S. A. Komolov

Total current spectroscopy

Abstract In total current spectroscopy (TCS) a beam of low energy (0–15 eV) electrons is directed upon a solid sample and secondary electron emission investigated by monitoring the current to the target, rather than by direct energy analysis of emitted electrons. A TCS signal is the derivative of the direct current with respect to the incident energy E1 displayed as a function of that energy, and obtained experimentally in ultra high vacuum conditions using simple modulation and lock in amplification techniques. A typical experimental arrangement is described and factors influencing a typical spectrum outlined. A simple background theory is discussed and it is shown how TCS is a powerful tool for studying not only the surface and near surface regions, but also bulk physics. In particular the technique has already found application in studies of work function, chemi- and physisorption, epitaxy, interband transitions, densities of states, excitons, impurity atom excitation, plasmons, radiation damage and the measurement of thicknesses of very thin surface layers.

Low-energy electron mean free path in thin films of copper phthalocyanine

The formation of thin organic films of copper phthalocyanine (CuPc) deposited onto the surface of gold-coated quartz crystal resonator was studied in situ under ultrahigh vacuum conditions by means of total electron-beam-induced current spectroscopy in combination with deposit thickness determination by piezocrystal microbalance technique. Variations in the fine structure of the total current spectra of CuPc layers of var-ious thicknesses in the 0–8 nm interval have been analyzed and the electron mean free path in thin CuPc films was determined as a function of the electron energy. For electron energies of 5.0, 7.2, 14.4, and 18.0 eV above the Fermi level, the mean free path is 6.4, 3.9, 2.6, and 2.3 nm.

Potential barrier and photovoltage at interfaces of hexadecafluoro-copper-phthalocyanine and copper phthalocyanine films on the surface of tin dioxide

Potential barrier formation during the deposition of ultrathin coatings of copper phthalocyanine (CuPc) and hexadecafluoro-copper-phthalocyanine (F16CuPc) on the surface of polycrystalline tin dioxide and during the deposition of F16CuPc coatings over a CuPc film is studied. A photoinduced change in the surface potential of the prepared structures upon exposure to light in the visible wavelength region is detected. The surface photovoltage of the studied organic films has a positive sign with respect to the substrate, its spectral dependences correspond to the absorption spectra of the organic materials CuPc and F16CuPc. Surface potential measurements are performed using a probe beam of low-energy electrons, based on the total current spectroscopy technique. A total decrease in the work function by 0.2 eV is detected during the deposition of a CuPc film up to 8 nm in thickness on a SnO2 substrate; in the case of the F16CuPc/SnO2 interface, an increase in the work function by 0.55 eV is detected. At the initial deposition stage, at organic film thicknesses of up to 1.5 nm, the interfacial potential barrier corresponded to electron density transfer from the organic film to the substrate in both cases of CuPc/SnO2 and F16CuPc/SnO2. It is assumed that the photoinduced change in the surface potential is caused by charge-carrier separation in a boundary region up to 1.5 nm thick.

Gas-sensor properties of composite semiconductor films of substituted perylene and tin dioxide nanoparticles

Electrical conductivity of film samples of a composite constituted by a perylene derivative (3,4,9,10-perylenetetracarboxylic-dianhydride) and SnO2 nanoparticles was studied in adsorption of vapors of ammonia, toluene, aqueous hydrogen peroxide, ethanol, and water. A model of formation of the composite sample under study and a mechanism by which adsorbed molecules affect its electrical conductivity are suggested.

Laser stimulated fragmentation and desorption from the surface of organic films: Perylene derivatives

The mass spectra of laser desorption from the surface of perylene-tetracarboxylic dianhydride (PTCDA) films deposited in vacuum onto indium arsenide (InAs) and gallium arsenide (GaAs) substrates have been studied. The desorption was induced by 10-ns pulses of neodymium laser radiation (quantum energy, 2.34 eV) with the energy density E varied within 0.5–20 mJ/cm2. It is established that laser radiation produces fragmentation of PTCDA molecules and desorption of the molecular fragments. The main fragments observed in the mass spectra are identified. Initially, carboxy and dianhydride groups are detached, decomposed, and desorbed in the form of CO, CO2, and a small amount of atomic oxygen species. An increase in the laser pulse power leads to the desorption of the perylene nucleus (M = 248 amu) and its halves (M = 124 amu). No evidence for laser-stimulated desorption of intact PTCDA molecules was observed. The possible mechanisms of photostimulated fragmentation of PTCDA molecules and the subsequent desorption of fragments are discussed.

Modification of electronic properties during adsorption of conjugate organic molecules on the surface of polycrystalline SnO2

The modification of the electronic structure during adsorption of ultrathin copper phthalocyanine (CuPc) and 3, 4, 9, 10 perylene-tetracarboxylic-dianhydride (PTCDA) coatings on the surface of polycrystalline tin dioxide is traced. Auger electron spectroscopy is employed to find changes in the atomic composition of the surface. It is found with the help of low-energy electron total current spectroscopy using a testing beam of electrons with energies up to 30 eV that the total current spectra typical of organic films are formed when the thickness of the coating being deposited is 2–7 nm. The formation of an interface layer 1.5–2.0 nm in thickness is detected, in which the intensity of the structure of the total current spectra decreases and the effect of interaction of PTCDA molecules with the SnO2 surface is manifested.

Laser-induced desorption of atomic and molecular fragments from a tin dioxide surface modified by a thin organic covering of copper phthalocyanine

The systematic features of laser-induced desorption from an SnO2 surface exposed to 10-ns pulsed neodymium laser radiation are studied at the photon energy 2.34 eV, in the range of pulse energy densities 1 to 50 mJ/cm2. As the threshold pulse energy 28 mJ/cm2 is achieved, molecular oxygen O2 is detected in the desorption mass spectra from the SnO2 surface; as the threshold pulse energy 42 mJ/cm2 is reached, tin Sn, and SnO and (SnO)2 particle desorption is observed. The laser desorption mass spectra from the SnO2 surface coated with an organic copper phthalocyanine (CuPc) film 50 nm thick are measured. It is shown that laser irradiation causes the fragmentation of CuPc molecules and the desorption of molecular fragments in the laser pulse energy density range 6 to 10 mJ/cm2. Along with the desorption of molecular fragments, a weak desorption signal of the substrate components O2, Sn, SnO, and (SnO)2 is observed in the same energy range. Desorption energy thresholds of substrate atomic components from the organic film surface are approximately five times lower than thresholds of their desorption from the atomically clean SnO2 surface, which indicates the diffusion of atomic components of the SnO2 substrate to the bulk of the deposited organic film.

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).

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.