Total Current Spectroscopy Research Articles

Photovoltaic properties of interfaces of organic films of substituted perylene with TiO2 and SnO2 surfaces

The photovoltaic effect has been detected and studied in structures based on ultrathin vacuum-deposited organic films of perylene-3,4,9,10-tetracarboxylic acid dianhydride on the titanium and tin dioxide surfaces. The interfacial potential barrier shape in these structures is studied by low-energy electron total current spectroscopy. Changes in the surface potential upon exposure to visible light are recorded in situ using an electron-beam probe with energies from 0 to 25 eV. The photovoltage is detected at incident photon energies of 1.5–2.5 eV, which corresponds to the organic film absorption range and simultaneously to the transmission band of titanium and tin dioxides. An analysis of the spectral distributions and transient responses shows that two components of the observed photovoltage can be distinguished. The relation of one of the components to the excitation of interband transitions in the organic film and another component to electronic transitions involving interfacial energy states are discussed.

Formation and electron properties of the interface between organic (TPD) and inorganic (ZnO) materials

The initial stage of formation of an organic coating of N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine (TPD) on a crystalline (ZnO) surface in the course of thermal deposition in high vacuum has been studied using the low-energy total current spectroscopy technique. A change in the work function and the density of unoccupied electron states in an energy interval of 0–20 eV above the vacuum level was traced as the organic coating thickness increased up to 8 nm. The energy positions of the bands of unoccupied electron states in the TPD film have been determined, including π* band (7–9 eV above the Fermi level), σ1* band (10–12 eV), σ2* band (14–16 and 18–20 eV).

Influence of the substrate properties on the electronic structure of organic film-inorganic substrate interfaces

The structure of electronic states at the interfaces between perylene tetracarboxyldianhydride films and ZnO, Cu, GaAs, and InAs substrates is studied by the method of total current spectroscopy. It is found that the physicochemical properties of the substrate have a strong effect on the electronic state configuration in the substrate and film. On the basis of the experimental data, potential diagrams of the interfaces are constructed, which illustrate the formation of sharp and extended dipole layers, change in the charge state of surface levels, and modification of the electronic structure of the film.

Formation of organic films on the indium arsenide surface

The formation of interfaces between the single-crystalline InAs(001) surface and organic semiconductor molecules (PTCDA and NTCDA) is investigated. With the methods of total current spectroscopy and Auger electron spectroscopy, it is found that indium atoms diffuse from the substrate into the molecular film. Indium atoms present in the film change the structure of its π-electron states and decrease the energy of unoccupied molecular orbitals.

Interface formation between two organic films based on phthalocyanine and perylene derivatives

The process of interface formation between two organic films composed of donor (copper phthalocyanine, CuPc) and acceptor (perylene-3,4,9,10-tetracarboxylic dianhydride, PTCDA) molecules has been studied in situ using the total current spectroscopy technique. It is established that the donor-acceptor interaction between CuPc and PTCDA molecules do not distort the energy structure of the density of electron states. The main π*, σ*1, and σ*2 bands of antibonding (unoccupied) electron states are identified, which are determined both by C-C bonds in the aromatic rings and by additional C-N and C-O bonds. The width of the interface potential barrier is evaluated and its relation to the limiting polarizability of molecules is demonstrated. The interface potential barrier is formed in the course of negative charge transfer between donor (CuPc) and acceptor (PTCDA) molecules.

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 (EF) 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 structure at the PTCDA/GaAs and NTCDA/GaAs interfaces

The formation of the interface between the GaAs(100) single-crystal surface and PTCDA and NTCDA organic semiconductors is investigated. The method of total current spectroscopy makes it possible to trace the formation of the interfacial electronic structure. The two organic materials and the GaAs substrate are bonded together when the π electron cloud of an aromatic ring spreads toward the substrate. This modifies the electronic states of interfacial organic molecules and generates a dipole at the interface.

Direct interface states observation by total-current spectroscopy

Direct interface states observation by total-current spectroscopy

Interface formation between Cu-phthalocyanine films and CdS and GaAs semiconductor surfaces

Total current electron spectroscopy (TCS) is applied as a key experimental technique to studies of electronic structure of organic semiconductor films interfacing inorganic semiconductors. Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on CdS(0 0 0 −1) and GaAs(1 0 0) substrates. The surface potential, the surface work function and the structure of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F ) was monitored during the film deposition, using an incident beam of low-energy electrons according to the TCS method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. Electronic work function of the CuPc films changed during the film deposition until it reached a stable value 4.5±0.1 eV at the film thickness 8–10 nm. The deposition of the CuPc under 1.5 nm resulted in formation of the intermediate DOUS structures that were different for the cases of the two substrates used in the study. Fragments of the CuPc molecules were found to constitute mainly the intermediate deposit layers. Along with the increase of the deposit thickness to 8–10 nm, the intermediate DOUS structures were replaced by the DOUS structure typical for the CuPc film, although the S, Ga and As atoms migrated from the substrates were present in the CuPc film. Electron charge transfer layer at the CuPc/GaAs interface extended more than 5 nm from the GaAs surface, while the interaction at the CuPc/CdS interface resulted in band bending in the CdS subsurface region.

Direct interface states observation by total-current spectroscopy

Interface electronic states resulting from interaction between Cu(1 1 1) surface and naphtalene-1,4,5,8-tetracarboxylic dianhydride (NTCDA) and N, N′-dibenzyl-naphtalene-tetracarboxylic diimide was investigated using the total-current spectroscopy. The evolution of these states with the film thickness increasing was observed directly upon the film growth. The contribution of different parts of the molecules into the formation of particular interface states was clarified.

Interface formation between Cu-phthalocyanine films and CdS and GaAs semiconductor surfaces

Total current electron spectroscopy (TCS) is applied as a key experimental technique to studies of electronic structure of organic semiconductor films interfacing inorganic semiconductors. Thin films of Cu-phthalocyanine (CuPc) were thermally deposited in UHV on CdS(0 0 0 −1) and GaAs(1 0 0) substrates. The surface potential, the surface work function and the structure of unoccupied electron states (DOUS) located 5–25 eV above the Fermi level ( E F) was monitored during the film deposition, using an incident beam of low-energy electrons according to the TCS method. Auger electron spectroscopy (AES) was used to monitor atomic composition of the surfaces under study. Electronic work function of the CuPc films changed during the film deposition until it reached a stable value 4.5±0.1 eV at the film thickness 8–10 nm. The deposition of the CuPc under 1.5 nm resulted in formation of the intermediate DOUS structures that were different for the cases of the two substrates used in the study. Fragments of the CuPc molecules were found to constitute mainly the intermediate deposit layers. Along with the increase of the deposit thickness to 8–10 nm, the intermediate DOUS structures were replaced by the DOUS structure typical for the CuPc film, although the S, Ga and As atoms migrated from the substrates were present in the CuPc film. Electron charge transfer layer at the CuPc/GaAs interface extended more than 5 nm from the GaAs surface, while the interaction at the CuPc/CdS interface resulted in band bending in the CdS subsurface region.

Electron spectroscopy study of NO 2 adsorption on the Pd/MgO/Cu system

An evolution of an energy structure of the unoccupied electronic states and of a surface composition during the formation of a Pd/MgO/Cu (polycrystalline) thin layer system and NO 2 adsorption was studied using Auger electron spectroscopy, total current spectroscopy (TCS) and work function measurements. All investigations were performed in situ under UHV conditions. A 10nm thick MgO film was prepared via oxidation of a Mg overlayer on a clean polycrystalline Cu substrate. The MgO electronic structure was completely screened after deposition of a few monolayers of Pd and the Pd-related TCS fine structure appeared. We observed an irreversible NO 2 adsorption on the Pd layer. A negative charge transfer to adsorbed molecules and appearance of new adsorption-induced unoccupied bands were revealed. These new bands are considered as a result of hybridisation of palladium and unoccupied oxygen orbitals.

Electronic charge distribution at interfaces between Cu-phthalocyanine films and semiconductor surfaces

Total current electron spectroscopy (TCS) that uses a probing beam of low energy electrons was applied to study electronic charge transfer at interfaces between Cu-phthalocyanine (CuPc) films thermally deposited in situ onto ZnO, oxidized and crystalline silicon substrates. Analysis of the TCS data provided us also with new data on the density of unoccupied electron states (DOUS) of the CuPc films at 0–25 eV above E F. A most significant electronic charge transfer from the CuPc film to SiO 2/n-Si and n-Si(1 0 0) was observed and the polarization layer extended up to 10 nm into the CuPc film bulk. The electronic structure of the CuPc molecules on n-Si(1 0 0) and on ZnO(0 0 0 1) was perturbed within 1–2 nm of the deposit due to interaction with the substrates. Admission of O 2 and NO 2 at 10 −5 Pa and 300 K resulted in a reversible decrease and increase of the film surface potential, respectively. Formation of TC peaks related to O-orbitals on the gas adsorption on the CuPc surface was also registered.

Stepwise ionization of the 2,4-dioxybenzaldehyde and 3,4-dimethoxypropiophenone vapors

The processes of stepwise ionization of the 2,4-dioxybenzaldehyde and 3,4-dimethoxypropiophenone vapors by radiation in the range of wavelengths down to 266 nm were studied using total-current spectroscopy, mass spectrometry, and photoelectron spectroscopy. The process of two-step ionization and the resulting generation of molecular ions are dominant if the laser-radiation intensity amounts to ≈106 W/cm2. These molecular ions have appreciable (up to 1.8 eV) energy of vibrational excitation. As the laser-radiation intensity increases, progressively more pronounced fragmentation occurs owing to dissociation of molecular (and, possibly, fragmentation) ions as a result of absorption of at least a single additional photon. The processes leading to fragmentation of ions are suggested. It is found that the dissociation-ionization mechanism is important for the dimethoxypropiophenone. Dissociation with a breakage of the α bond in the carbonyl group with subsequent two-photon ionization of the fragments occurs when the S 2(1 A′) state of the ππ* type is excited.

Stepwise photoionization of 1,2- dimethoxybenzene vapor

Processes of stepwise ionization of 1,2-dimethoxybenzene vapor by radiation in the range 295-275 nm were studied by the techniques of total current spectroscopy, mass spectrometry, electronic absorption spectroscopy, and threshold electron spectroscopy. A two-step ionization process via the first electronically excited state of the molecule yielding the molecular ion was found to prevail at an average laser radiation intensity less than 106 W/cm2. Molecular ions possess a considerable (up to 1 eV) vibrational excitation energy. As the radiation intensity increases, the progressively stronger and deeper degradation takes place via dissociation of molecular and, probably, fragment ions due to absorption of at least one additional photon.

A total current spectroscopy study of metal oxide surfaces: II. Unoccupiedelectronic states on TiO2(110) and SrTiO3(100) surfaces

Unoccupied densities of states (DOS) for the ordered and atomicallyclean TiO2(110) and SrTiO3(100) surfaces were studied using the total currentspectroscopy (TCS) technique. Obtained TC spectra reveal the Ti-derived(Ti 3d(eg) - 5.2 eV, Ti 4s - 23.5 eV) and Sr-derived (Sr 3d - triplet: 7.3, 9.0 and10.5 eV; Sr 5p/O 2p - 15 eV) maxima of unoccupied DOS. Main oxygen-derived maximaare located at 11.2 and 15.0 eV for TiO2 and at 11.8, 18.0 and 19.6 eV forSrTiO3. The results are discussed in comparison with the previously reporteddata on DOS of these materials and they provide additional detailedinformation on the energy structure of unoccupied DOS on TiO2(110) andSrTiO3(100) surfaces in the 4-25 eV range above the Fermi level.

Electron scattering states for low-energy spectroscopies

In contrast to bound-state calculations for the valence band regime, the continuous states of half-space systems have not attained a level where a broad variety of methods ensures large applicability and sufficient accuracy for the needs of highly resolved spectroscopy. The multiple-scattering procedure works well in the high-energy range, but lacks either accuracy or convenience when applied to lower energies, where the anisotropic parts of the potential are strongly effective. The concept of Bloch-wave procedures in its original form is less well adapted to scattering boundary conditions, but requires additional theoretical adjustment, such as matching conditions, which often leads to an ill-posed boundary problem of the elliptic equation. The solution then suffers from instabilities preventing the development of stand-alone algorithms. The current state of the subject together with some promising solutions will be presented. Consideration will be given to the multiple scattering approach with anisotropic potentials and to the Bloch-wave development relying on pseudopotential, as well as on singular-potential formulations. The importance and quality of procedures is estimated through a comparison with very low energy electron diffraction (VLEED), low-energy electron diffraction (LEED) and total current spectroscopy (TCS) experiments.

PTCDA film formation on Si(111):H-1×1 surface: total current spectroscopy monitoring

The process of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) film formation on the Si(111):H-1×1 surface was studied by means of total current spectroscopy (TCS) and low-energy electron diffraction (LEED). Film deposition was performed under ultrahigh vacuum (UHV) conditions at room temperature. Analysis of the evolution of total current (TC) spectra, combined with average film thickness measurements performed by atomic force microscopy (AFM), revealed island growth under the given experimental conditions. This growth type is the result of a very weak interaction between PTCDA molecules and the passivated silicon surface. A preliminary explanation for the fine structure in the TC spectra is given in terms of variation of the elastic electron reflection on the edges of empty electronic bands located above the vacuum level of PTCDA.

A total current spectroscopy study of metal oxide surfaces: I. Unoccupied electronic states of ZnO and MgO

Unoccupied density of states (DOS) for the polar (0001) Zn, (000) O and non-polar (100) ZnO surfaces and for a MgO(001)-c(2 × 2) thin film were studied using total current spectroscopy (TCS). TC spectra of the ZnO surfaces reveal the Zn 4s-O 2p band (6.2 eV) and two DOS maxima in the Zn 4p-O 2p band which exhibited different energy positions on the Zn-terminated (8.1 eV) and O-terminated (11.0 eV) surfaces. In the TC spectra of the MgO thin film, the energy location of Mg-derived (8.0, 14.8 and 21.0 eV) and O-derived (11.6, 14.2 and 18.5 eV) maxima of DOS were determined. Previously reported data on DOS of these materials is discussed, and it is shown that additional detailed information on the energy structure of unoccupied DOS in ZnO and MgO in the 5-25 eV range above EF has been found.

Oxidation of ultrathin copper layers on zinc oxide polar surfaces: unoccupied electronic states

Total current spectroscopy (TCS) was used to study an evolution of unoccupied states through the process of thin-layer copper deposition and its oxidation on polar ZnO(0001) and surfaces. Zn-derived and O-derived unoccupied bands were identified in the TC spectrum at 7.7 and 9.8 eV, respectively, from the clean and well ordered polar ZnO faces, while the spectra from a thin CuO layer reveal an intensive fine structure in the 6 - 25 eV energy range above the Fermi level, which is discussed in comparison with previously reported results of experimental and theoretical investigations. The obtained data provide additional information on the energy location of the edges of unoccupied bands in CuO in the 6 - 25 eV range relative to .