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Core-Hole Screening, Electronic Structure, and Paramagnetic Character in Thin Films of Organic Radicals Deposited on SiO2/Si(111) Reza Kakavandi,† Sabine-Antonia Savu,† Lorenzo Sorace,‡ Donella Rovai,‡ Matteo Mannini,‡ and Maria Benedetta Casu*,† †

Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Baden-Württemberg, Germany ‡ Department of Chemistry “Ugo Schiff” and RU INSTM, University of Florence, Via della Lastruccia 3-13, I-50019 Sesto Fiorentino, Florence, Italy S Supporting Information *

ABSTRACT: We investigate thin films of a pyrene derivative of the nitronyl nitroxide radical deposited on SiO2/Si(111). By using X-ray photoelectron spectroscopy and near-edge X-ray absorption fine structure spectroscopy, we study the electronic structure of the system. From photon energy-dependent analysis, we suggest the occurrence of a shake-up transition consisting of a dipole excitation of a core electron into a valence orbital and a monopole ionization of a valence electron. The persistence of the paramagnetic character is explored by electron spin resonance spectroscopy.

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by studying both temperature and magnetic field dependence of the resonance. Grillo et al.28 investigated the adsorption of the tripara-carboxylic polychlorotriphenylmethyl radical on a Cu/Au(111) surface: the presence of copper favors the formation of metal−organic assemblies, which were analyzed using STM and high-resolution electron energy loss spectroscopy. They also used density functional theory methods to show that the unpaired electron survives after adsorption. As a complement to these techniques, electron spin resonance (ESR) spectroscopy can be used to verify the persistence of the paramagnetic character of nanostructures and thin films based on purely organic molecules and to evidence the presence of ordered phases.7,29−31 We have recently focused our attention on a pyrenederivative of the nitronyl nitroxide radical (4,4,5,5-tetramethyl2-(pyrenyl)-imidazoline-1-oxy-3-oxide, NitPyn, Figure 1), demonstrating its thermal stability upon OMBD32 and investigating its thin-film processes on technologically relevant substrates33−35 under controlled conditions in the nanoscale regime. In the present work, we use soft X-ray techniques such as Xray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to explore in situ the electronic structure and the paramagnetic character of

INTRODUCTION Stable metal-free magnetic compounds have been the focus of intense multidisciplinary efforts in view of their possible use in a number of applications ranging from medicine to material science and electronics.1−5 This requires advancements in the synthesis of new organic radicals1,6−10 with the final aim of exploiting the spin degree of freedom of the unpaired electrons in real devices.11−16 The preparation of thin films of these compounds can be based on their chemical functionalization with specific linkers promoting the formation of chemical bonds with the substrate.14,17 Alternatively, organic molecular beam deposition (OMBD) guarantees the higher level of purity of the ultra high vacuum (UHV)-based methods and a fine control of the properties of the deposited film.18−24 Several research groups have focused their attention on organic radicals deposited in UHV on Au(111) single crystals, investigating them by scanning tunnelling microscopy (STM). Work in this field is flourishing: a Kondo resonance has been observed for a stable neutral organic radical, 1,3,5-triphenyl-6-oxoverdazyl, adsorbed on an Au(111) surface because of the spin-electron interaction.25 Müllegger et al.26 reported on a low-temperature STM study of a new type of Kondo system based on metal-free stable hydrocarbon π− radicals, BDPA (α,γ-bisdiphenylene-βphenylallyl) that organize by self-assembly into different 1-D nanostructures on Au(111) surfaces while maintaining their spin-1/2 state. Zhang et al.27 investigated a radical nitronylnitroxide side group, adsorbed on Au(111) surfaces. They demonstrated that Kondo physics can be fully understood only © 2014 American Chemical Society

Received: February 7, 2014 Revised: March 14, 2014 Published: March 17, 2014 8044

dx.doi.org/10.1021/jp5013736 | J. Phys. Chem. C 2014, 118, 8044−8049

The Journal of Physical Chemistry C

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Figure 1. NitPyn molecular structure. (a−c) C1s core level spectra (together with their fits) of a 6 nm nominally thick film of NitPyn taken at different photon energies, as indicated. (d) A zoom into the higher binding energy range to evidence the shake-up satellite intensities at different photon energies is also shown.

thin films of NitPyn deposited on SiO2/Si(111). Ex situ ESR measurements demonstrate the persistence of the paramagnetic character of the molecules in the thin films, and they are used to evaluate their degree of organization.

normalization and molecular orientation calculations are given elsewhere.23,37 All necessary precautions were taken to avoid radiation damage, i.e., short beam exposure, short acquisition time, a freshly prepared film for each spectrum (see ref 32 for details on the pattern of XPS radiation damage in NitPyn). No degradation of the samples was observed on the time scale of all discussed in situ experiments. X-band EPR spectra were recorded using a Bruker E500 continuous-wave spectrometer equipped with a SHQ cavity and, for the surface deposited samples, a digital programmable goniometer (ER218PG1, Bruker BioSpin) to measure the angular dependence in steps of 2°. Care was taken to avoid saturating condition and to reduce the modulation amplitude to values where no distortion of the resonance line may occur. Resonance lines were fitted to Lorentzian line shape by the dedicated function of the XEPR (Bruker Biospin) software.

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EXPERIMENTAL METHODS NitPyn was synthesized according to the procedure reported in ref 32. Thin films of NitPyn were deposited in situ under UHV conditions by OMBD using a Knudsen cell (evaporation rate, 1 Å/min) and keeping the substrate at room temperature (RT). The evaporation rate was determined by a quartz microbalance. The surface was native SiO2, grown on single-side polished pSi(111) wafers with a doped resistivity of 5−10 Ω cm (borondoped). Without any previous ex situ treatment, the substrates were outgassed in UHV by several cycles of annealing at around 500 K (i.e., much below the temperature at which the oxide is removed) for several hours. Their cleanness was verified by XPS and NEXAFS. The experiments were carried out at the UE52-PGM undulator beamline at the synchrotron radiation facility BESSY II (Berlin). The end-station had separated preparation and analysis chambers (base pressure, 2 × 10−10 mbar) equipped with a SCIENTA R 4000 electron energy analyzer. The measurements were performed in low-alpha (ring current at injection, 13/20 mA (mode B/mode A); cff, 2.5; analyzer resolution, 0.1 eV). The XPS spectra were recorded for incident photon energies of 330, 640, and 1000 eV. The peak fit analysis was carried out with the Unifit package.36 The NEXAFS spectra were measured in total and partial electron yield, normalized by the clean substrate signals. All spectra were scaled in order to give an equal edge jump. Details on NEXAFS

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RESULTS AND DISCUSSION The thin-film XPS C1s core level spectra (together with their fitting curves) measured at different photon energies are shown in Figure 1. The spectra exhibit a main peak at 284.3 eV and a second feature at 286.1 eV, which are analyzed following the procedure discussed elsewhere.32,38 In particular, the four nonequivalent C1s core level contributions, expected because of the different atomic local chemical environments, are classified into two groups: the aromatic (C−C and C−H) and the methyl group (CH3) carbon sites contribute to the lower binding energy feature, while the shoulder at higher binding energies can be attributed to the signal from nitrogenbonded carbon atoms. The fit assignment of the XPS C1s 8045



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Figure 2. N1s core level spectra (together with their fits) of a 6 nm nominally thick film of NitPyn taken at (a) 640 and (b) 1000 eV photon energies, as indicated.

substituted pentacenes,38 too. Recently, an oscillating ratio of the XPS intensity versus photon energy has been observed by Söderström et al.43 in the gas phase of small chlorine-based molecules. The phenomenon is explained in terms of scattering of the outgoing electrons and considering the effects of losses due to monopole shake-up and shake-off and to intramolecular inelastic scattering processes.43 To shed light on the occurring mechanisms in NitPyn thin films, we analyzed the C1s XPS core level spectra for the three photon energies used by performing different series of fit, assuming different trends for S1 intensity (see the Supporting Information for further details). The structure of the C1s spectrum of NitPyn is far more intricate than that of analogous systems.38 The least squares fit procedure did not provide any defined trend on S1 intensity as a function of the photon energy, so that we could not gain definite information from its satellite intensity. On the contrary, experimental data (Figure 1d) clearly show a decrease of the intensity of S2 (the satellite at 288.3 eV) on increasing the photon energy, suggesting the occurrence of a shake-up transition consisting of a dipole excitation of a core electron into a valence orbital and a monopole ionization of a valence electron.42,44 The satellite intensities show a rather different behavior in NitPyn thin films, depending on the substrate chosen for deposition: no satellite intensities are observed when depositing NitPyn films on Au(111) single crystals.32 On the contrary, a clear satellite structure is seen in films deposited on Al2O3(11−20) single crystals;33 this indicates that different satellite intensities are correlated with the structure and morphology of the films, that is with solid-state properties. Note further that the experimental and theoretical works present in the literature are carried out either in gas phase or with very small molecules, i.e., in systems characterized by no, or negligible, intermolecular interactions.41−44 In this respect, additional experiments and the support of robust theoretical models are certainly needed to deepen the knowledge of these mechanisms. To complete the picture of the photoemission mechanisms in NitPyn films deposited on SiO2, we analyze the N1s core level spectra. The N1s core level spectra of an intact mesomeric nitronyl nitroxide radical are characterized by a single peak,32 as is the case for the N1s core level spectra in Figure 2, having an intense peak centered at around 402 eV, and a clear shake-up satellite at higher binding energies. This is due to the chemical equivalence of the two nitrogen atoms of the nitronyl nitroxide group.45 N1s core level spectra are very sensitive to the perturbation of the organic radical upon either degradation or molecule−substrate chemical interaction. The consequent alteration in the electronic structure of NitPyn is mirrored by a complex spectral line shape in the N1s core level spectra.32,35 NitPyn degradation leads to imino-nitroxide or diimino

contributions is in good agreement with the stoichiometric ratio, demonstrating that the deposition occurs without degradation of the molecule (see tables in the Supporting Information). A widely spread satellite structure, due to the charge redistribution within the molecule upon the photoemission event, is also visible at higher binding energy. The shake-up satellites are characteristic in photoemission core level spectra of aromatic hydrocarbon ring molecules.39 Different contributions to the satellite spectra are given by the large number of nonequivalent carbon sites originating from the symmetry reduction due to the core-hole formation. The satellite structure (S1) that appears at 285.5 eV (1.2 eV higher than the binding energy of the C1s main line) is assigned to the aromatic carbon atoms. The S1 satellite can be related to the first highest occupied molecular orbital−lowest unoccupied molecular orbital (HOMO−LUMO) shake-up.39 Note that 1.2 eV is lower than the corresponding HOMO−LUMO gap characterized by optical spectroscopy33 because of the enhanced screening of the core-hole due to its delocalization over the large aromatic system. The value of 1.2 eV is in agreement with the value obtained for tetracene (1.48 eV).39 We also notice the clear presence of a second satellite feature: according to stoichiometry arguments, the S2 satellite may be related to C−N contributions. The analysis of the C1s core level spectra, obtained for different photon energies, shows a slightly variable intensity for the C−N contribution. This may be due to inhomogeneous broadening related with local morphological and structural changes in the thin films,40 in agreement with the reported atomic force microscopy (AFM) characterization (see the Supporting Information), and with changes in the C1s cross section (see below). A change in photon energy implies a change in the C1s cross section, on the one hand increasing the number of effects that must be taken into account, and on the other, giving the opportunity to identify the core-hole screening mechanisms. In previous works,41−43 the intensity dependence of the first HOMO−LUMO shake-up satellite on the exciting photon energy has been correlated with the relaxation mechanisms of the system upon photoemission. The observation of a decrease of the S1 satellite intensity on increasing the photon energy accompanied by an opposite behavior of the remaining satellites indicates that the S1 intensity is related to the dipole excitation of a core electron to the LUMO accompanied by the monopole ionization of the valence electron. This shake-up contribution is near to the ionization threshold region and decreases with the increase in energy.41,42 This has been observed in small molecules like carbon monoxide41 and benzene.42 We observed this behavior in larger molecules, i.e. 8046



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of the pyrene substituent with respect to the substrate.46 Surprisingly, the pyrene does not show any preferential orientation, indicating films with rather small structural domains almost randomly oriented, contrary to what is observed in NitPyn films deposited on rutile TiO2(110) single crystals under the same preparation conditions. This may be explained by the fact that the topography of rutile TiO2(110) single crystals acts as a template for morphology and structure in organic thin films, as for example for para-sexiphenyl47 and diindenoperylene.48 The amorphous and flat SiO2 surface, on the contrary, does not favor any preferential morphology and/ or structure leading to thin films with strong polycrystalline texture. The persistence of the paramagnetic character of the deposited sample is clearly evidenced by the observation of an ESR signal typical for an organic radical, centered at g = 2.0067 (Figure 4a). This is in agreement with what was reported for other thick films based on nitronyl nitroxide radicals.29,49 The 4 nm thick sample has an average line width of 8.6 Oe, with a clear Lorentzian line shape, which suggests that exchange-narrowing processes are dominant.50 The observed line width is, however, somewhat larger than that of a powder of the pristine radical, indicating that some residual low-dimensional magnetic character is maintained in the deposited structure. Indeed, a small angular dependence on the line width and, more importantly, on the apparent integrated intensity of the lines is clearly visible. This is possibly due to the formation of locally ordered domains of the nitronyl nitroxide groups29 (Figure 4c,d) that cannot be easily detected by the C−K edge NEXAFS experiment. This might be due to the fact that the NEXAFS signal gives structural information integrated over the area sampled by the incident spot, whereas ESR is sensitive only to the interaction involving the paramagnetic function.

derivative products, which contribute with spectroscopic lines at lower binding energies with respect to the N1s spectroscopic line of the intact radical.32 In Figure 2, we observe a weak signal in the binding energy range between 398 and 400 eV. Consequently, we cannot exclude that this contribution originates from a small amount of degraded molecules, whose percentage corresponds to 4.8%, as determined by a XPS line analysis. To further characterize the thin films, we performed NEXAFS spectroscopy, aiming at the investigation of the unoccupied states. The typical thin-film C−K edge NEXAFS spectra, for two different polarization directions of the incident light, as indicated, are shown in Figure 3. They are

Figure 3. C−K edge NEXAFS spectra obtained from a 15 nm nominally thick film. The spectra were taken in grazing incidence for p-pol (black curve) and s-pol (gray curve) polarization.

characterized by two main regions: the π* region up to around 290 eV and the σ* region in the photon energy range above 290 eV.34 The resonances resemble closely those observed for NitPyn thin films deposited on rutile TiO2(110) single crystals.34 The C−K edge NEXAFS dichroism is also useful for investigating the average orientation of the molecular plane

Figure 4. Room-temperature ESR characterization of the samples. (a) Spectra of a nominally 4 nm thick film (lower trace) and of the residual powder after sublimation (upper trace). (b) Spectra obtained by dissolving the residual powder in CH2Cl2 (upper trace) and by washing the Si deposited sample in the same solvent (lower trace). (c) Angular dependence of the line width obtained by best fit of the spectra by Lorentzian line shape for the 4 nm thick film. θ = 0° when the magnetic field is applied parallel to the Si surface. (d) Angular dependence of the double integral of the Lorentzian curves obtained by best fit of the spectra of the 4 nm thick film; intensities are normalized as 1.00 at θ = 0°, when the magnetic field is applied parallel to the Si surface. 8047



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The nature of the deposited radical is investigated by washing the SiO2/Si(111) supported sample in CH2Cl2, and measuring the spectra of the resulting solution, by which the hyperfine pattern is observed (Figure 4b, lower trace). In addition to the 1:2:3:2:1 intensity pattern expected for a nitronyl nitroxide radical, weaker intensity lines are observed, which can be safely attributed to the iminonitroxide radical. This is known to yield a seven-line pattern with intensity 1:1:2:1:2:1:1 due to the superposition of two of the hyperfine lines, resulting from the condition 2aimino= anitrox.51 Interestingly, the residue after sublimation, once dissolved in solution, provides an ESR spectrum (Figure 4b, upper trace) which is due only to a nitronyl nitroxide radical, without traces of product degradation. This indicates that iminonitroxide is not formed during the molecular beam evaporation process but either after the condensation or because of the exposure of the prepared sample to the atmosphere. The former possibility is in line with the reported XPS data, showing a small amount of degraded molecules, even if one has to consider that ESR is only sensitive to paramagnetic degradation products.

CONCLUSIONS We have investigated the core-hole screening, electronic structure, and paramagnetic character of thin films of NitPyn deposited on SiO2/Si(111) by using XPS, NEXAFS, and EPR measurements. Our work confirms the correlation between these techniques. Their combined use allows a direct investigation of the electronic and paramagnetic character in thin films of organic radicals. In addition, some aspects of the core-hole screening mechanisms occurring in the NitPyn thin film are also elucidated. The adopted approach not only is uncommon for organic thin films, because it requires synchrotron-based measurements, but also is certainly new to this class of materials. In this respect, with our work we intend to stimulate further discussion and the necessary theoretical support. ASSOCIATED CONTENT

S Supporting Information *

Tables with fit parameters for XPS C1s core level spectra, a typical AFM image, fit results with different S1 intensities, and N−K edge NEXAFS spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel. +49 7071 29 76252. Fax: +49 7071 29 5490. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank BESSY for providing beamtime; W. Neu, E. Nadler, A. Vollmer, M. Oehzelt, S. Krause, M. Bauer, R. Ovsyannikov, and S. Pohl for technical support; and Prof. A. Caneschi for helpful discussions. We also thank Prof. Thomas Chassé for his critical reading of the manuscript. Financial support from Helmholtz-Zentrum Berlin, DFG under contract CA852/5-1, Ente Cassa di Risparmio di Firenze, Italian MIUR through FIRB project RBFR10OAI0, and EC through FP7People-2011-IAPP (286196) ESN-STM project is gratefully acknowledged. 8048



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