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A Derivative of the Blatter Radical as a Potential Metal-Free Magnet for Stable Thin Films and Interfaces F. Ciccullo,† N. M. Gallagher,‡ O. Geladari,† T. Chassé,† A. Rajca,*,‡ and M. B. Casu*,† †

Institute of Physical and Theoretical Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany Department of Chemistry, University of Nebraska, Lincoln, Nebraska 68588-0304, United States

‡

S Supporting Information *

ABSTRACT: Organic radicals are fascinating materials because of their unique properties, which make them suitable for a variety of applications. Their synthesis may be challenging, and big efforts have focused on chemical stability. However, introducing a new material in electronics not only requires chemically stable molecules but also stable monolayers and thin films in view of their use in devices. In this work, we have investigated the thin films of a derivative of the Blatter radical that was synthesized bearing in mind the thermodynamic factors that govern thin film stability. We have proved our concept by investigating the electronic structure, the paramagnetic character, and stability of the obtained films under UHV and ambient conditions by in situ X-ray photoelectron spectroscopy, ex situ atomic force microscopy, and electron paramagnetic resonance spectroscopy. KEYWORDS: organic radicals, organic magnetism, thin film processes, spinterface, organic electronics

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INTRODUCTION

However, design and synthesis have so far focused solely on chemical stability at the molecular level. The transition from the lab to the market introducing a new material in electronics requires not only a chemically stable molecule but also stable molecular monolayers and thin films in view of their use in devices. A controlled method to grow organic thin films onto a surface that will later work as a device contact/substrate is based on the use of organic molecular beam deposition (OMBD).29 To obtain deposition from the vapor phase, it is necessary to work in nonequilibrium conditions, i.e., overcoming the thermodynamic equilibrium between adsorption and desorption establishing no net growth on the surface.30,31 This is achieved by OMBD working in supersaturation, Δμ. As a first approximation, it can be shown that supersaturation is a function of substrate temperature, Tsub, and deposition rate, Φ,30,32 and it is given by

Organic radicals, which contain one unpaired electron, have long been considered reactive materials. However, during the last few decades different families of stable radicals have been synthesized,1−5 paving the way for radical functionalization of organic materials,6 graphene,7 carbon nanotubes,8 and other substrates.9−12 Their redox properties have been used in memory elements,13 batteries,14 sensors,15 organic electronic devices,14,16,17 and biomedicine.18,19 Recently, they have been shown to protect pentacene derivatives against photodegradation20 as well as to quench the photoexcited triplet-excited state,21 thus increasing their potential for applications in organic electronics. Organic radicals also find use in spinlabeling, spin-trapping, magnetic resonance imaging (MRI), spintronics and as building blocks for organic magnets.6,9,18−23 These unique properties allow radicals to be considered a “capital” suitable for groundbreaking applications from energy storage to quantum computing, having strong social impact with low costs, chemical flexibility, energy-saving technologies, and eco-friendly production playing major roles. These characteristics have inspired the synthesis of radicals that possess sufficient chemical stability which is a fundamental prerequisite for room-temperature applications. Two main strategies exist for the stabilization of organic radicals: protection of the spin centers with steric hindrance and delocalization of the unpaired electron. A large variety of novel, stable N,O- and N-centered radicals were synthesized in recent years, such as nitroxides, nitronyl and imino nitroxides, aminyls, verdazyls, and Blatter radicals (Figure 1).2−4,24−28 © 2016 American Chemical Society

Δμ = ΔHsub + RTsub[ln( 2πMRTsub Φ) − A]

(1)

where ΔHsub is the heat of sublimation, R is the gas constant, M is the molecular weight, and A is an empirical constant related to the entropy of sublimation.32 In principle, it is possible to control supersaturation to achieve film formation by varying substrate temperature and deposition rate. In the case of organic radicals, the range of allowed values for these two parameters may be very limited; for example, lowering the substrate temperature may drastically reduce the azimuthal Received: October 12, 2015 Accepted: January 4, 2016 Published: January 4, 2016 1805

DOI: 10.1021/acsami.5b09693 ACS Appl. Mater. Interfaces 2016, 8, 1805−1812

Research Article

ACS Applied Materials & Interfaces

Using DFT computations, we considered two different modes of fusing pyrene and 1,3-diphenyl-1,4-dihydro-1,2,4-triazin-4-yl as illustrated by radicals 1 and 2 (Figure 1). The UB3LYP/631G(d,p)-optimized geometries indicate a significant out-ofplane twisting of the pyrene moiety in 2; constitutional isomer 2 is also ∼7 kcal mol−1 higher in energy compared to 1 (Table S1, 1 kcal mol−1 = 4.184 kJ mol−1).42 Therefore, we selected constitutional isomer 1, with a more planar structure and with greater delocalization of spin density into the pyrene moiety, as the target for the synthesis.

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EXPERIMENTAL SECTION

NMR, EPR, and IR Spectroscopy, Mass Spectrometry, Computations, and Synthesis. NMR spectra (1H, 400 MHz) were obtained using acetone-d6 and chloroform-d (CDCl3) as solvent. The chemical shift references were as follows: (1H) acetone-d5, 2.05 ppm; (13C) acetone-d5, 29.92 ppm (acetone-d6); (1H) chloroform, 7.26 ppm. CW X-band EPR spectra for radical 1 in benzene or in the films were acquired at ambient temperature on a Bruker EMX instrument equipped with a frequency counter. The solution samples were contained in 4 mm EPR sample tubes, and the film samples were placed in 5 mm EPR sample tubes. A goniometer was used for the film samples. The spectra were obtained using a dual-mode cavity, with an oscillating magnetic field perpendicular (TE102) to the swept magnetic field. DPPH powder (g = 2.0037) was used as a g-value reference. Spin concentrations (by EPR spin counting) were calculated by comparing the double integrals of a solution of known concentration (1−3 mM) of radical 1 in either benzene or CHCl3 with the double integral of a standard solution of nitroxide (1−3 mM of either TEMPOL or 3-carboxy-PROXYL) in the same solvent. A sweep width of 100 G was used for both radical and standard measurements, and double integrals were normalized to account for different receiver gains. IR spectra were obtained using an instrument equipped with an ATR sampling accessory. DFT computations were carried out using an 8-CPU workstation running Gaussian 09. Optimized geometries of 1 and 2 had rms forces in Cartesian coordinates of less than 9.8 × 10−6 a.u. All reported computed structures are minima on the gas-phase potential energy surface, as determined by vibrational analyses (Table S1). Standard techniques for synthesis were employed. Chromatographic separations were carried out using normal phase silica gel. Further description of synthesis may be found in the Supporting Information. Thin Film Growth and Characterization. Thin film growth and XPS measurements were performed in an UHV system consisting of a substrate preparation chamber and an OMBD-dedicated chamber connected to an analysis chamber (base pressure 4 × 10−10 mbar) equipped with a SPECS Phoibos 150 hemispherical electron analyzer and a monochromatic Al Kα source (SPECS Focus 500). Native SiO2, grown on single-side-polished n-Si(111) wafers with a doped resistivity of 107−118 Ω·cm (phosphorus-doped) was used as a substrate. The clean substrates were prepared by cleaning in ultrasonic baths of acetone and ethanol, followed by multiple 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. Thin films of pyrene−Blatter radical 1 were deposited in situ by OMBD using a Knudsen cell (evaporation rate = 2 Å /min, substrate at room temperature). The evaporation rate was measured with a quartz crystal microbalance, and the nominal thickness was cross-checked by using the attenuation of the XPS substrate signal (Si 2p) after deposition of radical 1. Survey and detailed XPS spectra were measured with electron pass energy of 50 and 20 eV, respectively. The binding energy was calibrated by using the Si 2p XPS signal (Si 2p at 99.3 eV). Because radicals are radiation-sensitive, we have taken all precautions necessary to avoid radiation damage (e.g., short beam exposure and freshly prepared films for each spectrum). Thus, no beam-induced degradation of the samples was observed on the time scale of all discussed experiments. Atomic force microscopy (AFM) studies were performed under ambient conditions in tapping mode with a Digital

Figure 1. Upper panel: Drawings of parent 1,3-diphenyl-1,4-dihydro1,2,4-benzotriazin-4-yl (Blatter radical) and pyrene-fused 1,3-diphenyl1,4-dihydro-1,2,4-triazin-4-yl radicals 1 and 2. Lower panel: spindensity maps for radicals 1 and 2 at the UB3LYP/6-31G(d,p)// UB3LYP/6-31G(d,p) level of theory. Positive (blue) and negative (green) spin densities are shown at the isodensity level of 0.001 electron/Bohr.

molecular order.33−35 This may represent a disadvantage when designing an active layer for electronics, and it may hinder possible magnetic ordering. In contrast, higher substrate temperatures complicate the achievement of necessary nonequilibrium conditions for deposition by increasing the desorption probability. Also, the radical may suffer from decomposition at these higher temperatures. Designing the molecules in such a way that the tuning of their heat and entropy of sublimation give them good film-forming and stability properties especially in the temperature range of interest for a working device, i.e., at/slightly above room temperature, is a way to address this issue. Inspired by this concept, we have designed a stable Blatter radical (1,3-diphenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl) derivative,36,37 taking into account the thermodynamics that govern thin film processes. Pyrene is a well-known chromophore characterized by relatively high vapor pressure at room temperature (∼5.4 × 10−4 Pa),38 i.e., with a relatively low tendency to stick and to stay onto a surface. (See eq 1.) Note that according to the Clausius−Clapeyron equation sublimation enthalpy and vapor pressure are directly connected30. We reasoned that fusing a Blatter’s radical, a stable radical25 with lower vapor pressure (analogous 1,2,3-benzotriazine- and triazine-derivatives are characterized by vapor pressure in the low 10−5 Pa range at room temperature),38,39 to pyrene would lower the overall vapor pressure and give a molecule with good film-forming properties. Note that choosing the appropriate preparation conditions is possible to obtain films of both molecules; however, because of the above argument of the vapor pressure, the films are not so thermally stable at room temperature as the films obtained by fusing the Blatter radical with pyrene and are therefore affected by a complex series of postgrowth phenomena30,31,33,40,41 such as desorption, readjustment of the molecular orientation leading to structural changes, island ripening that would hinder their use in thin-film devices. 1806


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ACS Applied Materials & Interfaces Instruments Nanoscope III Multimode AFM. Image processing was performed with WSxM 5.0 Develop 7.0.43

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RESULTS AND DISCUSSION Synthesis of radical 1 (Scheme 1) followed the typical route to Blatter radicals.25,44−46Reaction of 1-aminopyrene47,48 with NScheme 1. Synthesis of Radical 1

phenyl-benzenecarbohydrazonoyl chloride49 provided amidrazone 3. Subsequently, 3 was subjected to reductive cyclization to radical 1, following an established route.36,50,51 EPR spin counting and paramagnetic 1H NMR spectra confirm the 100% spin purity of radical 1 (Supporting Information). X-ray structure of radical 1 was found to be identical to that already reported for 1.36 The EPR spectrum of 1 in benzene at room temperature revealed hyperfine coupling to three nonequivalent nitrogens (∼4−7 G) as well as a smaller coupling to one hydrogen (1.55 G). Hyperfine splittings, aN(1) ≫ aN(4) > aN(2) (Scheme 1), were in qualitative agreement with DFT-computed values at the UB3LYP/6-31G(d,p)/IEF-PCM-UFF (benzene) level of theory and with the previous EPR/ENDOR studies on 15Nlabeled benzotriazinyls.52,56 DFT computations also predict significant delocalization of spin density into the pyrene moiety (Figure 1) and a 1H hyperfine splitting of 1.85 G attributed to one of the hydrogens of the pyrene moiety, which is in agreement with our simulated value (1.55 G) (Figure S12). To address the film properties, the first point to be clarified is whether it is possible to deposit thin films of radical 1 by using OMBD without molecular degradation. To answer this question, following a well-established approach,40,41 we use the element-sensitivity of the XPS signal and its correlation with the molecular stoichiometry to analyze the core level spectra of radical 1 thin films deposited on the SiO2/Si(111) surfaces (Figure 2). The C/N intensity ratio obtained from the XPS curves nicely agrees with the expected stoichiometric value (Table S3). Therefore, we can infer that the radical 1 molecules are intact upon evaporation and deposition. This result is confirmed by the comparison of the thin film spectra with those of the powder embedded in indium foil (Figure S1) that show the same spectroscopic lines, apart from small broadening and energy shifts due to typical charging effects occurring in organic crystals. The C 1s core level spectra of the thick films are dominated by a main line at around 284.5 eV. A careful best-fit procedure with constraints based on stoichiometry and electronegativity is a powerful tool to gain a deeper insight into the XPS core level spectra. (A detailed description of the best-fit procedure is given in ref 40.) Figure 2c shows the peak fit results for the C 1s core level spectrum of a 4.9 nm nominally thick film. Because of their different chemical environment, inequivalent carbon atoms should give different features in the XPS signal. Distinguishing all of them would make the fit quite speculative,

Figure 2. Thickness dependent (a) C 1s and (b) N 1s core level spectra of pyrene−Blatter thin films deposited on SiO2/Si(111) surfaces. The thickness is given in nanometers. The peak-fit analysis for the 4.9 nm nominally thick film is also shown (c and d). Fit details are given in the Supporting Information.

especially considering the experimental resolution. Hence, we take into account contributions from carbon atoms bound to nitrogen atoms and contributions from aromatic carbon atoms, identifying carbon atoms bound only to carbon (C−C) or also to hydrogen (C−H) atoms. The best-fit curves fully satisfy the stoichiometric requirements with the different contributions (Table S4). The N 1s core level spectra yield the information on the paramagnetic function of the radical 1 because the unpaired electron is delocalized mainly over the triazine ring. As expected, the N 1s spectra are characterized by three different main lines, related to the three chemically inequivalent nitrogen atoms (Figure 1). In the thicker films, the peak at around 399 eV is assigned to the pyridine-like nitrogen (N2) that has a carbon atom and a nitrogen atom as neighbors, whereas the peak at 400.9 eV is attributed to the nitrogen bound also to the phenyl ring (N1) in agreement with previous works.53−58 Because of the delocalization of the unpaired electron, the peak at lower binding energy (398.2 eV) can be related to photoelectron emitted from the nitrogen radical (Nrad). By our peak fit analysis (Figure 2d), we find that the single contributions agree with the stoichiometry of the molecule (Table S5). This result indicates that the radical is intact. Stability of radical 1 in the films is confirmed by ex situ EPR spectra, obtained after the films were exposed to air for 5 days and 3 months (Figure S4). Note that in addition to the abovediscussed components, we observe a rich satellite structure (denoted with Si i = 1, 2, 3, ... in Figure 2). The shakeup satellites are expected in photoemission core level spectra of aromatic hydrocarbon ring molecules.59 They are related to electron-relaxation effects arising after the core−hole creation. 1807


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the initial layers, and it is consistent with the AFM images (Figure 3, right side), clearly showing island morphologies. As discussed, a thorough understanding of the long-term stability of the films is an essential prerequisite for the successful development of applications based on new molecules. To check the vacuum stability, we monitor substrate and film XPS signals as a function of time, keeping the sample in UHV (base pressure ≈ 4 × 10−10 mbar) for 12 h (720 min). Figure 4a shows the time dependence of the C 1s and Si 2p signals. The signals stay substantially constant, except for a minor increase of the C 1s signal after 240 min, probably related to adsorption of carbon from the environment. Thus, there is no evidence of dewetting or ripening processes in the films. In Figure 4b, we report a comparison of N 1s core level spectra acquired upon deposition and after 12 h. The spectra do not show any change, indicating chemical stability of the Blatter radical 1 in UHV. To monitor the film morphology in air, we carry out XPS, AFM, and EPR spectroscopy. We measure the XPS core level spectra of a 6.1 nm thick film immediately after evaporation and after air exposure. The results are shown in Figure 5: Figures 5a,b show the N 1s core level spectra of a freshly evaporated film and after 45 h air exposure. From the direct comparison of the experimental data, we can confidently rule out relevant changes after 45 h. This observation is further supported by the fit procedure (Figure 5b) that is performed adopting the same parameters as for the freshly prepared films. However, the N 1s signal presents changes after keeping the sample for 6 days (∼144 h) under air conditions. (See also Figure S3.) Also in this case, a fit procedure is a useful way to gain a deeper insight into the ongoing phenomena (Figure 5d). We note that the contribution assigned to the radical at around 398 eV decreases, whereas a signal at around 400 eV grows. Notably, the decrease in intensity (∼9−10%, Tables S6 and S7) is equivalent to the increase of the signal at around 400 eV. These phenomena are related to a (slow) degradation of the films under ambient conditions due to changes impacting the radical part of the molecules. The binding energy value hints at a possible oxidation or hydrogenation of the radical. Hydrogenation of pyridinic nitrogen atoms in edge configuration is likely to occur in nitrogen dopants.54,55,58 The typical binding energies of the photoemitted electrons are in that case in the 398−405 eV range.54 We do not observe changes in the C 1s signal (Figure S3); thus, we can exclude pyrene degradation and/or radical dissociation. Note that the signal due to contributions from the oxidized or hydrogenated nitrogen atoms overlaps the energy of the first shake-up satellite S1 in the case of the radical 1 molecules. It would be speculative to fit both contributions; that is, at the present state of the art it is not possible to identify/calculate the two phenomena/contributions separately. How the radical degradation affects the shakeup satellite intensities is also largely unknown. Therefore, in our fit procedure we cumulate the oxidation/hydrogenation and the shakeup satellite intensities in a unique contribution (feature S1* in Figure 5d). To investigate the film morphology under ambient conditions, we perform ex situ tapping-mode AFM measurements for four consecutive days. Figure 5e shows the evolution of the root-mean-square (rms) roughness measured on 10 × 10 μm2 AFM images as a function of time. The roughness experiences a 7% change over 4 days (from ∼8.7 to ∼9.4 nm). This change can be considered very small and confirms the good stability of the films not only in vacuum but also in air.

Their intensity contributes to the stoichiometric analysis of the XPS signals and it cannot be neglected.60,61 The analysis of thickness-dependent XPS spectra gives information about the nature of the interaction between 1 and the substrate.62,63 Thickness-dependent C 1s and N 1s spectra (Figure 2) do not show any significant variation with increasing the film thickness, i.e., the molecular orbitals of radical 1 remain unperturbed at the interface. In fact, in case of chemisorption, strong changes are expected in the photoemission spectra of molecules close to the interface,64,65 contrary to the observation here. This finding indicates that molecules of 1 are weakly physisorbed on SiO2. We do not observe any relevant difference with film thickness in the binding energies of the main lines, apart from a rigid energy shift (0.15 eV) toward higher binding energies when 0.7 nm thickness is reached. This thickness corresponds to the regime where island growth starts. (See discussion below.) Morphological changes may be concomitant with structural changes, impacting the photoemission line.66 These observations indicate that a small image-charge screening occurs at the interface, as observed for other organic molecules deposited on metal oxides67,68 and, in particular, for a fluorophore−nitroxide radical recently investigated.41 These results, together with an annealing experiment, which shows that the molecules are completely desorbed from the SiO2 surface at 500 K (Figure S2) further support the conclusion of a weak molecule/ substrate interaction. During the annealing experiment, we have also monitored the N 1s core level spectra in order to gain information on the radical thermal stability. The result shows surprisingly good thermal stability for a thin film of a radical: no changes in the spectra are visible up to around 400 K (Supporting Information). Note that a small change in intensity of the peak at 398.2 eV is visible with increasing thickness (Figure 2b). This may be due to inhomogeneous broadening of the spectroscopic lines because of the mentioned locally different thickness-dependent structural and morphological film characteristics66,69 or to charge redistribution at the film surface because of the electronegativity of the N atoms.61,69 The growth mode of radical 1 on SiO2, under the present preparation conditions, can be identified by monitoring the attenuation of the XPS signal substrate upon deposition (Figure 3, left side). The attenuation curve shows two different slopes with a crossover between two regimes that corresponds to a nominal thickness of 0.7 nm. This intensity decay is typical of Stranski−Krastanov growth mode, i.e., first one or more layers grow on the substrate followed by islands which nucleate on

Figure 3. (a) Attenuation of the Si 2p XPS signal, normalized to the corresponding saturation signal, as a function of time during pyrene− Blatter deposition at room temperature. (b) A typical 10 μm × 10 μm AFM image of a 5.6 nm nominally thin film. 1808


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Figure 4. (a) Temporal evolution at room temperature of the Si 2p and C 1s XPS signals for a 6.6 nm nominally thick film after deposition. (b) Comparison of N 1s core level spectra of a freshly evaporated film and after 12 h, keeping the sample at room temperature in vacuum conditions.

Thus, at a lower temperature, the increased number of nucleation sites would lead to more homogeneous films, whereas at room temperature, diffusion is facilitated, with the consequent decrease of nucleation sites and a more pronounced island morphology. Additionally, an important role is also played by the energetic barriers to diffusion and to traverse step edges, i.e., Ehrlich−Schwö bel barriers.72,73 Ehrlich−Schwöbel barriers depend on temperature, orientation of the underlying molecules, torsion potential, distance of the landing molecule from the step,74 and substituents.75 The hypothesis of a high Ehrlich−Schwöbel barrier leading to rough films in radical 1 is highly plausible. Considering that the molecules in films show the tendency to approach an orientation close to the one they assume in their single crystal once they overcome the substrate interaction and the presence of the phenyl rings, we may infer the presence of high barriers for the molecules to jump downward and thus find it energetically more convenient to stay close to the point where they land, causing in such way the growth of high 3D islands. Finally, we performed ex situ EPR spectroscopic analyses after 5 days and 3 months of exposure of the films to air at room temperature (Figure S4). EPR spectra show an exchangenarrowed peak of radical 1 and a broader peak ascribed to the silicon substrate. The spectra are isotropic, without significant dependence of the spectral shape or intensity on the angle between the substrate and the applied magnetic field. The ratios between peak heights for the narrow peak and the broad peak increase with the thickness of the film. Notably, the ratios of the peak heights are practically constant over 3 months, indicating good stability of radical 1 under ambient conditions (Figure S4).

Figure 5. Comparison of N 1s core level spectra of a freshly evaporated film and after (a) 45 h and (c) 6 days of air exposure. (b and d) Peak fit analyses for a and c. The peak analysis in b is the same as that adopted in Figure 2 for the fresh film. The details are given in the Supporting Information. (e) Evolution of rms-roughness as a function of time for 10 μm × 10 μm images of a 6.6 nm nominally thick film kept under ambient conditions. The time 0 indicates when the sample has been taken out of the vacuum chamber. (f) Comparison of height distributions for a sample (6.6 nm nominally thick film) immediately exposed to air (first day) and after 4 days of air exposure.

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CONCLUSIONS

We have synthesized and characterized radical 1 while bearing in mind the thermodynamic factors that govern thin film stability with the purpose of obtaining not only a chemically stable radical but also stable thin films. We have proved our concept by investigating the electronic structure, the paramagnetic character of the obtained films, and their stability under UHV and ambient conditions. Our results indicate that the radical 1 has very good vacuum and air stability properties contrary to already existing radical derivatives10,40,76 and small molecules such as perylene33 and tetracene35 synthesized with the sole purpose of chemical stability. In addition, EPR measurements show that the paramagnetic character of the molecules is also preserved in the films over a long period of time. We believe that this class of materials has immense

This is also mirrored by the time evolution of the height distributions (Figure 5f); the widening of the height distribution is mainly due to a slight rearrangement of the islands and to adsorption of environmental contaminants, two phenomena that are usually hindered in real devices by means of encapsulation. We observe that the rms roughness value is quite large. Rapid roughing in thin films of organic materials has been reported for a variety of molecules.70,71 This is not surprising if we consider that the films are grown at room temperature: the nucleation density depends on the substrate temperature.30 1809


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technological potential and that our approach may pave the way to their systematic use in real devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b09693. General procedures and materials, additional experimental details, and computational data. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +49 7071 29 76252. Fax: +49 7071 29 5490. *E-mail: [email protected]. Tel.: 1-402-472-9196. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank H. Adler and E. Nadler for technical support and Dr. Maren Pink for the X-ray structure of radical 1. Financial support from DFG under the contract CA852/5-2 (MBC) and from National Science Foundation under the grant CHE1362454 (AR) is gratefully acknowledged.

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REFERENCES

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