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May 23, 2013 - Ultrathin films of trioligo(phenylene-vinylene) and of ... above EF a significant difference between DOUS of the two films studied was ...
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Unoccupied Electronic States at the Interface of Oligo(phenylenevinylene) Films with Oxidized Silicon A. S. Komolov,* E. F. Lazneva, S. N. Akhremtchik, N. S. Chepilko, and A. A. Gavrikov Physics Faculty, St. Petersburg State University, Uljanovskaja ul.1, St. Petersburg, Russia 198504 ABSTRACT: Ultrathin films of trioligo(phenylene-vinylene) and of dinitro-substituted trioligo(phenylene-vinylene) end terminated by dibutyl-thiole were thermally deposited in an ultrahigh vacuum (UHV) on an oxidized silicon substrate (SiO2)n-Si. The surface work function and the density of the unoccupied electron states (DOUS) located 5−20 eV above the Fermi level (EF) were studied during the film deposition using the very low energy electron diffraction (VLEED) method and the total current spectroscopy (TCS) measurement scheme. The electronic structure typical for the organic films studied was formed, while the organic deposit thickness increased up to 5−6 nm. The DOUS peak structure of the organic films studied obtained from the TCS results showed good correspondence to the DOUS peaks obtained from the density functional theory (DFT) calculations. The calculations were made for the model oligo(phenylene-vinylene) films where the neighboring molecules had relatively weak interaction, which is also characteristic of the disordered films studied experimentally. The comparison of the DOUS spectra of the unsubstituted film and of the dinitro-substituted film showed that the substitution had a minor effect on the DOUS for the energies higher than 10 eV above EF, while in the energy range from 5 to 10 eV above EF a significant difference between DOUS of the two films studied was observed. The 0.3 and 0.7 eV increase of the surface work function values was observed during the formation of the unsubstituted trioligo(phenylene-vinylene) and of the dinitro-substituted trioligo(phenylene-vinylene) deposits on the substrate surface, respectively. The interfacial charge transfer was characterized by the formation of the 2 nm thick polarization layer in the organic films and by the negative charge transfer to the organic layer from the (SiO2)n-Si substrate.

1. INTRODUCTION Thin and ultrathin films of phenylene-vinylene oligomers (OPVs) and their heterojunctions with metals and inorganic semiconductors have shown interesting electronic properties which can be used in organic electronics device applications as well as in single-molecule devices.1−5 The surface science techniques were used successfully to determine the formation of the interface dipole layers and chemical interaction at a number of the OPV film interfaces with metal and semiconductor surfaces.6−8 The electronic structure of the organic layers can be tuned by introducing polar substituents into the OPV molecules.9−12 The electron-withdrawing effect upon the substitution by fluorine- or nitrogen-containing molecular groups would lead to stabilization of the energy positions of the edges of the forbidden energy gap and to narrowing of the bandgap.11,12 The electron acceptor substitution of the conjugated rings affects the whole peak structure of the density of the valence and of the unoccupied electronic states (DOS and DOUS, respectively).13,14 It has been reported that the π* and low-lying σ* DOUS maxima were restructured and shifted 0.5−1.5 eV toward lower electron energies upon fluorination of benzene.13 Studies of the unoccupied electronic states can provide information about the interface formation complementary to the information on valence electronic states, obtained traditionally by photoelectron spectroscopy. A large amount of data on DOUS of organic molecules obtained by © XXXX American Chemical Society

means of X-ray absorption spectroscopy (NEXAFS) have been collected.15−17 Studies of DOUS by means of NEXAFS have certain limitations with respect to smaller organic molecules.17 Inverse photoemission spectroscopy and a number of techniques for energy structure calculations have also been used for analysis of DOUS of organic films.16,18,19 However, inverse photoemission studies and some of the theoretical calculations often do not concern the electron energy higher than 2−3 eV above the vacuum level. The electronic structure of the unoccupied states can be obtained by monitoring secondary electrons backscattered from the sample surface using very low-energy electron diffraction (VLEED) using the total current spectroscopy (TCS) as a measurement scheme.20 TCS has been successfully used in studies of band alignment at interfaces, and a direct correspondence between TCS results and DOUS in the energy range 5−20 eV above the EF has been shown for organic films, metal oxide films, and some other materials.20−25 The electron attachment spectroscopy provides an analogous method for testing the vacant energy bands of the small organic molecules of the gas phase.25−28 The thiole end-capping of the OPV(1) and OPV(2) molecules under present study (Figure 1) was preserved for Received: March 19, 2013 Revised: May 15, 2013

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−d2J(E)/dE2 = −dS(E)/dE, as its peaks have in many cases been shown to represent the DOUS peaks.18,19,21 To prepare the (SiO2)n-Si substrates, the silicon wafers were pretreated in HF and in H2O2/H2SO4 solutions and then were placed into the experiment chamber. After such short exposure to air one should expect that a 3−4 nm thick silicon oxide is present on the surface. The purity of the substrate was tested by Auger electron spectroscopy, and the ratio between the silicon and oxygen atoms was determined as 1:1.8, which corresponds to a small deviation from a stoichiometric SiO2. Trioligo(phenylene-vinylene) end terminated by dibutyl-thiole (OPV(1)) and dinitro-substituted trioligo(phenylene-vinylene) end terminated by dibutyl-thiole (OPV(2)) (Figure 1) have been synthesized and characterized as described in refs 3−5. These molecules were used for the film deposition after a few hours of baking out at 100 °C in UHV. The films were thermally deposited in situ from a Knudsen cell on the surfaces of the substrates kept at room temperature. The deposition rate was kept at 0.1 nm/min, and the thickness of the deposit was controlled using a quartz microbalance. The LEED measurements during the film deposition did not show any distinct pattern, which suggested that disordered films were formed. Calculations of the electronic structure of the OPV films were performed using a density functional theory (DFT), generalized gradient approximation (GGA), and linearized augmented plane waves (LAPW) basis set as is implemented in the program code WIEN2k.32 Knowing that the molecular films under study were disordered and were composed from relatively small molecules, one might expect an essentially weak energy dependence on the wave-vector k, so a few k-mesh points were sufficient for the calculation. Analogous results were obtained when 4 and 25 k-mesh points were used. DOS and DOUS were calculated by integrating the solutions of the Kohn−Sham equations using the modified tetrahedron method.33 Convolution of the DOS and DOUS calculated with a Gaussian function which had 0.75 eV full width at halfmaximum was used to suit it for comparison with the experimental results. WIEN2k calculations can be applied to infinite periodic crystal structures. To suit this calculation method for the disordered OPV films under study, the authors constructed the model films from the OPV molecules separated far enough from each other to neglect the intermolecular interaction, though positioned periodically. Within an OPV molecule, the 1.39, 1.35, 1.50, and 1.08 Å bond-length values were used for aromatic C bonds, double CC bonds, single C−C bonds, and C−H bonds, respectively, as they are known from the literature.15,18,34 The model film was constructed by translations along the direction normal to the molecular plane and the two perpendicular directions within the molecular plane.

Figure 1. Chemical structure of the molecules used in the study. Trioligo(phenylene-vinylene) end terminated by dibutyl-thiole (OPV(1)) and dinitro-substituted trioligo(phenylene-vinylene) end terminated by dibutyl-thiole (OPV(2)).

covalent bonding to gold surfaces to be used in single-molecule transport devices.5,6 It has been shown that −NO2 substitution changes the electron tunnelling probability through the monolayer of OPV(2) by a factor of 5 compared to the case of the OPV(1) monolayer. The authors have earlier studied the interface formation of the OPV(1) films with gold and with highly ordered pyrolytic graphite and with germanium surfaces.20 It was found out that besides the interface dipole phenomena a fragmentation of the OPV(1) molecules occurred within the OPV(1) film layer up to 3 nm near the substrate surface. Oxidized silicon surfaces are of technological interest. Mechanisms of the interface formation between oxidized silicon and an organic film may involve interfacial charge transfer and chemical interaction, and they are dependent on particular features of the interfacing layers.7,21,29−31 In this paper, the authors present the results of the TCS studies of the OPV(1)/ (SiO2)n-Si and OPV(2)/(SiO2)n-Si interfaces and of the DOUS of the OPV films under study. The DOUS obtained on the basis of the TCS experiments and the DOUS obtained for the model OPV films using density functional calculations are compared.

2. EXPERIMENTAL SECTION The experiment was performed in an UHV system at a base pressure of 10−7 Pa. The LEED unit installed was used as a main instrument for the measurements according to the TCS method described in detail in ref 17−19. A result of a TCS measurementTCS spectrumconsists of a primary TCS peak and a TCS fine structure. The energy position of the primary TCS peak is determined by the surface potential of the surface under study. The energy scale of the measurement system was calibrated using a Au film freshly deposited at pressure lower than 10−8 Pa, which is known to have a 5.2 eV work function.26 The Fermi energy (EF) in the organic film and in the silicon oxide layer is assumed to be a continuation of EF in the conducting substrate. The TCS fine structure S(E) is located in the energy interval 0−25 eV above the vacuum level and is determined by the energy dependence of the elastic scattering of the incident electrons from the sample surface, which is closely related to the density of the unoccupied electron states (DOUS) of the sample surface. DOUS analysis is usually carried out using the negative second derivative

3. RESULTS AND DISCUSSION The structure of the unoccupied electronic states and the surface potential were monitored by means of the measurement of the series of the TCS spectra during the OPV(1) and OPV(2) film deposition process onto the (SiO2)n-Si substrate (Figure 2). The organic deposit thickness changed from 0 to 6 nm. The TCS fine structure of the (SiO2)n-Si substrate that we measured at 0 nm film coverage corresponds well to the results of the earlier TCS studies.21 During the organic film deposition the peak structure from the substrate was attenuated, and the new TCS peak structure appeared (Figure 2). The TCS fine structure obtained from the OPV(1) films at 6 nm coverage has B

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Figure 2. TCS fine structure S(E) during the process of the (a) OPV(1) deposition and (b) OPV(2) deposition on the (SiO2)n-Si surface. Numbers at the curves point to the corresponding thickness in nanometers of the organic deposit.

Figure 3. (a) DOUS of the OPV(1) film (curve 1) and of the OPV(2) film (curve 2) based on the TCS results obtained for the OPV(1) and OPV(2) films of 6 nm thickness. In curve 1 the low-energy peak (5.5 eV) is scaled 0.3. (b) DOUS of the model OPV(1) film (curve 1) and of the OPV(2) film (curve 2) obtained by means of the DFT calculations.

main peaks V1 (5.5 eV), V2 (7.5 eV), V3 (11.5 eV), and V4 (17 eV) (Figure 2a), and they are almost identical to the peaks of the TCS fine structure obtained for the OPV(1) films on a number of other substrates.20 The TCS fine structure obtained from the OPV(2) films had main peaks N1 (6.5 eV), N2 (8 eV), N3 (12 eV), and N4 (16.5 eV) (Figure 2a). The TCS fine structures obtained at 6 nm coverage remained with a further increase of the deposit thickness up to 10−15 nm until the charging of films by the incident electrons started to affect the measurement. Considering the broadness of the peaks at the energies above 10 eV (Figure 2), the peaks N3 and N4 of the OPV(2) film correspond well to the peaks V3 and V4 of the OPV(1) film, which indicates that in this energy region the −NO 2 substitution had a little effect on the TCS fine structure. In the energy range from 5 to 10 eV above EF, a more significant difference was observed between the TCS fine structure of the two films under study (Figure 2). The peaks N1 and N2 of the OPV(2) film are shifted 0.5−1 eV toward higher electron energies compared to the peaks V1 and V2 of the OPV(1) film. To perform the DOUS analysis of the films studied, a negative derivative of the TCS fine structure of the 6 nm thick OPV films (Figure 2) was used, according to the relevant part of the discussion presented in Section 2. The DOUS curves in the form of −dS(E)/dE are presented in Figure 3a. The DOUS peak assignment of the OPV(1) and OPV(2) films may be made with a reference to the DOUS of condensed benzene and its derivatives,15,18 as if one may expect an essential contribution of the aromatic ring into the electron structure of the whole molecule. The DOUS peaks of the OPV(1) and OPV(2) films at 12 and at 18 eV (Figure 3a) have positions analogous to σ*(C−C) and σ*(CC) peaks of the unoccupied bands of condensed benzene.15,18 The energy region below 10 eV in Figure 3a corresponds to π* electronic bands.15 One can also observe that the low-lying DOUS peak at 6 eV from the OPV(1) film has a much larger intensity compared to the other peaks (Figure 3a, curves 1), which can be considered as a typical feature of the OPV(1) film under study. The DOUS of the OPV films was also obtained using the quantum-chemical calculations. The model periodic structures were chosen so that the separation distance between the

aromatic molecular planes was 4 Å and the distance between the closest carbon atoms of the adjacent molecules was 4.5 Å. The choice of the separation distance can be explained using the following. Interatomic interaction can be characterized by the transfer integrals which can be considered proportional to inverse square of the distance between the centers of the atomic orbitals.35 Using this proportionality, one may obtain that the interaction of electrons on orbitals with centers spaced 4 Å is 7 times weaker than the one of those separated by a typical intramolecular distance of 1.5 Å. Values of intermolecular distance 3.0−3.5 Å are known for a number of conjugated molecular films.36−39 It was also shown that when the distance between a couple of small conjugated molecules stacked parallel is increased from 3.3 to 4.0 Å the energy of the intermolecular interaction is decreased approximately 5 times.38,39 The model OPV structures were therefore assumed to have electronic band structure similar to the disordered OPV(1) and OPV(2) films studied experimentally. Let us compare the calculated DOUS curves of the OPV(1) and OPV(2) films (Figure 3b, curves 1 and 2) and the DOUS of these films obtained experimentally (Figure 3a, curves 1 and 2). The two broader σ* peaks in the energy region above 10 eV are shifted 1−2 eV toward lower energies in the calculated spectra (Figure 3b) compared to the experimental ones (Figure 3a). The authors attribute this shift to the fact that DFT calculations are known to underestimate the higher DOUS peak energies.27,28 In the energy range from 5 to 10 eV a good correspondence between the calculated and the experimental DOUS of the OPV(1) films (Figure 3a and b, curves 1) and of the OPV(2) films (Figure 3a and b, curves 2) was observed. The calculated DOUS (Figure 3b) demonstrate also the bands at the energies below 5 eV. The experimental DOUS analysis in this energy region using the TCS technique is limited because the Evac is located at 4.3 eV (4.7 eV) in the case of the OPV(1) (OPV(2)) films studied, respectively. As a result one can observe that −NO2 substitution had a minor effect on the DOUS for the energies higher than 10 eV above EF, while in the in the energy range from 5 to 10 eV above EF a significant difference between DOUS of the OPV(1) and OPV(2) films was observed. To further analyze the evolution of the TCS spectra at the interfaces studied (Figure 2), we plotted the intensity (I) of the TCS peaks originating from the SiO2 substrate and from the OPV(1) and OPV(2) films as a function of the organic deposit C

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molecular deposit interfacing the (SiO2)n-Si surface there is a valuable deviation of the charge transfer features including the molecular polarization depending on the particular site, to which the deposit molecule is attached. Such deviation in the molecular energetic state might result in smearing of the spectral features out.20,29 The changes of the Evac level position during the OPV(1) and OPV(2) film deposition were monitored by the changes of the energy positions of the primary TCS peak (Figure 4b and 4c). The value of the Evac −E F of the (SiO2)n-Si substrate surface was measured as 4.0 ± 0.1 eV, which is typical for this surface.21,40 Along with the deposition of the OPV(1) film, the Evac − EF values changed monotonically until they reached 4.3 ± 0.1 eV as is illustrated in Figure 3b. Upon the OPV(2) deposition the Evac − EF values increased to 4.7 ± 0.1 eV. The increase of the work function corresponds to the negative charge transfer to the organic layer from the substrate. The major changes of the Evac − EF values occur within 2 nm of the tOPV deposit. According to the discussion in refs 20−22 these changes of the surface potential were determined by the changes of the surface work function due to the change of the contents of the surface layer along with the film deposition. The −NO2 substituted OPV(2) film demonstrated a larger work function value than the OPV(1) film, which corresponds well to the results of the work function changes upon the introduction of polar substituents to the small organic molecules.13

thickness (Figure 4a). An estimate of the SiO2 signal intensity can be obtained from the height of the peak located at 13 eV

Figure 4. Analysis of the OPV(1)/(SiO2)n-Si and OPV(2)/(SiO2)n-Si interface formation. (a) Decrease of the intensity (I) of the TCS fine structure from the (SiO2)n-Si substrate (curve 1) and the increase of the intensity of the TCS fine structure from the OPV(1) film (curve 2) and from the OPV(2) film (curve 3). (b) Changes of the surface work function during the OPV(1)/(SiO2)n-Si interface formation. (c) Changes of the surface work function during the OPV(2)/(SiO2)n-Si interface formation.

4. CONCLUSIONS The modification of the structure of the unoccupied electronic states and upon deposition of the ultrathin OPV(1) and OPV(2) films onto the (SiO2)n-Si surface was determined using the incident beam of low-energy electrons according to the TCS method. The electronic structure typical for the organic films studied was formed, while the organic deposit thickness increased up to 5−6 nm. In the case of the OPV(2)/ (SiO2)n-Si interface and when the OPV(2) deposit thickness was below 1 nm, an intermediate layer was formed, which was characterized by a featureless peak-free DOUS structure. DOUS spectra of the OPV(1) film and of the −NO2substituted OPV(2) film obtained using the TCS results were compared. It was shown that the −NO2 substitution had a minor effect on the DOUS for the energies higher than 10 eV above EF, while in the energy range from 5 to 10 eV above EF a significant difference between DOUS of the OPV(1) and OPV(2) films was observed. This observation is supported by the DFT-calculated DOUS spectra of the model OPV film structures. The 0.3 and 0.7 eV increase of the surface work function values was observed during the OPV(1) and OPV(2) deposition, which corresponds to the negative charge transfer to the organic layer from the (SiO2)n-Si substrate.

relative to the minimum located at 18 eV (Figure 2). The SiO2 substrate signal intensity was attenuated exponentially upon the deposit growth (curve 1 in Figure 4a), which corresponds to formation of the uniform organic coverage.20,22 The intensity (I) of the TCS peaks of the organic films can be traced using the intensity of the peak V2 of the TCS fine structure from the OPV(1) film and of the peak N2 in the case of the OPV(2) film (curves 2 and 3 in Figure 4a, respectively). The OPV(1) signal started to appear at the earlier stages of the deposition process when the deposit thickness was under 0.5 nm, and it reached saturation at 6−7 nm film thickness (curve 2, Figure 4a). This observation suggests that the influence of the (SiO2)n-Si substrate surface under study does not change the unoccupied electronic structure of the OPV(1) deposit, which corresponds well to the results of the photoelectron spectroscopy studies of another oligo(phenylene-vinylene) film interfacing SiO2 layer.40 An analogous result has been earlier reported for the interface of Cu-phthalocyanine films on the (SiO2)n-Si substrate.21 We should note that the intactness of the OPV(1) molecules has not been the case for the interfaces with Ge(111) and the polycrystalline Au surfaces.20 In those cases the influence of the substrate surface led to formation of the TCS peaks different from peaks V1−V4 within a 2−3 nm thick intermediate OPV(1) layer. The TCS fine structure typical for the OPV(2) film started to appear after the deposit layer of approximately 1 nm was formed when the substrate signal was already substantially attenuated (curves 3 and 1 in Figure 4a). This observation may correspond to the formation of the intermediate OPV(2) layer characterized by a featureless peak-free TCS fine structure as we have observed for the interfaces of small conjugated molecular films with the tin dioxide surface.23 It may be suggested that in the case of the −NO2 substituted OPV(2)

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+7 812) 428 45 39. Fax: (+7 812) 428 72 40. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Russian Foundation for Basic Research (11-03-0533a). The authors thank Dr. N. StuhrD

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Hansen and Prof. T. Bjørnholm from the Nano Science Centre of the University of Copenhagen for providing the OPV material.

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