Unoccupied electronic states in quaterphenyl oligomer films and at the film-gold and film-oxidized silicon interfaces

Thin films of 4-quaterphenyl (4-QP) are thermally deposited in an ultrahigh vacuum on polycrystalline gold and oxidized silicon substrates. In the process of deposition, the structure of unoccupied electron states 5–20 eV above the Fermi level (E _F) and the surface potential are monitored by the method of total current spectroscopy (TCS) using an incident beam of low-energy electrons. During the deposition, the electron work function of the surface changes because of a change in the surface layer composition, reaching a steady-state value of 4.3 ± 0.1 eV at a 4-QP film thickness of 8–10 nm. The density of valence states (DOVS) and the density of unoccupied states (DOUS) in model 4-QP films are calculated using the linearized augmented plane wave method in the generalized gradient approximation of the density functional theory. In the model 4-QP structure, the minimal spacing between carbon atoms of neighboring 4-QP molecules is taken to be 0.4 nm in order that intermolecular interaction can be assumed to be relatively weak, which is observed in disordered 4-QP films. The TCS-measured DOUS and the DOUS predicted theoretically are in good agreement.     

Electronic properties of the surface of perylene tetracarboxylic acid dianhydride film upon deposition of the ultrathin conjugated layers of Pyronine B

Ultrathin conjugated layers of Pyronine B were thermally deposited in UHV on the surface of perylene tetracarboxylic acid dianhydride (PTCDA) film. The structure of unoccupied electron states located 5-20 eV above the Fermi level (E_F) and the surface potential were monitored during the Pyronine B overlayer deposition, using an incident beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. Electronic work function of the PTCDA surface changed from 4.9 ± 0.1 eV, during the Pyronine B deposition due to the change of the contents of the surface layer, until it reached a stable value 4.6 ± 0.1 eV at the Pyronine B film thickness 8-10 nm. The interface dipole corresponds to electron transfer from the Pyronine B overlayer to the PTCDA surface and the polarization in the Pyronine B overlayer was found confined within approximately 1 nm near the interfaces. The edges of major bands of density of unoccupied electronic states (DOUS) of PTCDA substrate and of the Pyronine B overlayer were unaffected by the process of the interface formation. The major TCS spectral features of the Pyronine B film corresponding to the DOUS band edges were identified and the assignment of the π^*, σ^*(C-C) and σ^*(CC) character was suggested.     

Density of Electronic States in the Conduction Band of Ultrathin Films of Naphthalenedicarboxylic Anhydride and Naphthalenetetracarboxylic Dianhydride on the Surface of Oxidized Silicon

The results of examination of the electronic structure of the conduction band of naphthalenedicarboxylic anhydride (NDCA) films in the process of their deposition on the surface of oxidized silicon are presented. These results were obtained using total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The energy position of the primary maxima of the density of unoccupied states (DOUS) of an NDCA film was determined based on the experimental TCS data and calculated data and compared with the position of the DOUS maxima of a naphthalenetetracarboxylic dianhydride (NTCDA) film. The theoretical analysis involved calculating the energies and the spatial distribution of orbitals of the molecules under study at the B3LYP/6-31G(d) DFT (density functional theory) level and correcting the obtained energies in accordance with the procedure that was proven effective in earlier studies of the conduction band of films of small conjugated organic molecules. It was found that the DOUS maxima of the NTCDA film in the studied energy interval from 5 to 20 eV above the Fermi level are shifted toward lower electron energies by 1–2 eV relative to the corresponding DOUS maxima of the NDCA film Subdivision of the Ufa Federal Research Centre of the.

     

Structure of vacant electronic states of an oxidized germanium surface upon deposition of perylene tetracarboxylic dianhydride films

This paper presents the results of the investigation of the interface potential barrier and vacant electronic states in the energy range of 5 to 20 eV above the Fermi level (E_F) in the deposition of perylene tetracarboxylic dianhydride (PTCDA) films on the oxidized germanium surface ((GeO₂)Ge). The concentration of oxide on the (GeO₂)Ge surface was determined by X-ray photoelectron spectroscopy. In the experiments, we used the recording of the reflection of a test low-energy electron beam from the surface, implemented in the mode of total current spectroscopy. The theoretical analysis involves the calculation of the energy and spatial distribution of the orbitals of PTCDA molecules by the density functional theory (DFT) using B3LYP functional with the basis 6-31G(d), followed by the scaling of the calculated values of the orbital energy according to the procedure well-proven in the studies of small organic conjugated molecules. The pattern of changes in the fine structure of the total current spectra with increasing thickness of the PTCDA coating on the (GeO₂)Ge surface to 6 nm was studied. At energies below 9 eV above E_F, there is a maximum of the density of unoccupied electron states in the PTCDA film, formed mainly by π* molecular orbitals. The higher density maxima of unoccupied states are of σ* nature. The formation of the interface potential barrier in the deposition of PTCDA at the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface, E_vac–E_F, from 4.6 ± 0.1 to 4.9 ± 0.1 eV. This occurs when the PTCDA coating thickness increases to 3 nm, and upon further deposition of PTCDA, the work function of the surface does not change, which corresponds to the model of formation of a limited polarization layer in the deposited organic film.     

Inverse photoemission of pyridine on silver (111)

Inverse photoemission, together with UPS and EELS, is used to obtain information on the position of the affinity level of pyridine adsorbed on Ag(111). The unresolved a₂ and b₁ affinity levels (AL)(1.20 and 0.62 eV above the vacuum level for free pyridine) are found at E_F+(2.9±0.2) eV (indicating a Coulomb relaxation of about 2.1 eV). The reported phase transition in the first adsorbed monolayer of pyridine had no influence on the position and halfwidth of the AL nor on the electronic excitation intensities. The assignment of UPS structure is discussed. Relaxation is different for unoccupied and occupied states. The second layer “initial state” model of photoionization is not confirmed.     

Modification of the electronic properties of the TiO2 (1 1 0) surface upon deposition of the ultrathin conjugated organic layers

A few nm thick 3,4,9,10-perylenetetracarboxylic acid dianhydride (PTCDA) and Cu-phthalocyanine (CuPc) overlayers were thermally deposited in situ in UHV onto TiO₂ (1 1 0) surface. Atomic composition of the surfaces under study was monitored using Auger electron spectroscopy (AES). The formation of the interfacial potential barrier and the structure of the unoccupied electronic states located 5-25 eV above the Fermi level (E_F) was monitored using a probing beam of low-energy electrons according to the total current electron spectroscopy (TCS) method. The work function values upon the overlayer deposition changed from 4.6 to 4.9 eV at the PTCDA/TiO₂ (1 1 0) interface and from 4.6 to 4.3 eV at the CuPc/TiO₂ (1 1 0) interface. Band bending in the TiO₂ substrate, molecular polarization in the organic film and changes in the work function due to the change in the surface composition were found to contribute to the formation of the interfacial potential barriers. Oxygen admixture related peaks were observed in the AES and in the TCS spectra of the CuPc overlayers. A mechanism of the transformations in the PTCDA and CuPc overlayers on the TiO₂ (1 1 0) upon elevating temperature from 25 to 400 °C was suggested.     

Electronic structure of the conduction band of the interface region of ultrathin films of substituted perylenedicarboximides and the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band and the interfacial potential barrier during the formation of interfaces of dioctyl-substituted perylenedicarboximide (PTCDI-C₈) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films with the oxidized germanium surface have been presented. The experimental results have been obtained using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode at energies in the range from 5 to 20 eV above the Fermi level E_F. The positions of the maxima of the fine structure of total current spectra (FSTCS) of the PTCDI-C₈ and PTCDI-Ph films differ significantly in the energy range from 9 to 20 eV above the Fermi level E_F, which can be associated with the difference between the substituents of the chosen molecules, dioctyl- and diphenyl-, respectively. At the same time, the positions of the lowenergy maxima in the FSTCS spectra at an energy 6–7 eV above the Fermi level E_F for the PTCDI-C₈ and PTCDI-Ph films almost coincide with each other. It has been suggested that these maxima are attributed to the electronic states of the perylene core of the molecules under investigation. The process of the formation of interfacial potential barriers of the PTCDI-C₈ and PTCDI-Ph films with the oxidized germanium surface has been analyzed. It has been found that the work functions of the surface, E_vac–E_F, differ little from 4.6 ± 0.1 eV over the entire range of organic coating thicknesses from 0 to 6 nm.     

Electronic structure of the conduction band upon the formation of ultrathin fullerene films on the germanium oxide surface

The results of the investigation of the electronic structure of the conduction band in the energy range 5–25 eV above the Fermi level E_F and the interfacial potential barrier upon deposition of aziridinylphenylpyrrolofullerene (APP-C₆₀) and fullerene (C₆₀) films on the surface of the real germanium oxide ((GeO₂)Ge) have been presented. The content of the oxide on the (GeO₂)Ge surface has been determined using X-ray photoelectron spectroscopy. The electronic properties have been measured using the very low energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode. The regularities of the change in the fine structure of total current spectra (FSTCS) with an increase in the thickness of the APP-C₆₀ and C₆₀ coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C₆₀ and APP-C₆₀ films has made it possible to reveal the energy range (6–10 eV above the Fermi level E_F) in which the energy states are determined by both the π* and σ* states and the FSTCS spectra have different structures of the maxima for the APP-C₆₀ and unsubstituted C₆₀ films. The formation of the interfacial potential barrier upon deposition of APP-C₆₀ and C₆₀ on the (GeO₂)Ge surface is accompanied by an increase in the work function of the surface E_vac–E_F by the value of 0.2–0.3 eV, which corresponds to the transfer of the electron density from the substrate to the organic films under investigation. The largest changes occur with an increase in the coating thickness to 3 nm, and with further deposition of APP-C₆₀ and C₆₀, the work function of the surface changes only slightly.     

Three-photon photoemission from Cu(100): observation of an unoccupied surface state near gG

The photon energy dependence has been measured for three-photon photoemission from Cu(100). A narrow resonance (FWHM = 0.02 eV) was observed centered at 2.02 eV photon energy. The resonance can be explained by the presence of an unoccupied surface state near the center of the surface Brillouin zone at gG and 2.0 eV above E_F.     

Electronic properties of clean and oxygen exposed GaAs (100) surfaces

Both Photoemission Yield Spectroscopy (PYS) and Auger Electron Spectroscopy (AES) have been used in the study of the electronic properties of the clean GaAs(100) surface prepared by IBA procedure and subsequently exposed to oxygen. For the clean GaAs(100)c(8 × 2) surface, the values of the work function and the absolute band bending were 4.20 ± 0.02 eV and −0.23 ± 0.06 eV, respectively, which confirms the pinning of the Fermi level E_F, and two filled electronic surface state bands localized in the band gap below the Fermi level were observed. After exposition of this surface to 10³ L of oxygen, the electronic surface state band localized just below the Fermi level E_F disappeared, and the work function and the absolute band bending increased by only 0.12eV, whereas for the higher oxygen exposures of 10⁴L and 10⁵L, only small increases in the values of the work function and the absolute bending by 0.04 eV and 0.03 eV, respectively, were observed.     

Electron states on the (111) surface of copper

A calculation of electronic states at the (111)-surface of Cu in the energy region around E_F is presented. The calculation is based on the empirical pseudopotential method. A proper position of an abrupt potential barrier ensures overall charge neutrality. A surface state is found at 0.13 eV below E_F, being localized midway between atomic layers. The effective mass of the associate bands is 0.39m_e. Our results are in good agreement with directional photoemission data.     

Unoccupied surface states on the (111) surface of silver: Evidence for broadening

We have studied Ag(111) withk-resolved inverse photoemission spectroscopy athv=9.7 eV. In normal incidence we find image-state emission atE _vac–(0.4±0.1) eV and the unoccupied part of an intrinsic surface-state band as a huge emission peak cut byE _F. The energy dispersion of the intrinsic surface-state band and in particular its crossing ofE _F predicted by Ho et al. cannot be observed because of broadening effects as is shown by a theoretical simulation. The broadening is due to the vicinity of the surface state to the bulk continuum nearE _F as suggested by Kevan.     

Electronic properties of the polycrystalline tin dioxide interface with conjugated organic layers

The results on the electronic structure of the unoccupied electronic states of the polycrystalline SnO₂ in the energy range from 5 eV to 25 eV above the Fermi level are presented. The modification of the electronic structure and of the surface potential upon deposition of the ultrathin films of copper phthalocyanine (CuPc) and of perylene tetracarboxylic acid dianhydride (PTCDA) film onto the SnO₂ surface were studied using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. A substantial attenuation of the TCS signal coming from the SnO₂ surface was observed upon formation of a 1.5–2 nm thick organic deposit layer while no new spectral features from the deposit were distinguishable. It was observed that the electronic structure typical for the organic films was formed within the organic deposit thickness range from 2 nm to 7 nm. The interfacial charge transfer was characterized by the formation of the polarization layer up to 5 nm thick in the organic films. The PTCDA deposition on SnO₂ was accompanied by the negative charge transfer onto the organic layer and to the 0.65 eV increase the surface work function. At the CuPc/SnO₂ interface, the negative charge was transferred to the SnO₂ surface and the overall surface work function decreased by 0.15 eV.     

Spin-resolved photoelectron spectroscopy of the MgO/Fe(110) system

The surface structure and electronic properties of ultrathin MgO layers grown on epitaxial Fe(110) films were investigated at room temperature by means of electron diffraction, Auger electron spectroscopy, scanning tunneling microscopy, and spin-resolved photoelectron spectroscopy. The spin polarization at the Fermi level (E_F) of the Fe(110) film decreases sharply with increasing thickness of the MgO layer. This behavior arises from the formation of a thin FeO layer at the MgO(111)/Fe(110) interface, as revealed by structural and spectroscopic investigations. The strong attenuation of the intrinsic spin polarization is qualitatively attributed to the scattering of spin-polarized electrons at the unoccupied d-orbitals of Fe²⁺. PACS 68.35.-p; 68.55.-a; 73.20.r; 75.70.Cn; 79.60.-I     

N-doping of pentacene by decamethylcobaltocene

We demonstrate n-type doping of pentacene with the powerful reducing molecule decamethylcobaltocene (CoCp₂^*). Characterization of pentacene films deposited in a background pressure of CoCp₂^* by X-ray photoemission spectroscopy and Rutherford backscattering confirm that the concentration of incorporated donor molecules can be controlled to a level as high as 1%. Ultraviolet photoemission spectroscopy show Fermi level (E _F) shifts toward unoccupied pentacene states, indicative of an increase in the electron concentration. A 1% donor incorporation level brings E _F to 0.6 eV below the pentacene lowest unoccupied molecular orbital. The corresponding electron density of ∼10¹⁸ cm⁻³ is confirmed by capacitance–voltage measurements on a metal–pentacene–oxide–silicon structure. The demonstration of n-doping suggests applications of CoCp₂^* to pentacene contacts or channel regions of pentacene OTFTs.     

Density of unoccupied electronic states of vapor-deposited films of dioctyl-substituted and diphenyl-substituted perylenedicarboximides

Physics of the Solid State - The results of the investigation of the density of unoccupied electronic states (DOUS) in the energy range from 5 to 20 eV above the Fermi level (E F) in...     

Electronic properties of TiC(100) and polar TiC(111) surfaces

The energy bands of films of TiC have been calculated using the linear-combination-of-atomic-orbitals method with parameters obtained by a fit to the bulk band structure. The Madelung potentials and charge redistribution have been determined self-consistently. For the neutral TiC(100) surface, the density of states (DOS) is similar to that of the bulk. For the non-neutral Ti-covered TiC(111) surface, Ti 3d-derived surface states appear around the Fermi energy E_F. The long-range electric field produced by the polar surfaces is screened by the charge redistribution, and the polar surfaces are stabilized. Characteristic features of TiC(111) compared to other surfaces of TiC are attributed to the high surface DOS at E_F.     

Study of the occupied and unoccupied electronic states of the Y-substituted (Bi,Pb)-2223 high temperature superconductor

Spectra for the filled and unfilled electronic states of the (Bi,Pb)-2223 high temperature superconductor were recorded by photoemission and fluorescence X-ray absorption in the entire doping range achieved by substitution of bivalent Ca ions with trivalent Y. In photoemission these samples show diminishing spectral intensity near E _F and at 1.5 eV binding energy with increasing Y content. Parallel to the observations for the filled states the O1s X-ray absorption spectra show a decrease and a shift to higher energies of the empty states just above E_F The spectral structures are identified in the framework of the Hubbard model.     

Interpretation of photoemission from Na (1 1 0)

A self-consistent calculation of the electronic structure of the Na (1 1 0) surface, using an embedding method to treat the semi-infinite system, gives a prominent surface resonance peak at 0.75 eV above the Fermi energy. The tail of this resonance extends below E_F at the surface, and it is suggested that surface photoemission from this tail is responsible for the peak in the photocurrent at E_F observed experimentally, A photoemission calculation, with a non-self-consistent surface potential, reproduces the enhancement of the peak as a direct transition moves through it.     

CaWO4晶体中F型色心电子结构的研究

运用相对论的密度泛函离散变分法(DV-Xα)研究了CaWO₄晶体中F型色心的电子结构. 计算结果表明，F和F⁺心在禁带中引入了新的施主能级；分析了晶体内可能存在的光学跃迁模式，并通过过渡态的方法计算了F，F⁺心跃迁到导带底的能量分别为1.92eV和2.42eV. 因此，从理论上推断了F和F⁺心在CaWO₄晶体中可能引起650nm和515nm的吸收，由此说明CaWO₄晶体中650nm和515nm吸收带起源于晶体中的F和F⁺心. 关键词： 4晶体')" href="#">CaWO₄晶体 +心')" href="#">F和F⁺心 DV-Xα