Ultraviolet Photoemission Spectroscopy Data Research Articles

Surface analysis and surface doping of graphene on indium-tin-oxide

Surface doped indium tin oxide (ITO)/graphene systems as a hole transporting transparent front electrode in electro-optical applications are potential alternatives to conventional ITO with wet chemically prepared polymers.In this paper we investigate, firstly, chemical impurities of chemical vapor deposition prepared graphene transferred onto glass/ITO substrates under ambient conditions using x-ray photoemission spectroscopy (XPS) at the oxygen and carbon peak positions. For each of the two elements the intensity contribution of the graphene layer could be extracted from a mathematical simulation of the respective XPS spectra. Impurities from the supporting poly(methyl methacrylate) layer for the transfer were identified. Secondly, we study effects of graphene surface-doping on the work function of such transparent front electrodes using ultraviolet photoemission spectroscopy data. As doping material we have deposited ultra-thin layers of low-cost electron acceptors, iron chloride, vanadium oxide, molybdenum oxide (MoOx) or tetracyano-2,3,5,6-tetrafluoroquinodimethane in order to increase its initial low surface work function of Φ = 4.3 eV up to Φ = 6.3 eV.Finally, we fabricated under vacuum conditions a bulk heterojunction organic photovoltaic device based on a Zn-phtalocyanine:C60 absorber layer thermally deposited on an ITO/graphene front electrode, which has been electronically adjusted to the Zn-phthalocyanine donor (Φ = 5.2 eV) by applying the matching dopant MoOx. The current-voltage characteristics of the device shows improved open circuit voltage and fill factor, hence improved power conversion efficiency compared to that with pristine ITO/graphene.

Thermionic emission properties and the work function determination of arrays of conical carbon nanotubes

Thermionic emission properties of conical carbon nanotubes (CCNTs) grown on platinum wires and planar graphite foils were investigated. The work function (Φ) values extracted from the thermionic emission data range from 4.1 to 4.7eV. The range of Φ values is attributed to the morphological characteristics, such as tip radius, aspect ratio, density, and wall structure of CCNTs. The observed lower values for Φ are significantly smaller than that of multi-walled carbon nanotubes (MWNTs). The reduced Φ values are attributed to field penetration effect as a result of the local field enhancement from these structures having high aspect ratio and an excellent field enhancement factor. The high amplification of the external field at the apex of the nanostructures is capable of reducing both the barrier height and the width, in turn contributing to the improved emission current at lower temperatures. The ultraviolet photoemission spectroscopy data of CCNTs grown on Pt wires are in reasonable agreement with the thermionic emission data. The conical carbon nanotubes may be potential candidates for thermionic cathodes with superior performance over conventional cathodes.

Work-Function Decrease of Graphene Sheet Using Alkali Metal Carbonates

A chemical approach was applied to decrease the work function of few-layer graphene. Li2CO3, K2CO3, Rb2CO3, and Cs2CO3 were used as n-doping materials. The sheet resistance of graphene doped with carbonate salt slightly increased from 1100 to 1700–2500 Ω/sq, and the transmittance of doped graphene with 0.1 M alkali metal at 550 nm decreased from 96.7 to 96.1–94% due to the formation of metal particles on the surface of graphene. A higher sheet resistance and lower transmittance were obtained at a higher concentration of alkali metal carbonate. The G peak in the Raman spectra was shifted to a lower wavenumber after alkali metal carbonate doping and the intensity ratio of the carbon double bond to the carbon single bond decreased with doping in the X-ray photoemission spectroscopy spectra, suggesting the charge transfer from metal ions to graphene sheets. Ultraviolet photoemission spectroscopy data showed that the work function of the graphene sheets decreased from 4.25 eV to 3.8, 3.7, 3.5, and 3.4 eV for graphene doped with Li2CO3, K2CO3, Rb2CO3, and Cs2CO3, respectively. This suggested that spontaneous chemical combination occurred between the carbon atoms and alkali metal, thereby decreasing the work function.

Extension of stability in organic photovoltaic cells using UV/ozone-treated graphene sheets

Organic photovoltaic (OPV) cells with a longer lifetime than that of poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS)-based OPV cells were fabricated using UV/ozone-treated graphene sheets as hole extraction layers (HELs). The sheet resistance increased from 1.1kΩ/sq to infinity and the transmittance at 550nm decreased from 95.2% to 93.8% after UV–ozone treatment for 9min due to the formation of carbon–oxygen functionalization layers. The intensity of the C–C, C–O, and C=O bonds in X-ray photoemission spectroscopy spectra increased with increasing UV–ozone treatment time, indicating the bonding formation between carbon and oxygen. Ultraviolet photoemission spectroscopy data showed that the work function of graphene increased from 4.3eV to 4.85eV after UV/ozone treatment for 9min. However, the OPV cell with 5min-treated graphene HEL displayed the best efficiency of 3.0% because the graphene sheet was damaged by UV–ozone exposure after 7min. The efficiency of the PEDOT:PSS-based device rapidly decreased to 0% after 14h exposure in humid conditions while the UV/ozone-treated device continued to operate for 26h. These results suggested that UV/ozone-treated graphene under optimized condition is a good HEL candidate in OPV cells as a replacement for the conventional PEDOT:PSS layer.

Increased Work Function in Few‐Layer Graphene Sheets via Metal Chloride Doping

AbstractA chemical approach to controlling the work function of few‐layer graphene is investigated. Graphene films are synthesized on Cu foil by chemical vapor deposition. Six metal chlorides, AuCl3, IrCl3, MoCl3, OsCl3, PdCl2, and RhCl3, are used as dopants. The sheet resistance of the doped graphene decreases from 1100 Ω/sq to ≈500–700 Ω/sq and its transmittance at 550 nm also decreases from 96.7% to 93% for 20 mM AuCl3 due to the formation of metal particles. The sheet resistance and transmittance are reduced with increasing metal chloride concentration. The G peak in the Raman spectra is shifted to a higher wavenumber after metal chloride doping, which indicates a charge transfer from graphene to metal ions. The intensity ratio of ICC/IC−C increases with doping, indicating an electron transfer from graphene sheets to metal ions. Ultraviolet photoemission spectroscopy data shows that the work function of graphene increases from 4.2 eV to 5.0, 4.9, 4.8, 4.68, 5.0, and 5.14 eV for the graphene with 20 mM AuCl3, IrCl3, MoCl3, OsCl3, PdCl2, and RhCl3, respectively. It is considered that spontaneous charge transfer occurs from the specific energy level of graphene to the metal ions, thus increasing the work function.

Mechanism for the direct electron injection from Al cathode to the phosphine oxide type electron transport layer

A high efficiency blue fluorescent organic light-emitting diode without LiF electron injection layer was developed. Aluminum electrode was directly deposited on a phosphine oxide type electron transport layer and the observed quantum efficiency was as high as 6.13%. The ultraviolet photoemission spectroscopy data clearly indicated that the electron injection barrier (the offset between Al Fermi level and the lowest unoccupied molecular orbital of the organic layer) is less than 0.1 eV, which led us to believe that more efficient electron injection through the lower barrier is mainly responsible for the high efficiency.

The effect of molybdenum oxide interlayer on organic photovoltaic cells

Both small molecule and polymer photovoltaic cells were fabricated with molybdenum oxide interlayer at the indium tin oxide electrode. Enhancement in power efficiencies was observed in both small molecule and polymer cells. Specifically, the power conversion efficiencies of small molecule cells with the molybdenum oxide interlayer were enhanced by a maximum of 38% due to a significant enhancement in the fill factor. The improved fill factor is attributed to the reduction in series resistance. Our ultraviolet photoemission spectroscopy data indicate that the formation of band bending and the built-in field at the interface due to the interlayer leads to enhancement in hole extraction from the photoactive layer toward the anode.

Work function studies of rare-gas/noble metal adsorption systems using a Kelvin probe

We present a systematic study of the change in work function of noble metal (111) surfaces (Cu, Ag, Au) when being covered with a monolayer of a rare gas (Ar, Kr, Xe). Using a Kelvin probe the experiments show a decrease of the work function of the adsorbate covered system compared to the clean metal surface, in agreement with physisorption theory. The difference in work function depends both on the substrate and adsorbate being the highest for the $\mathrm{Xe}∕\mathrm{Cu}$ system and the lowest for $\mathrm{Ar}∕\mathrm{Au}$. A comparison is performed with angle-resolved ultraviolet photoemission spectroscopy data using the binding energy shift of the (111) surface states of the same adsorbate/substrate systems, and with density functional calculations on simple model systems.

Preparation and characterization of clean, single-crystalline YHx films (0⩽x⩽2.9) on W(110)

Yttrium can be loaded with hydrogen up to high concentrations causing dramatic structural and electronic changes of the host lattice. We report on the preparation of clean, single-crystalline YHx films (0⩽x⩽2.9). The films have been characterized in situ combining angle-resolved photoelectron spectroscopy (ARPES) and low energy electron diffraction. Direct Y dihydride growth, i.e., Y evaporation under a H2 partial pressures of ≈5×10−6 mbar at 500 K on W(110), is the most convenient starting point for the preparation of clean single-crystalline Y hydride films covering H concentrations from the “clean metal” (x≈0) up to the lower boundary of the pure trihydride phase (x≈2.9). Upon annealing Y dihydride films the desired H concentration can be adjusted within the α-phase or the (α+β) two-phase regime. On the other hand, the extension of our photoelectron spectrometer with an homemade ultrahigh vacuum (UHV) compatible hydrogenation system allows to induce the transition from Y dihydride to Y trihydride within a few minutes. The hydrogenation system combines a high-pressure reaction cell with hydrogen permeation through a Pd–24%Ag tube. The overall design is such that the sample never gets in contact with non-UHV compartments. For direct Y dihydride growth on W(110) two equally populated face-centered- cubic(111) domains rotated by 180° with respect to each other are observed. In the α- and γ-phase the Y atoms form a hexagonal-close-packed(0001) oriented lattice. Furthermore, the previously established model for in situ H concentration estimation in Y [J. Hayoz et al., Phys. Rev. B 58, R4270 (1998)] is extended successfully from the α to β to the β to γ-phase transition. Ultraviolet photoemission spectroscopy data unequivocally reveal the opening of a gap extending as far as 1 eV below EF for normal electron emission upon the phase-transformation from Y dihydride to Y trihydride. It also appears that the H absorption rate strongly depends on the H2 purity. Our experimental results demonstrate the capability of this setup for in situ preparation and investigations on the geometrical and electronic structure of Y hydride films and, more generally, rare-earth hydride films using ARPES.

Ab initioinvestigation of Bi-covered GaSb(110) surfaces

We present theoretical studies for the Bi chemisorbed GaSb(110) surface using ab initio pseudoptential calculations. Our results give strong support that the metastable $(1\ifmmode\times\else\texttimes\fi{}1)$ phase is best described by the epitaxially continued layer structure. We also investigate a number of structures for the stable $(1\ifmmode\times\else\texttimes\fi{}2)$ phase and conclude that the system is best described by the modified epitaxially continued layer structure mechanism reported by van Gemmeren et al. The calculated electronic structure is found to be in excellent agreement for both phases with angle-resolved ultraviolet photoemission spectroscopy data presented by McIlroy et al., whereas geometric parameters agree excellently with those obtained by van Gemmeren et al.

Comparative photoemission study ofPt(111)(3×3)R30°CeandPt(111)(2×2)La

Careful examination of the low-energy electron diffraction image from $\mathrm{Pt}(111)(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\mathrm{Ce}$ shows the designation to be only approximate and suggests a close structural similarity to $\mathrm{Pt}(111)(2\ifmmode\times\else\texttimes\fi{}2)\mathrm{La}.$ This is backed up by angle-resolved photoemission spectroscopy, which reveals the valence-band structure of the two alloys to be remarkably similar. The use of the La alloy to identify the non-$f$ features in the photoemission spectra of $\mathrm{Pt}(111)(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\mathrm{Ce}$ is thus put on a firm footing. The Kondo peak in He II angle-resolved ultraviolet photoemission spectroscopy data of $\mathrm{Pt}(111)(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})R30\ifmmode^\circ\else\textdegree\fi{}\mathrm{Ce}$ is observed at the first and second $\overline{\ensuremath{\Gamma}}$ points, suggesting the presence of surface umklapp scattering.

Photoemission with high angular and energy resolution: Temperature dependent exchange splitting in nickel

Ultraviolet photoemission spectroscopy data from a Ni(110) crystal taken with high angular and energy resolution at room temperature (RT) and near the ferromagnetic Curie temperature are presented. For a polar scan in the (001) plane we recorded energy spectra within 1.5 eV from the Fermi energy (E F) in 1° steps. Distinct bands crossing E F can be well separated and, by comparing the RT data with a recent band structure calculation [H. Eckardt, L. Fritsche, J. Phys. F 17, 925 (1987)], the bands can be assigned to a minority d-band and two exchange split sp-bands. While their exchange splitting can hardly be extracted from the energy spectra, the peaks are well separated in k-space, i.e., in polar angle. We determine ΔE ex = 204±8 meV for the sp-band in the examined k-region and find that the angular line width of the majority sp-band is significantly smaller than that of the minority sp-band. Upon approaching T C the exchange splitting of the sp-band vanishes and the d-band becomes very broad.

X-ray induced processes in SrTiO3

The x-ray induced visible luminescence of SrTiO3 has been studied in the range 15–300 °K. An apparently single-emission band at 500 nm has been observed whose efficiency abruptly decreases at ∼30 °K. This luminescence band is proposed to be intrinsic and associated with the decay of a relaxed Ti+3O−53 exciton. After 15 °K irradiation, a visible absorption spectrum which increases towards the infrared is produced and anneals out at ∼30 °K. It is suggested that the absorption is due to electron transition from a Ti+3 center to the conduction band, in accordance with recent ultraviolet photoemission spectroscopy data. The thermoluminescence spectrum after 15 °K irradiation has also been determined. The first glow peak at 30 °K appears to correlate well with the thermal decay of the irradiation-induced absorption.

Photoemission studies of clean and oxidized Nb and Nb3Sn

In conjunction with Auger electron spectroscopy measurements, we have developed a procedure for obtaining a clean Nb surface using an electron bombardment heating technique. We have then studied the valence band photoemission as a function of photon energy. We find that calculations for the bulk theoretical density of states are in basic agreement with the ultraviolet photoemission spectroscopy data. We have also studied the initial oxidation of the clean Nb surface. Structure in the energy distribution curves suggests three stages in the oxidation process. Furthermore, the valence band energy distribution curves for clean Nb3Sn prepared ’’in situ’’ were measured and showed a strong resemblance to pure Nb as has been observed previously in x-ray photoemission. Evidence is found for the existence of Nb2O5 and SnO2 in the surface oxides of Nb3Sn.