed during TiO2 deposition. This structure enables light harvesting for a high photocurrent. The high photocurrent and ease of production of chemically grown TiO2 imply that it is a suitable candidate for future low-cost, high-efficiency solar cell applications.

Electronically Passivated Hole‐Blocking Titanium Dioxide/Silicon Heterojunction for Hybrid Silicon Photovoltaics

Carrier‐selective heterojunctions are important for low‐cost silicon‐based photovoltaic applications. A low temperature (<100 °C) chemical vapor deposition technique is used here to deposit ultrathin (n‐type) titanium dioxide (TiO2) layers onto hydrogen‐passivated surfaces of crystalline‐silicon (c‐Si). Energy level alignment and chemical composition at these abrupt, interfacial layer‐free TiO2/Si heterojunctions are investigated via ultraviolet, X‐ray, and inverse photoemission spectroscopy, for c‐Si doping ranging from p++(1019) to n++(1019). The interface Fermi level position and device‐relevant TiO2/Si band offsets are found to shift monotonically as a function of the Si doping, revealing the absence of Fermi level pinning at the c‐Si interface and pointing to simple Fermi level equilibration as the driving mechanism behind the interface energy level alignment. Electrical transport measurements performed on TiO2/Si‐based diodes confirm the energy level alignment yielded by spectroscopic measurements and the hole‐blocking properties of the TiO2/Si heterojunction, exclude hole conduction in the TiO2 as a transport mechanism, and show carrier recombination at the TiO2/p‐Si heterojunction.

Difficulty of carrier generation in orthorhombic PbO

Polycrystalline β-PbO films were grown by pulsed laser deposition in atmospheres ranging from oxygen-poor (the oxygen pressure of 0.01 Pa) to oxygen-rich (13 Pa) conditions, and the oxygen chemical potential was further enhanced by ozone annealing to examine hole doping. It was found that each of the as-grown β-PbO films showed poor electrical conductivity, σ < 1.4 × 10−7 S cm−1, regardless of the oxygen pressure. The density functional calculations revealed that native defects including Pb and O vacancies have deep transition levels and extremely high formation enthalpies, which indicates difficulty of carrier generation in β-PbO and explains the experimentally observed poor electrical conductivity. The analysis of the electronic structures showed that the interaction between Pb 6s and O 2p orbitals is weak due to the deep energy level of Pb 6s and does not raise the valence band maximum (VBM) level unlike that observed in SnO, which is also supported by ultraviolet photoemission spectroscopy measurements. The deep acceptor transition levels of the native defects are attributed to the deep VBM of β-PbO. On the other hand, annealing β-PbO films in reactive oxygen-containing atmospheres (i.e., O3) led to a significantly enhanced electrical conductivity (i.e., σ > 7.1 × 102 S cm−1) but it is the result of the formation of an n-type PbO2 phase because oxygen chemical potential exceeded the phase boundary limit. The striking difference in carrier generation between PbO and SnO is discussed based on the electronic structures calculated by density functional theory.

Electronic structure study of wide band gap magnetic semiconductor (La0.6Pr0.4)0.65Ca0.35MnO3 nanocrystals in paramagnetic and ferromagnetic phases

X-ray circular magnetic dichroism (XMCD), X-ray photoemission spectroscopy (XPS), and ultraviolet photoemission spectroscopy (UPS) techniques were used to study the electronic structure of nanocrystalline (La0.6Pr0.4)0.65Ca0.35MnO3 near Fermi-level. XMCD results indicate that Mn3+ and Mn4+ spins are aligned parallel to each other at 20 K. The low M-H hysteresis curve measured at 5 K confirms ferromagnetic ordering in the (La0.6Pr0.4)0.65Ca0.35MnO3 system. The low temperature valence band XPS indicates that coupling between Mn3d and O2p is enhanced and the electronic states near Fermi-level have been suppressed below TC. The valence band UPS also confirms the suppression of electronic states near Fermi-level below Curie temperature. UPS near Fermi-edge shows that the electronic states are almost absent below 0.5 eV (at 300 K) and 1 eV (at 115 K). This absence clearly demonstrates the existence of a wide band-gap in the system since, for hole-doped semiconductors, the Fermi-level resides just above the valence band maximum.

Fabrication of Graphene–Alumina Heterostructured Films with Nanotube Morphology

The search for highly scalable techniques for the fabrication of promising nanostructured materials is one of the key challenges in modern nanotechnology. Herein, we report on the formation of novel alumina–graphene composite films with an unprecedentedly low optical gap (1.53 eV) and an exciting chemical inertness via Al anodizing in a tartaric acid solution at extremely high current densities that exceed burning initiation. These properties were ascribed to the entrapping of a significant amount of graphene-based materials at the top and back sides of the film. Moreover, nanotube morphology and other properties untypical for alumina films were revealed using scanning electron microscopy, X-ray photoelectron spectroscopy, Auger electron spectroscopy, ultraviolet photoemission spectroscopy, and reflectance spectroscopy techniques. Because of novel, exciting optical properties and chemical stability, aluminum anodic oxide films heterostructured with a high amount of graphene material open up opportunities ...

Surface spectroscopy and structural analysis of nanostructured multifunctional (Zn, Al) layered double hydroxides

Two types of (Zn, Al) layered double hydroxide were prepared by a hydrothermal process at room temperature using Zn salt precursors on Al foils. The examined LDHs differ for the hosted anions in the interlamellar space, namely Cl− and NO3−. Scanning electron microscopy, X‐ray photoelectron spectroscopy and ultraviolet photoemission spectroscopy have been used to characterize four types of the samples, representative of the two hosted anions (Cl− and NO3−) and two times of growth (6 and 24 h). X‐ray photoelectron spectroscopy results permitted to describe the interactions between inorganic anions hosted in the interlamellar space and the metallic cations on the brucite layer. They also allowed giving a tentative explanation of the different morphologies observed by scanning electron microscopy. Copyright © 2016 The Authors Surface and Interface Analysis Published by John Wiley & Sons Ltd.

Interfacial electronic structure of C60/ZnPc/AZO on photoemission spectroscopy for organic photovoltaic applications

The interfacial electronic structure of a bilayer of fullerene (C60) and zinc phthalocyanine (ZnPc) grown on aluminum-doped zinc oxide (AZO) substrates has been evaluated by X-ray and ultraviolet photoemission spectroscopy. The energy difference between the highest occupied molecular orbital (HOMO) level of the ZnPc layer and the lowest unoccupied molecular orbital (LUMO) level of the C60 layer (EDHOMO−EALUMO) was determined and compared to that grown on an indium tin oxide (ITO) substrate. The EDHOMO−EALUMO value of the heterojunction on AZO was 1.4eV, while that on ITO was 1.1eV. This result is discussed in terms of the differences of the work function and resistivity of each transparent conductive oxide. We also obtained complete energy level diagrams of C60/ZnPc/AZO and C60/ZnPc/ITO.

Inherently-Forced Tensile Strain in Nanodiamond-Derived Onion-like Carbon: Consequences in Defect-Induced Electrochemical Activation.

We analyzed the nanodiamond-derived onion-like carbon (OLC) as function of synthesis temperature (1000~1400 °C), by high-resolution electron microscopy, electron energy loss spectroscopy, visible-Raman spectroscopy, ultraviolet photoemission spectroscopy, impedance spectroscopy, cyclic voltammetry and differential pulse voltammetry. The temperature dependences of the obtained properties (averaged particle size, tensile strain, defect density, density of states, electron transfer kinetics, and electrochemical oxidation current) unanimously coincided: they initially increased and saturated at 1200 °C. It was attributed to the inherent tensile strains arising from (1) the volume expansion associated with the layer-wise diamond-to-graphite transformation of the core, which caused forced dilation of the outer shells during their thermal synthesis; (2) the extreme curvature of the shells. The former origin was dominant over the latter at the outermost shell, of which the relevant evolution in defect density, DOS and electron transfer kinetics determined the electrochemical performances. In detection of dopamine (DA), uric acid (UA) and ascorbic acid (AA) using the OLC as electrode, their oxidation peak currents were enhanced by factors of 15~60 with annealing temperature. Their limit of detection and the linear range of detection, in the post-treatment-free condition, were as excellent as those of the nano-carbon electrodes post-treated by Pt-decoration, N-doping, plasma, or polymer.

Substrate Polarity Effects on the Interface Electronic Structure in Organic Light Emitting Diodes

Organic layers deposited on various polarity substrates and the electronic structures of (PTCDA/TiOPc) on hydrophobic and hydrophilic substrates have been studied by ultraviolet photoemission spectroscopy. The difference between work function and polarity of the substrates induce the formation of an interface dipole with corresponding shift in the relative position of molecular levels across the interface. While the vacuum level and open circuit voltage show vastly difference respectively, the barrier between anode-organic or organic-cathode also changes from 0.75eV to 1.13eV or 0.35eV to 0.65eV. The results show the possibility of tuning the electronic structure by the modification of substrate and potential applications on performance enhancement in organic electronic devices.

Ultrawide band gap amorphous oxide semiconductor, Ga–Zn–O

We fabricated amorphous oxide semiconductor films, a-(Ga1–xZnx)Oy, at room temperature on glass, which have widely tunable band gaps (Eg) ranging from 3.47–4.12eV. The highest electron Hall mobility ~7cm2V−1s−1 was obtained for Eg=~3.8eV. Ultraviolet photoemission spectroscopy revealed that the increase in Eg with increasing the Ga content comes mostly from the deepening of the valence band maximum level while the conduction band minimum level remains almost unchanged. These characteristics are explained by their electronic structures. As these films can be fabricated at room temperature on plastic, this achievement extends the applications of flexible electronics to opto-electronic integrated circuits associated with deep ultraviolet region.

Coverage-dependent two-photon photoexcitation at the H2O/TiO2 interface

Excited electrons and holes are crucial for redox reactions on metal oxide surfaces. However, precise details of this charge transfer process are not known. We report two-photon photoemission (hν=3.23eV) measurements of rutile TiO2(110) as a function of exposure to water below room temperature. The two-photon resonance associated with bridging hydroxyls is enhanced following water exposure, reaching a maximum at a nominal coverage of one monolayer. Higher coverages attenuate the observed resonance. Ultraviolet photoemission spectroscopy (hν=21.22eV) of the initial, band gap states shows little change up to one monolayer water coverage. It is likely that the enhancement arises from dissociation within the adsorbed water monolayer, although other mechanisms cannot be excluded.

Improved quantum dot light-emitting diodes with a cathode interfacial layer

Colloidal quantum dot light-emitting diodes (QLEDs) are reported with improved external quantum efficiencies (EQE) and efficiency roll-off under high current densities by introducing a thermally-evaporated organic cathode interfacial material (CIM) Phen-NaDPO. QLEDs with this new CIM modified Al cathode were fabricated, giving an upwards of 25% enhancement in the EQE relative to the bare Al device. Ultraviolet photoemission spectroscopy (UPS) suggests that this material can effectively lower the work function of Al, therefore facilitating the electron injection in QLEDs. Furthermore, Phen-NaDPO was introduced into the LiF/Al device to afford better balanced hole/electron injection in the emitting layer. Consequently, the QLEDs with the organic CIM/LiF/Al cathode further increased EQE and current efficiency by 44% and 52%, respectively, with higher luminance and lower efficiency roll-off under high current densities.