Незаполненные электронные состояния и потенциальный барьер в пленках замещенных дифенилфталидов на поверхности высокоупорядоченного пиролитического графита

The results of a study of the unoccupied electronic states of ultrathin films of bis-carboxyphenyl-phthalide (DCA-DPP) and bis-methylphenyl-phthalide (DM-DPP) up to 8 nm thick are presented. The studies were carried out by total current spectroscopy (TCS) technique in the energy range from 5 eV to 20 eV above EF during thermal vacuum deposition of these organic films on the surface of highly oriented pyrolytic graphite (HOPG). The energy Evac relative to EF, that is, the electronic work function of the DM-DPP films, at a film thickness of 5–8 nm was 4.3 ± 0.1 eV. The electronic work function of the DCA-DPP films was 3.7 ± 0.1 eV. The structure of the maxima of the unoccupied electronic states of DCA-DPP films and DM-DPP films in the studied energy range is determined. The properties determined of DCA-DPP and DM-DPP films are compared with the properties of films of unsubstituted diphenylphthalide (DPP). According to our analysis, –CH3 substitution of the DPP molecule practically did not affect the height of the potential barrier between the film and the HOPG surface, and –COOH substitution of the DPP molecule led to an increase in the height of the potential barrier between the film and the HOPG substrate surface by 0.5–0.6 eV. Substitution of DPP molecules with –COOH functional groups which represents formation of DCA-DPP molecules led to a shift of two peaks of the experimental total current spectra located at energies in the range from 5 eV to 8 eV above EF, by about 1 eV towards lower electron energies.

Незаполненные электронные состояния ультратонких пленок тиофен-фенилен со-олигомеров на поверхности поликристаллического золота

The results of studying unoccupied electronic states in the energy range from 5 eV to 20 eV above the Fermi level of ultrathin films of dimethyl-substituted thiophene-phenylene co-oligomers CH3-phenylene-thiophene-thiophene-phenylene-CH3 (CH3-PTTP-CH3) are presented. The films were deposited on two types of surface of polycrystalline Au: ex situ Au layer thermally deposited in a separate chamber, and on the in situ Au surface prepared inside the analytical chamber. The structure of the films was studied by X-ray diffraction (XRD). The formation of a superposition of the amorphous and crystalline phases with a period of 3.8 nm is discussed. Investigations of the energy positioning of the maxima of unoccupied electronic states and of the process of the formation of the potential boundary barrier were carried out by the method of total current spectroscopy (TCS). The maxima of the fine structure of the total current spectra (FSTCS) of the CH3-PTTP-CH3 film 5–7 nm thick did not differ when using different types of Au substrates and the surface of the ZnO semiconductor prepared by the atomic layer deposition (ALD) method. When the CH3-PTTP-CH3 layer was deposited both on the ex situ Au surface and on the in situ Au surface, a slight (about 0.1 eV) increase in the electronic work function was observed with an increase in the coating thickness to 5–7 nm. At such CH3-PTTP-CH3 film thicknesses, the electron work function was determined as 4.7 ± 0.1 eV for the ex situ Au substrate and 4.9 ± 0.1 eV for the in situ Au substrate. The possible influence of the processes of physicochemical interaction at the film – substrate interface on the formation of the potential boundary barrier in the structures under study is discussed.

Квантовая теория эмиссии электронов из структуры "металл-диэлектрик" в сильных электрических полях

A generalized formula is derived for the electron emission current in relation to the temperature, the electric field, and the electronic work function for a “metal–dielectric” system. The formula takes into account the quantum nature of the image forces. In deriving it, the Fermi–Dirac distribution and the quantum image potential obtained in terms of the electron–polaron theory are used. In the limit of the classical potential of image forces, the well-known Richardson–Schottky and Fowler–Nordheim formulas are obtained for thermionic emission and field emission, respectively. It is shown that at high temperatures and electric fields E ≥ 10 MV/cm, the polaron contribution to the electron emission current increases with increasing field and decreases with increasing temperature. The decrease in current is related to an increase in effective electronic work function due to the electron-polaron effect. Extrapolation formulas convenient to obtain theoretical estimates are derived for the thermionic and the field emission current.

Determination of Contact Potential Difference by the Kelvin Probe (Part II) 2. Measurement System by Involving the Composite Bucking Voltage

The present research is devoted to creation of a new low-cost miniaturised measurement system for determination of potential difference in real time and with high measurement resolution. Furthermore, using the electrode of the reference probe, Kelvin method leads to both an indirect measurement of electronic work function or contact potential of the sample and measurement of a surface potential for insulator type samples. The bucking voltage in this system is composite and comprises a periodically variable component. The necessary steps for development of signal processing and tracking are described in detail.

Determination of Contact Potential Difference by the Kelvin Probe (Part I) I. Basic Principles of Measurements

Determination of electric potential difference using the Kelvin probe, i.e. vibrating capacitor technique, is one of the most sensitive measuring procedures in surface physics. Periodic modulation of distance between electrodes leads to changes in capacitance, thereby causing current to flow through the external circuit. The procedure of contactless, non-destructive determination of contact potential difference between an electrically conductive vibrating reference electrode and an electrically conductive sample is based on precise control measurement of Kelvin current flowing through a capacitor. The present research is devoted to creation of a new low-cost miniaturised measurement system to determine potential difference in real time and at high measurement resolution. Furthermore, using the electrode of a reference probe, the Kelvin method leads to both the indirect measurement of an electronic work function, or a contact potential of sample, and of a surface potential for insulator type samples. In the article, the first part of the research, i.e., the basic principles and prerequisites for establishment of such a measurement system are considered.