UV Photoemission Spectroscopy Research Articles

Temperature-dependent angular resolved UV-photoemission spectroscopy from CeNi2Ge2

Abstract Pronounced temperature effects in angular resolved ultraviolet photoelectron spectroscopy from the (001) surface of the ternary heavy fermion compound CeNi 2 Ge 2 are presented. The measurements were performed on atomically clean and well-ordered thin films grown on a W(110) substrate. A strongly enhanced intensity at the Fermi edge ( ϵ F ) is observed at low-temperatures if the spectra are excited by means of HeI light ( hν =21.2 eV). In addition, the work function is dramatically increased with temperature, exhibiting an unusually high positive temperature coefficient of about 0.65 meV/K. The observed temperature dependency suggests a strong redistribution of the states near the Fermi-energy with increasing temperature. A simple model will be given that correlates the observed intensity changes at ϵ F with the change of the work function.

Effects of Pressure on the Vibrational, Structural and Electronic Properties of C60 Powder and Thin Films

ABSTRACTWe present results from a study of the vibrational, structural, and electronic properties of C60 powder and thin films. Raman spectroscopy and diamond anvil cell have been used to study pressure dependence of the Raman active modes of C60 powder. The material undergoes structural phase transition between 9 and 15 GPa. Some of the Raman modes soften, while others harden with increasing pressure. Thin films of C60 and La-doped C60 have also been studied by using Raman scattering, x-ray diffraction, x-ray photoelectron spectroscopy and uv photoemission spectroscopy. Whereas the powder and La-doped C60 films exhibit fcc crystalline structure, the C60 film appears disordered. Further, we observe a significant difference in the electronic valence bands of the doped and undoped films.

Energy level alignment and band bending at model interfaces of organic electroluminescent devices

The basic concepts of common vacuum level and band bending in Mott–Schottky (MS) model was experimentally examined for the model interfaces of organic electroluminescent devices by using UV photoemission spectroscopy (UPS) and Kelvin probe method (KPM). We found that interfacial dipole cannot be neglected at most organic/metal interfaces and that the potential shift at the interface due to such dipole sometimes reaches over 1 eV in contrast to the assumption of the common vacuum level in MS model. Based on the observed data, possible origins of the interfacial dipole and general trends of the potential shift against the work function of the electrode metal were proposed. The band bending and Fermi level alignment were also examined at the interfaces between metals (Au, Cu, Ag, Mg, Ca) and TPD( N, N′-bis(3-methylphenyl)- N, N′- diphenyl-[1,1′-bisphenyl]-4,4′-diamine).

Structure and property relationships of amorphous CNx: a joint experimental and theoretical study

Amorphous CNx and CNx:H have been prepared by the ion beam assisted deposition technique. Samples were characterized through X-ray and UV photoemission, IR absorption and Raman spectroscopies. These spectra have been interpreted with the aid of quantum chemical calculations based upon the Hartree-Fock theory on several molecular models. The understanding of the electronic and structural properties of the amorphous alloy as a function of nitrogen content could help in the task of synthesizing the metastable silicon-nitride like-phase b-C3N4 , a solid which has been predicted to be as hard as diamond. The physical picture emerging from the present study helps to clarify the difficulties in obtaining the crystalline phase of the material, suggesting new experimental directions for syntheses.

Negative electron affinity on polycrystalline diamond surface induced by lithium fluoride deposition

Lithium fluoride (LiF) was deposited on hydrogenated and oxygenated polycrystalline diamond surfaces. Lowering of the work function induced by the dipole effect of LiF was clearly observed using UV photoemission spectroscopy and Kelvin probe. The decrease of the work function was sufficiently large so that negative electron affinity was induced. This shows that the LiF overlayer on diamond acts as an effective dipole layer to lower the surface work function of the diamond film.

Electronic structure of Alq 3/LiF/Al interfaces studied by UV photoemission

The electronic structure of tris (8-hydroxyquinolino) aluminum (Alq 3/LiF/Al system was studied in relation to the enhancement of electron-injection efficiency by the insertion of LiF insulating layer at Alq 3/Al contact, using UV photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and metastable atom electron spectroscopy (MAES). The observed energy separation between the HOMO of Alq 3 and the Fermi level of Al substrate increased from 2.7eV to 3.0eV by inserting 0.5nm thick LiF layer. This result indicates that the LiF layer induces the decrease of the electron injection barrier. We also found extra states probably caused by the interaction at the Alq 3/Al interface.The spectral intensity of this extra state decreased with increasing LiF thickness, and vanished at 0.5nm.

Electronic structure and molecular orientation of well-ordered polyethylene oligomer (n-C44H90) on Cu(100) and Au(111) surfaces studied by UV photoemission and low energy electron diffraction

Abstract The electronic structure and molecular orientation of tetratetracontane (n-C44H90) films on Cu(100) and Au(111) surfaces were investigated by angle-resolved UV photoemission spectroscopy (ARUPS) and low energy electron diffraction (LEED). The observed ARUPS spectra showed the drastic take-off angle dependence due to intramolecular band dispersion. A 2×1-like LEED pattern was observed for both substrates. From these results and theoretical simulation of ARUPS spectra based on independent-atomic center (IAC) approximation, we found that the C–C–C plane of the adsorbed TTC molecule is parallel to the substrate surface and its molecular axis is along a [110] direction for both substrates. We also measured the work function change by adsorption of TTC. The observed values were c.a. −0.3eV and −0.7eV for Cu(100) and Au(111) systems, respectively. Such decrease of the work function indicates the existence of a dipole layer at the interfaces in contrast to the traditional picture of energy level alignment at organic/metal interface assuming a common vacuum level at the interface. The dipole formation in such physisorbed systems can be explained by the polarization of the TTC molecule due to an image force.

Electronic properties of a pseudomorphic Cu-layer on Ni(111)

The band structure of a monolayer Cu on Ni(111) was studied both by angle resolved UV photoemission spectroscopy (ARUPS) and density functional (DF) calculations. The Cu 3d-bands at binding energies between 2 eV and 5 eV show a very pronounced k ‖ dispersion along the [11 2] and [ 110] substrate directions, but no dispersion with k ⊥. This proves the two-dimensional character of the Cu 3d-bands. The periodicity of the bands coincides with the periodicity of the Ni(111) surface. The experimentally determined two-dimensional band structure of the pseudomorphic Cu monolayer on Ni(111) is in very good agreement with the results of DF calculations on the system. From this comparison we obtain detailed information about the Cu/Ni interaction and the k-dependent coupling of Cu and Ni bands.

Electronic Structure of Organic/Metal Interfaces Studied by UPS and Kelvin Probe

ABSTRACTThe interfaces of organic materials with other solids play important roles in the function of various organic devices such as organic light-emitting diodes (OLEDs), spectral sensitization in photography, organic solar cells, and electrophotography. Also they should be important in future molecular devices, both in the central part of the device and at the connection with outside circuits. However, serious experimental examination of such interfaces has started only recently. In this talk we focus our attention on the organic/metal interfaces, and summarize our understanding about (1) the energy level alignment right at the interface, and (2) possible band bending within an organic layer, mainly using the techniques of UV photoemission spectroscopy (UPS) and Kelvin probe method. As for (1), the formation of electric dipole layer was observed in most organic/metal interfaces, and its origin is discussed. As for (2), recent examination of the existence/absence of band bending in ultrahigh vacuum will be reported. It is also pointed out that there can be much effects of (i) atmosphere at sample preparation and measurements, and (2) chemistry and interdiffusion at the interface.

Electronic properties of the Bi/Si(100) interface

An investigation of the electronic properties of the Bi/Si(100) interface in the early growth stage is presented. Bi-induced occupied electronic states and absorption features are determined through the complementary UV photoemission spectroscopy and high resolution electron energy loss spectroscopy in the energy gap region. The interface is clearly semiconducting at half-monolayer coverage, with a surface energy gap of 1.1 eV, greater than that of the clean Si(100) surface. This behavior can be understood by considering the saturation of the topmost layer silicon dimer dangling bonds by the Bi adatoms. The interface assumes a weak semimetallic character at the completion of one monolayer, probably because of the formation of Bi–Bi adatom bonds. Space charge layer formation and a slight downward band bending.

Electron affinity and Schottky barrier height of metal–diamond (100), (111), and (110) interfaces

The electron emission properties of metal–diamond (100), (111), and (110) interfaces were characterized by means of UV photoemission spectroscopy (UPS) and field-emission measurements. Different surface cleaning procedures including annealing in ultrahigh vacuum (UHV) and rf plasma treatments were used before metal deposition. This resulted in diamond surfaces terminated by oxygen, hydrogen, or free of adsorbates. The electron affinity and Schottky barrier height of Zr or Co thin films were correlated by means of UPS. A negative electron affinity (NEA) was observed for Zr on any diamond surface. Co on diamond resulted in NEA characteristics except for oxygen-terminated surfaces. The lowest Schottky barrier heights were obtained for the clean diamond surfaces. Higher values were measured for H termination, and the highest values were obtained for O on diamond. For Zr, the Schottky barrier height ranged from 0.70 eV for the clean to 0.90 eV for the O-terminated diamond (100) surface. Values for Co ranged from 0.35 to 1.40 eV for clean- and O-covered (100) surfaces, respectively. The metal-induced NEA proved to be stable after exposure to air. For the oxygen-terminated diamond (100) surface a field-emission threshold of 79 V/μm was measured. Zr or Co deposition resulted in lower thresholds. Values as low as 20 V/μm were observed for Zr on the clean diamond (100) surface. Results for Zr or Co on H- or O-terminated surfaces were higher. H or O layers on diamond tend to cause an increase in the Schottky barrier height and the field-emission threshold field of Zr– and Co–diamond interfaces. The value of the electron affinity and Schottky barrier were correlated with work function and different initial surface preparation. The results were largely consistent with a model in which the vacuum level was related to the metal work function and the measured Schottky barrier.

Electronic Structures and Chemical Bonding of Fluorinated Fullerenes Studied by NEXAFS, UPS, and Vacuum-UV Absorption Spectroscopies

The chemical bonding and the electronic structures of C60Fx and C70Fx were investigated by near edge X-ray absorption fine structure (NEXAFS) spectroscopy and UV photoemission spectroscopy (UPS), which are useful methods for examining the unoccupied and the occupied states, respectively. With these results and XPS measurements, we derived the electronic energy diagram of C60Fx and discussed the change of the electronic structure from that of C60 by fluorination. The energies of the LUMO and the Fermi level of solid C60F48 were estimated to be −5.0 and −5.4 eV below the vacuum level, indicating that highly doped C60Fx is a strong electron acceptor. The electronic absorption spectra of C60Fx solutions deep into the vacuum-ultraviolet region were also measured, and the isomerism of C60Fx was discussed by comparing the observed results with theoretical simulations.

Energy-level alignment at model interfaces of organic electroluminescent devices studied by UV photoemission: trend in the deviation from the traditional way of estimating the interfacial electronic structures

The electronic structures of model interfaces of organic electroluminescent (EL) devices were investigated with UV photoemission spectroscopy (UPS). Interfaces of TTN (tetrathianaphthacene) and TCNQ (tetracyanoquinodimethane) were also studied as extreme cases for hole transport and electron transport material, respectively. For all organic/metal interfaces studied, the work function of metal electrode was changed by deposition of organic layer, i.e., the vacuum level was shifted at the interface, indicating the invalidity of the traditional energy level alignment model where a common vacuum level was assumed at organic/metal interface. At TCNQ/Au, DP-NTCI/Al, which are acceptor/metal interfaces, upward shift of the vacuum level of organic layer relative to that of metal was observed, suggesting the formation of interfacial dipole due to electron-transfer from metal to acceptor. At other organic/metal interfaces, TPD(N, N'-diphenyl-N, N'-(3-methylphenyl)-1, 1'-biphenyl-4, 4'-diamine)/Au or ITO (indium tin oxide), ALq/sub 3/ (tris(8-hydroxyquinolino) aluminum)/Al, DP-NTCl(N, N'-diphenyl-1,4,5,8- naphthyltetracarboxylimide)/Al or Au, downward shift of the vacuum level was observed. Such downward shift has been also observed in our previous study for porphyrin/metal interfaces, and seems to be a trend for organic/metal interfaces at which no electron-transfer from metal to organic layer occurs. This trend suggests that the traditional model tends to underestimate (overestimate) the barrier height for hole (electron) injection. On the other hand, the vacuum level shift at ALq/sub 3//TPD interface was less than 0.1 eV, leading to an apparent applicability of the traditional model. However, it is not always the case for organic/organic interfaces: finite shift of 0.2 eV was observed at TTN/TCNQ interface due to electron-transfer from TTN to TCNQ. Possible origins of vacuum level shift at organic/metal interfaces were also discussed.

Tilted adsorption of benzene on Pt(110) 1 × 2

The adsorption of benzene on the reconstructed Pt(110) 1 × 2 surface was studied by temperature-programmed desorption (TPD), low-energy electron diffraction (LEED), angle-resolved UV photoemission (ARUPS) and near-edge X-ray absorption spectroscopy (NEXAFS). At 100 K, benzene is molecularly adsorbed on the Pt(110) 1 × 2 surface. The saturated layer is disordered and corresponds to a coverage of ∼0.28 ML. Annealing to 280 K leads to partial benzene desorption and to the formation of a well-ordered (4 × 2) layer (relative to the unreconstructed surface) with a coverage of 0.25 ML. The missing ( (2n + 1) 4 , 0) adsorbate LEED spots along [11̄0] indicate a glide plane in the adsorbate structure and the existence of two molecules per unit cell. In this layer, benzene adsorbs with the molecular plane tilted by ∼30° in the [001] direction, as determined from ARUPS and NEXAFS. This geometry corresponds to a nearly flat adsorption on the (111) microfacets of the troughs of the reconstructed surface. The molecules have a well-defined azimuthal orientation with a CH bond along the [11̄0] azimuth. For dilute layers ( ϑ = 0.12 ML) and a disordered layer with the same coverage as the ordered layer ( ϑ = 0.25 ML), the molecules are also titled into the troughs, a preferential azimuthal orientation is, however, not observed.

Investigation of line shapes and line intensities by high-resolution UV-photoemission spectroscopy — Some case studies on noble-metal surfaces

Line shapes in angle-resolved photoemission spectra from solid surfaces contain a wealth of information about many-body effects in the electron system. However, an analysis of experimental data depends on a very detailed understanding of the contributions to line width due to various other effects. This report reviews photoemission line shape studies of bulk- and surface-emission peaks measured on noble-metal surfaces and their interpretation in terms of electron and photohole lifetimes, electron-phonon interaction and defect scattering. Special attention is focussed on the possibility to measure the photohole lifetime in the vicinity of the Fermi level. Different channels of photohole decay like Auger processes, phonon creation and elastic scattering at surface defects and impurities are discussed along with high-resolution spectra from surface states. We discuss to which extent the various effects can be distinguished experimentally. Information can also be extracted from experimental data concerning properties of the many-electron system at elevated temperature.

Energy level alignment at organic/metal interfaces studied by UV photoemission

Energy level alignment at various organic compound/metal interfaces was investigated by UV photoemission spectroscopy (UPS). The systems measured were: (i) model interfaces of organic electroluminescence (EL) devices, (ii) porphyrin/metal interfaces, and (iii) tetratetracontane/Au interfaces. At all the measured interfaces, a shift of the vacuum level at the interface was observed, showing the formation of an interfacial electric dipole layer. For Alq 3 (tris(8-hydroxyquinolino)aluminum), TPD ( N,N′-diphenyl- N,N′-(3-methylphenyl)-1,1′-biphenyl-4, 4′-diamine), and DP-NTCI ( N,N′-diphenyl-1,4,5,8-naphthyltetracarboxylimide) /metal interfaces, interfacial energy diagrams determined by UPS correspond well with the actually observed carrier-injecting characters at the interfaces. For ZnTPP (5,10,15,20-zinctetraphenylporphyrin), H 2TPP (5,10,15,20-tetraphenylpotphyrin), and H 2T(4-Py)P (5,10,15,20-tetra(4-pyridyl)porphyrin)/metal interfaces, the shifts of the vacuum level as well as the energies of the levels in porphyrins could be expressed as linear functions of the work function of the metal substrate. The slope of the linear function depends on the compound. The existence of the shift of the vacuum level at tetratetracontane (TTC. n-C 44H 90/Au interfaces, where TTC molecules are physically adsorbed on Au substrate, suggests that the vacuum level shift is an almost general phenomenon at organic/metal interfaces even without strong chemical interaction at the interface. These findings clearly demonstrate the invalidity of the traditional model for energy level alignment with the assumption of vacuum level alignment at the interface.

Electronic Structures of Porphyrins and their Interfaces with Metals Studied by UV Photoemission

The electronic structures of porphyrins and their interfaces with metals were investigated by UV photoemission spectroscopy (UPS). The UPS spectral features of porphine, 5,10,15,20-tetraphenylporphynatozinc(II) (ZnTPP), 5,10,15,20-tetra(4-pyridyl)porphyrin (H2T(4-Py)P) and 5,10,15,20-tetraphenylporphyrin (H2TPP) could be assigned by comparison with MOPAC PM3 calculations. The electronic structures of the porphyrins can be regarded as the superposition of those of porphine and the substituents. The UPS results for ZnTPP/metal (Mg, Al, Ag, Au) interfaces indicated that the energies of the levels of ZnTPP relative to the Fermi level of substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface. The slope of the linear functions was about unity. This indicates that the energy levels of ZnTPP are fixed to the vacuum level of substrate metal with constant interfacial dipole. The level positions of ZnTPP at the interface exhibited drastic change by exposure to oxygen. This change could be explained by the change of the work function of metal surface upon oxidization. For H2T(4-Py)P and H2TPP/metal (Mg, Ag, Au) interfaces, similar linearity was observed between the energy levels of porphyrin and the work function of metal with the slope of much smaller than unity. This deviation of the slope from unity might be explained by the existence of interface state. These findings require the modification of the traditional concept of a common vacuum level at organi/metal interface. The relation between the observed electronic structures and the conduction type of the porphyrins are also discussed.

Characterization of metal-diamond interfaces: Electron affinity and Schottky barrier height

Abstract In this study, the electron affinity and Schottky barrier height of thin Cu and Zr films on diamond (100) substrates were correlated by means of UV photoemission spectroscopy (UPS) measurements. Prior to metal deposition the diamond crystals were cleaned by a 1150°C or 500°C anneal in UHV, and the surfaces were characterized by AES and AFM. This resulted in surfaces terminated with oxygen or free of chemisorbed species. By means of UPS it was found that whether a metal did induce a negative electron affinity (NEA) on a diamond surface was dependent on the surface preparation before metal deposition and on the metal work function. In particular, the Schottky barrier height for clean surfaces was lower than for surfaces terminated by oxygen. Metal-diamond interfaces exhibiting a NEA had a lower Schottky barrier height than those exhibiting a positive electron affinity. These effects were attributed to different interfacial layers. Field emission measurements were performed before and after metal deposition. For all cases a reduction in the threshold electric field was observed upon metal overgrowth.

The electronic structure of porphyrin/metal interfaces studied by UV photoemission spectroscopy

The electronic structures of the interface between porphyrin and metal (Mg, Al, Ag and Au) were studied by UV photoemission spectroscopy (UPS). The samples were 5,10,15,20-tetraphenylporphynatozinc(ZnTPP), 5,10,15,20-tetra(4-pyridyl) porphyrin(H 2T(4-Py)P) and 5,10,15,20-tetra-phenylporphyrin(H 2TPP) which are known as organic semiconductors. We found that the energy levels of these porphyrins relative to the Fermi level of the substrate metals could be expressed as linear functions of the work function of metals with a shift of the vacuum level at interface (Δ). This indicates that the energy levels of these porphyrins are fixed to the vacuum level of substrate metal with an interfacial dipole layer. We also found that the magnitude of Δ is constant at ZnTPP/metal interfaces, but depends on the work function of the metal at H 2T(4-Py)P and H 2TPP/metal interfaces. This work function dependence of Δ might be explained by the existence of interface state.

Electronic structure of organic carrier transporting material / metal interfaces as a model interface of electroluminescent device studied by UV photoemission

Electronic structures of N,N′-diphenyl- N,N′-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) / metal and N,N′-diphenyl-1,4,5,8-naphtyltetracarboxylimide (DP-NTCI) / metal interfaces were directly investigated as a model interface of organic electroluminescent (EL) devices using UV photoemission spectroscopy (UPS). At the organic / metal interfaces, abrupt shift of vacuum level was observed, in contrast to the traditional assumption of common vacuum level at the interface. For understanding EL devices, we see the necessity of the direct observation of the interfacial electronic structure by UPS or other techniques for understanding EL devices.