A comparative study has been performed for the structures of the maxima of the density of unoccupied electronic states (DOUS) in ultrathin films of the naphthalene-1,4,5,8-tetracarboxyl acid dianhydride (NTCDA) and naphthalene-1,8-dicarboxyl acid anhydride (NDCA) and two types of films based on phthalides: 3,3-diphenylphthalide (DPP) and 3,3-diphenylphthalide-4',4'-dicarboxyl acid (DPP–DCA). The structures of unoccupied electronic states of the 8–10-nm-thick films have been measured by total current spectroscopy (TCS) in the energy range from 5 to 20 eV above the Fermi level. The experimental results are analyzed by constructing the model total current spectra and the DOUS dependences using the results of calculation of the orbital energy of the molecules under study by the density functional theory (DFT) at the B3LYP/6-31G(d) level. The difference in the DOUS spectra of the NTCDA and NDCA films is the shift of the main DOUS maxima of the NTCDA film to lower energies approximately by 1 eV at energies lower than 12.5 eV; at higher energies, the DOUS maxima are shifted by 1.5–2 eV. The energy positions of the maxima of the total current spectra of the DPP–DCA and DPP films are changed only slightly as various substrates are used: highly ordered pyrolitic graphite (HOPG) and a CdS deposited layer-by-layer. The relative intensities of the maxima are different as various substrates are used. The characteristic shifts of the maxima of the total current spectra of the DPP–DCA films are approximately 1 eV at energies lower 12.5 eV above the Fermi level and 1.5–2 eV at higher energies, as compared to the positions of the corresponding maxima of the DPP films.

The results on comparing the peak structure of the density of unoccupied electronic states (DOUS) of ultrathin films of naphthalene anhydride-1,4,5,8-teracabonic acid (NTCDA) and naphthalene-1,8-dicarboxylic acid anhydride (NDCA) and of two types of phthalide-based films: 3,3-bis (phenyl) phthalide (DPP) and 3,3-bis (phenyl) phthalide-4 ′, 4′-dicarboxylic acid (DPP-DCA) are presented. The measurements of the structure of the unoccupied electronic states in the energy range from 5 eV to 20 eV above the Fermi level of the films studied having thickness of 8–10 nm were conducted using the total current spectroscopy (TCS) technique. Analysis of the experimental results was conducted using the model total current spectra and DOUS dependences generated using the calculated orbital energies of the studied molecules by means of the density functional theory (DFT) method at the B3LYP/6-31G(d) level. The difference in the DOUS spectra of NTCDA and NDCA films is characterized by the shift of the main DOUS maxima of the NTCDA film to lower energies by about 1 eV at energies less than 12.5 eV, and at higher energies the DOUS maxima are shifted by 1.5-2 eV. The energy positions of the maxima of the total current spectra of the DPP-DCA and DPP films practically do not change when using various substrates: highly ordered pyrolytic graphite (HOPG) and layer-by-layer deposited CdS. The relative intensities of the maxima differ when using different substrates. The characteristic shift of the maxima of the total current spectra of DPP-DCA films is about 1 eV at energies less than 12.5 eV above the Fermi level and 1.5-2 eV and at higher energies, compared with the position of the corresponding maxima of the DPP films.

AbstractThe 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 .

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 .

Приведены результаты исследования плотности незаполненных электронных состояний в энергетическом диапазоне от 5 до 20 eV выше энергии Ферми (EF) в сверхтонких пленках диоктил-замещенного и дифенил-замещенного перилен-дикарбоксимида. Экспериментальные результаты получены путем регистрации тока вторичных низкоэнергетических электронов с использованием методики спектроскопии полного тока. Теоретический анализ включал в себя расчет энергий и пространственного распределения орбиталей исследованных молекул методом теории функционала плотности и последующее масштабирование рассчитанных значений энергий орбиталей согласно процедуре, хорошо зарекомендовавшей себя ранее при исследованиях малых сопряженных органических молекул. Обнаружено, что для обоих видов исследованных пленок при энергиях ниже 8 eV над EF расположены по два основных максимума плотности незаполненных электронных состояний, образованных преимущественно pi*-орбиталями молекул. Более высоколежащие максимумы имеют преимущественно sigma*характер. Проведен анализ влияния диоктил- и дифенил-замещающих групп на плотность незаполненных электронных состояний при сравнении результатов для исследованных видов пленок. В случае pi*-максимумов наблюдается относительный сдвиг, около 1 eV, по энергии. В области sigma*-электронных состояний наблюдается незначительная перестройка структуры максимумов. Работа выполнена при поддержке научного гранта СПбГУ 11.38.219.2014, РФФИ (14-03-00087 и 15-29-05786). DOI: 10.21883/FTT.2017.02.44068.142

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 dioctyl-substituted perylenedicarboximide (PTCDI-C 8 ) and diphenyl-substituted perylenedicarboximide (PTCDI-Ph) ultrathin films have been presented. The experimental results have been obtained from measurements of the secondary low-energy electron current with the use of the total current spectroscopy (TCS) technique. A theoretical analysis has been performed, including the density functional theory calculation of the energies and spatial distribution of the orbitals of the molecules under investigation and the subsequent scaling of the calculated orbital energies according to the procedure well-proven previously in studies of small conjugated organic molecules. It has been found that, for each of the two types of the studied films, at energies below 8 eV above the Fermi level E F , there are two main maxima of the density of unoccupied electronic states predominantly formed by the π*-orbitals of the molecules. The higher-lying maxima have essentially a σ*-character. The influence of dioctyl- and diphenyl-substituent groups on the density of unoccupied electronic states has been analyzed in comparison with the results obtained for the studied types of films. In the case of the π*-maxima, the relative shift has been observed at an energy of approximately 1 eV. In the region of σ*-electronic states, there is a small transformation of the structure of the maxima.

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.

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 ((GeO 2 )Ge). The concentration of oxide on the (GeO 2 )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 (GeO 2 )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 (GeO 2 )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.

The interface chemical reactions, the deoxygenation and the overall thermal stability of the ex situ deposited graphene oxide (GO) thick film on the polycrystalline tantalum surface were studied by means of x-ray photoelectron spectroscopy (XPS) at room temperature and after annealing up to 600°С. The evolution of the Ta4f, C1s and O1s XPS core level spectra from the Ta/GO structure under study upon the increase of the annealing temperature was investigated with the reference to the uncovered Ta substrate surface. In the Ta/GO structure the Ta relative atomic concentration was about 1% at room temperature due to the low porosity of the GO layer while the thermal desorption of the overlayer at the elevated temperatures was accompanied by an increase of the Ta concentration up to 47% at 500°С. The C atoms relative concentration started decreasing after the annealing at 250°С accompanying the thermal desorption of the GO overlayer. The formation of the TaC compound starting from the 250°С annealing temperatures was determined. The relative oxygen concentration in the GO thick film was decreased upon increasing the annealing temperature accompanying both the thermal desorption of the GO material and the thermal reduction of the GO film.

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-C 60 ) and fullerene (C 60 ) films on the surface of the real germanium oxide ((GeO 2 )Ge) have been presented. The content of the oxide on the (GeO 2 )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-C 60 and C 60 coatings to 7 nm have been investigated. A comparison of the structures of the FSTCS maxima for the C 60 and APP-C 60 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-C 60 and unsubstituted C 60 films. The formation of the interfacial potential barrier upon deposition of APP-C 60 and C 60 on the (GeO 2 )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-C 60 and C 60 , the work function of the surface changes only slightly.

The ultrathin films of aziridinylphenylpyrrol-C-50 (APP-C-60) and of the unsubstituted C-60 thermally deposited in UHV on an oxidized silicon substrate (SiO2)n-Si were studied experimentally using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The density of the unoccupied electronic states (DOUS) located 2-20 eV above the Fermi level (E-F) of the films under study was determined using the TCS results and using the results of the density functional theory (DFT) calculations of the vacant electronic orbitals of the APP-C-60 and C-60 molecules. The DOUS peak structure obtained on the basis of the calculation results corresponds well to the structure of the major DOUS bands obtained from the TCS experiment. The comparison of the DOUS spectra of the APP-C-60 and C-60 films under study showed that the introduction of the APP substituent to the C-60 molecule was accompanied by the pronounced changes of the pi* DOUS bands in the energy range from 2 eV to 5 eV above E-F and of the DOUS band composed from both pi* and sigma* - type orbitals in the energy range from 5 eV to 9 eV above E-F. The formation of the low-lying sigma* DOUS band in the APP-C-60 film in the energy range from 8 eV to 13 eV above E-F can be explained by the superposition of the relevant DUDS maxima from the C-60 film and from the APP fragment. (C) 2015 Elsevier B.V. All rights reserved.

The results of the investigation of the conduction band electronic structure and the interfacial potential barrier during deposition of ultrathin dicarboximide-substituted perylene films (PTCBI-C 8 ) on the oxidized silicon surface have been presented. The measurements have been performed using the very low energy electron diffraction (VLEED) technique implemented in the total current spectroscopy (TCS) mode with a variation in the incident electron energy from 0 to 25 eV. Changes in the intensities of the maxima from the deposited PTCBI-C 8 film and from the substrate with an increase in the organic coating thickness to 7 nm have been analyzed using TCS measurements. A comparison of the structure of the maxima of PTCBI-C 8 and perylene-tetracarboxylic-dianhydride (PTCDA) films has made it possible to distinguish the energy range (8–13 eV above E F ) in which distinct differences in the structures of maxima for PTCDA and PTCBI-C 8 films are observed. This energy range corresponds to low-lying σ*-states of the conduction band of the films studied. The formation of the interfacial region of the PTCBI-C 8 film and (SiO 2 ) n -Si substrate is accompanied by an increase in the surface work function by 0.6 eV, which corresponds to the electron density charge transfer from the (SiO 2 ) n -Si substrate to the PTCBI-C 8 film.

The results of the investigation of the transmission of low-energy electrons through 8-nm-thick films of tin(IV) phthalocyanine oxide (SnOPc) on the surface of the oxidized silicon substrate are presented. The procedure of detecting the reflection of testing low-energy electron beam from the surface was implemented using the very low-energy electron diffraction (VLEED) technique in the total current spectroscopy (TCS) mode with a change in the incident electron beam energy from 0 to 25 eV. The structure of maxima in the total current spectra of SnOPc films was established and the variation in intensities of the maxima of total current spectra emitted from the deposited SnOPc film and (SiO 2 ) n -Si substrate during an increase in the thickness of the organic coating to 8 nm was analyzed. With such an increase in the thickness of the organic coating, the work function of the surface increases by 0.7 eV, which corresponds to the transfer of the electron density from the (SiO 2 ) n -Si substrate to the SnOPc film. Optical absorption spectra of SnOPc films were measured. The absorption spectra were compared with the spectra measured by the TCS method for the SnOPc films and the films of molecules of oxygen-free metal phthalocyanine.