Imaging Single Spin Probes Embedded in a Conductive Diamagnetic

Jan 21, 2009 - The detection of spin noise by means of scanning tunneling microscopy (STM) has recently been substantially improved by the work presen...
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Langmuir 2009, 25, 1885-1892

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Imaging Single Spin Probes Embedded in a Conductive Diamagnetic Layer Paolo Messina* and Frank Fradin Material Science DiVision, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed August 25, 2008 The detection of spin noise by means of scanning tunneling microscopy (STM) has recently been substantially improved by the work presented by Komeda and Manassen (Komeda, T.; Manassen, Y. Appl. Phys. Lett. 2008, 92, 212506). The application of this technique to molecular paramagnets requires the positioning and anchoring of paramagnetic molecules at surfaces. It also requires the possibility of tunneling high current densities into the STM -molecule-substrate tunneling junction. In this letter, we exploit the self-assembly of 1,10-phenantroline on the Au(111) surface to form a diamagnetic matrix that hosts individual molecules and dimers of diphenyl-2-picrylhydrazyl (DPPH). STM measurements are used to characterize the molecular layer. Electron spin resonance (ESR) measurements elucidate the role of thermal annealing in the preservation of the paramagnetic nature of the DPPH molecules.

Ensemble-detected magnetic resonances have proven to be an invaluable tool in the determination of structural properties of molecules.1,2 However, these techniques lack the necessary sensitivity to provide specific chemical information on the nanometer scale. Emerging areas of magnetosurface chemistry require specific structural and spin dynamics information to be provided on the nanometer scale.3-6 Force detected magnetic resonances have proven the ability to detect single spins and image 3D nanostructures7 although spatial resolution is limited to 25 nm and the acquisition time is very long. Electron spin noise scanning tunneling microscopy (ESN-STM) has emerged as a possible local probe for both electron spin and nuclear spin.8-10 Recent experiments have proven that the technique can provide information on the g factor of organic radicals such as R-γ-bisdiphenylene-β-phenylallyl (BDPA),11 1,1-diphenyl-2picryl-hydrazyl (DPPH)12 (Figure 1a,b) and nitroxyl derivatives13 at the single-molecule level. A crucial aspect in the further development of this technique is the precise positioning of spin probes at surfaces. Because these experiments take place at room * Corresponding author. E-mail: [email protected], paolonechicago@ gmail.com. Tel: +312 404 3379. (1) Chen, L.; Kaiser, J. M.; Polenova, T.; Yang, J.; Rienstra, C. M.; Mueller, L. J. J. Am. Chem. Soc. 2007, 129, 10650–10651. (2) Zhou, D. H.; Shah, G.; Cormos, M.; Mullen, C.; Sandoz, D.; Rienstra, C. M. J. Am. Chem. Soc. 2007, 129, 11791–11801. (3) Mannini, M.; Bonacchi, D.; Zobbi, L.; Piras, F. M.; Speets, E. A.; Caneschi, A.; Cornia, A.; Magnani, A.; Ravoo, B. J.; Reinhoudt, D. N.; Sessoli, R.; Gatteschi, D. Nano Lett. 2005, 5, 1435–1438. (4) Coronado, E.; Marti-Gastaldo, C.; Tatay, S. Appl. Surf. Sci. 2007, 254, 225–235. (5) Nickels, P.; Matsushita, M. M.; Minamoto, M.; Komiyama, S.; Sugawara, T. Small 2008, 4, 471–475. (6) Minamoto, M.; Matsushita, M. M.; Sugawara, T. Polyhedron 2005, 24, 2263–2268. (7) Seppe, K.; Steven, A. H.; John, A. M. J. Chem. Phys. 2008, 128, 052208. (8) Balatsky, A. V.; Fransson, J.; Mozyrsky, D.; Manassen, Y. Phys. ReV. B 2006, 73, 184429. (9) Balatsky, A. V.; Manassen, Y.; Salem, R. Phys. ReV. B 2002, 66, 195416.1195416.5. (10) Manassen, Y.; Hamers, R. J.; Demuth, J. E.; Castellano, A. J. Phys. ReV. Lett. 1989, 62, 2531–2534. (11) Durkan, C.; Welland, M. E. Appl. Phys. Lett. 2002, 80, 458–460. (12) Messina, P.; Mannini, M.; Caneschi, A.; Gatteschi, D.; Sorace, L.; Sigalotti, P.; Sandrin, C.; Prato, S.; Pittana, P.; Manassen, Y. J. Appl. Phys. 2007, 101. (13) Mannini, M.; Messina, P.; Sorace, L.; Gorini, L.; Fabrizioli, M.; Caneschi, A.; Manassen, Y.; Sigalotti, P.; Pittana, P.; Gatteschi, D. Inorg. Chim. Acta 2007, 360, 3837–3842.

Figure 1. (a) Three-dimensional molecular model of DPPH. (b) Structural diagram for DPPH. (c) EPR spectra of a solution of 1,10-phenanthroline and DPPH taken after the solution is heated to 80 °C for 10, 30, and 60 min. The inset shows the central line peak amplitude variation as a function of time as the solution is heated to 80 °C.

temperature, it is of paramount importance to anchor the spin probe at the surface, preventing electron migration or lateral diffusion during ESN-STM spectroscopy. The major requirement for addressing individual spin probes is to insert them in a conductive and diamagnetic matrix. ESN-STM spectroscopy is performed at tunneling currents typically of 0.1 to 4 nA,14 and the mechanism for the coupling between the tunneling current (14) Komeda, T.; Manassen, Y. Appl. Phys. Lett. 2008, 92, 212506.

10.1021/la8039863 CCC: $40.75  2009 American Chemical Society Published on Web 01/21/2009

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and the spin dynamics may be related to shot noise8,9,15 (shot noise emerges only at certain combinations of applied voltage and tunneling currents). In this respect, previously developed approaches based on the dispersion of molecules above a selfassembled monolayer (SAM) of aliphatic thiols16 is not applicable. The STM tip would penetrate and locally deform the layer. Also, spin probes are typically difficult to functionalize directly with thiol groups, and only certain specific molecular designs result in a nanometer-scale ordering.17-19 Furthermore, the presence of several paramagnetic radicals in the neighborhood of the spin probe of interest alters the local magnetic field and the spin dynamics of the probe.18 In fact, magnetic dipolar coupling between neighboring spins would result in a decrease in the spin phase coherent time (T2). T2 is an important parameter in the detection of magnetic resonances in small ensembles and single molecules as the signal-to-noise ratio is improved for longer T2.20,21 This is an additional reason to position spin probes away from one another. Although it is generally possible to deposit individual molecules in an ultrahigh vacuum (UHV) environment22,23 and anchor them via surface cooling,24 this approach limits the range application of the ESN-STM. In fact, there are other sectors of surface chemistry that may benefit from this novel technique. STM can be used to gain information on the governing parameters for self-assembly at the solid-air25 or solid-liquid interface.26-28 Inherently, the possibility of design a strategy for self-assembly to position spin probes in an airstable molecular matrix is also very appealing in preparation for intriguing local magnetic resonance measurements. The precise insertion of single molecules into alkanethiol selfassembled monolayers has been proven to be a viable option for individually positioning nonmagnetic molecules on the surface.29,30 In this work, the authors exploit the fact that alkanethiol SAMs have pits in their structures. The molecule of interest is inserted into the pit after an alkanethiol SAM has first formed on the surface. This technique has also successfully been applied to microcontact insertion printing.31 The latter proves that this approach is well suited to be combined with top-down lithographic approaches to position individual molecules over large surface areas and may be of interest in the development of certain optical (15) Kemiktarak, U.; Ndukum, T.; Schwab, K. C.; Ekinci, K. L. Nature 2007, 450, 85. (16) Banin, U.; Cao, Y. W.; Katz, D.; Millo, O. Nature 1999, 400, 542–544. (17) Shekhah, O.; Roques, N.; Mugnaini, V.; Munuera, C.; Ocal, C.; Veciana, J.; Woll, C. Langmuir 2008, 24, 6640–6648. (18) Mannini, M.; Sorace, L.; Gorini, L.; Piras, F. M.; Caneschi, A.; Magnani, A.; Menichetti, S.; Gatteschi, D. Langmuir 2007, 23, 2389–2397. (19) Crivillers, N.; Mas-Torrent, M.; Vidal-Gancedo, J.; Veciana, J.; Rovira, C. J. Am. Chem. Soc. 2008, 130, 5499–5506. (20) Budker, D.; Lamoreaux, S. K.; Sushkov, A. O.; Sushkov, O. P. Phys. ReV. A 2006, 73, -. (21) Taylor, J. M.; Cappellaro, P.; Childress, L.; Jiang, L.; Budker, D.; Hemmer, P. R.; Yacoby, A.; Walsworth, R.; Lukin, M. D. Nat. Phys. 2008, 4, 810–816. (22) Messina, P.; Dmitriev, A.; Lin, N.; Spillmann, H.; Abel, M.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2002, 124, 14000–14001. (23) Spillmann, H.; Dmitriev, A.; Lin, N.; Messina, P.; Barth, J. V.; Kern, K. J. Am. Chem. Soc. 2003, 125, 10725–10728. (24) Shoji, O.; Tanaka, H.; Kawai, T.; Kobuke, Y. J. Am. Chem. Soc. 2005, 127, 8598–8599. (25) Marchenko, A.; Katsonis, N.; Fichou, D.; Aubert, C.; Malacria, M. J. Am. Chem. Soc. 2002, 124, 9998–9999. (26) Wei, Y.; Tong, W.; Zimmt, M. B. J. Am. Chem. Soc. 2008, 3399–3405. (27) Palermo, V.; Samori, P. Angew. Chem., Int. Ed. 2007, 46, 4428–4432. (28) Liscio, A.; De Luca, G.; Nolde, F.; Palermo, V.; Muellen, K.; Samori, P. J. Am. Chem. Soc. 2008, 130, 780. (29) Donhauser, Z. J.; Mantooth, B. A.; Pearl, T. P.; Kelly, K. F.; Nanayakkara, S. U.; Weiss, P. S. Jpn. J. Appl. Phys., Part 1 2002, 41, 4871–4877. (30) Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.; Stapleton, J. J.; Price, D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 2001, 292, 2303–2307. (31) Mullen, T. J.; Srinivasan, C.; Hohman, J. N.; Gillmor, S. D.; Shuster, M. J.; Horn, M. W.; Andrews, A. M.; Weiss, P. S. Appl. Phys. Lett. 2007, 90, 3.

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approaches to detect magnetic resonances on the nanoscale.32,33 Nevertheless, the use of an electrically insulating layer (made up of alkanethiols) makes it impossible to use an STM tunneling current set point higher than 30 pA.34 This is not suitable for the application that we seek to fulfill in this work and for the broader scope of our research efforts. In this letter, we report on a self-assembly approach that results in isolated single spin probes inserted into a diamagnetic molecular matrix suited for subnanometer-scale STM investigation. We build upon the approach suggested by Echegoyen35 and co-workers whereby the Au-N bond36 is as strong as the Au-S bond in thiol-based SAMs. Indeed, pyridine-like compounds can self-assemble at the Au surface, generating ordered and electrically conductive monolayers.37,36 Echegoyen tried to use 1,10-phenanthroline to build ordered SAMs on Au(111) at the solid-liquid interface. In addition, the formation of an ordered monolayer is facilitated by the π-π interaction between the stacked aromatic rings of neighboring 1,10-phenantroline molecules.38 Monolayers of 1,10-phenantroline were found to be stable and resistant at the solid-liquid interface.35 We deposited 1,10-phenantroline on Au(111) following this procedure: Au(111) was immersed at room temperature for 12 h in a solution of 1 mM 1,10-phenantroline dissolved in dichloromethane. The sample was then rinsed quickly in dichloromethane and dried under nitrogen. The sample was then mounted on a sample holder for STM imaging at the solid-air interface. Optimal solid-air STM imaging was achieved with a tunneling current of 150 pA and a bias voltage of 0.3 V, as reported in the literature.35 We used a home-built instrument to carry out these measurements.39,40 Ordered arrays of 1,10-phenanthroline intercalated into disordered regions appearing in the STM image shown in Figure 2b,c and in the insets. Figure 2d,e shows a detailed STM image of a 10 × 6 nm2 region comprised within an ordered array. The ordered arrays consist of stripes 14 to 25 Å away from each another. We find different values for the stripe-to-stripe distance at the array boundaries from those in the middle of the arrays. Also, the larger the array, the shorter the distance between stripes. This value differs from the 10.7 Å reported in the literature.35,41,42 Also, the ordered arrays seem smaller than that observed at the solid-liquid interface.35 Cunha and co-workers first studied the self-assembly of 1,10-phenantroline on Au(111) in an electrochemical environment at the solid-liquid interface. They found that layer growth starts with the decoration of the herringbones of the reconstructed (22 × 3)Au(111) surface41,42 at the solid-liquid interface. By varying the reference electrode potential, the authors obtain a sequence of increasingly ordered domains where 1,10-phenantroline molecules are aligned in stripes.41,42 Also, Lux and co-workers studied the self-assembly (32) Meriles, C. A. Concepts Magn. Reson., Part A 2008, 32A, 79–87. (33) Meriles, C. A. J. Magn. Reson. 2005, 176, 207–214. (34) In most cases, proper, highly resolved STM imaging of alkanethiols is obtained only with set points less or equal to E1 pA. (35) Dominguez, O.; Echegoyen, L.; Cunha, F.; Tao, N. J. Langmuir 1998, 14, 821–824. (36) Lee, K. H.; Suh, Y.; Lee, C.; Hwang, Y. G.; Koo, H. J.; Whangbo, M. H. J. Phys. Chem. B 2005, 109, 15322–15326. (37) A typical thiol SAM can be imaged at a tunneling current as high as 10 pA. The monolayers described in this work tollerate tunneling currents as high as 250 pA. (38) Franco Cozzi; M. C.; Annunziata, R.; Siegel, J. S. J. Am. Chem. Soc. 1993, 115, 5330–5331. (39) Messina, P.; Pearson, J.; Vasserman, I.; Sasaki, S.; Moog, E.; Fradin, F. Meas. Sci. Technol. 2008, 19, 7. (40) Messina, P.; Fradin, F.; Pittana, P. Nanotechnology, 2008, . accepted for publication. (41) Cunha, F.; Jin, Q.; Tao, N. J.; Li, C. Z. Surf. Sci. 1997, 389, 19–28. (42) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376.

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Figure 2. (a) 200 × 200 nm2 STM image of the bare Au(111) surface. The bias voltage (BV) is 0.3 V, and the tunnelling current (It) is 10 pA. The terraces appear flat with no particular structure. (b) STM image of Au(111) covered with a 400 × 400 nm2 1,10-phenantroline surface. BV ) 0.6 V and It ) 150 pA. (c) STM image of Au(111) covered with a 200 × 200 nm2 1,10-phenantroline surface. BV) 0.6 V and It) 150 pA. Ordered islands are clearly distinguished. Inset 1 shows the pits and the nonordered regions in the 1,10-phenantroline monolayer. Insets 2 and 3 show the detail of ordered stripes of 1,10-phenantroline. (d) STM image of 1,10-phenantroline stripes on the 10 × 6 nm2 Au(111) surface. BV) 0.6 V and It) 150 pA. (e) Height profile of the stripes reported in d. The profile corresponds to the line highlighted in white in d. The distance between the stripes (peak to peak) in this particular image varies from 1.4 to 1.8 Å. (In other images, some of the stripes within the ordered arrays are separated by 2.5 Å.)

of 1,10-phenantroline derivatives on the (22 × 3)Au(111).43 They found the stripe to stripe distance to be 9.8 Å. In more recent studies, Cafe and co-workers conclude that the lateral interaction between 1,10-phenantroline molecules ultimately determines the stripe-to-stripe distance.44 Under certain conditions, they find the stripes to be separated by 12 to 13 Å. An understanding of the discrepancy between our data and those obtained from STM investigations at the solid-liquid interface is beyond the scope of this letter. However, if the anchoring of 1,10-phenantroline to the Au(111) surface takes place through a chemical bond to a pitted Au adatom (see, for example, Cafe´ and co-workers44), than different molecular architectures may arise at the solid-air interface where the density of pitted Au atoms may be different than in the electrochemical studies. STM imaging was obtained from more than 30 different monolayers. Results were consistent, and no impurities or molecular species were found in the 1,10-phenantroline layer. (43) Lux, F.; Lemercier, G.; Andraud, C.; Schull, G.; Charra, F. Langmuir 2006, 22, 10874–10876. (44) Cafe, P. F.; Larsen, A. G.; Yang, W.; Bilic, A.; Blake, I. M.; Crossley, M. J.; Zhang, J.; Wackerbarth, H.; Ulstrup, J.; Reimers, J. R. J. Phys. Chem. C 2007, 111, 17285–17296.

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Figure 2b shows a 400 × 400 nm2 view of the 1,10-phenantroline layers with no impurities on it (which will be relevant in the following text). To further prove that 1,10-phenantroline forms a uniform layer on the Au(111) surface, we obtained laser desorption mass spectra from several monolayers and different surface spots of area equal to 1 µm2. We repeatedly found a peak at m/z ) 180 in agreement with the 1,10-phenantroline molecular mass (Supporting Information). The long-range ordering of the arrays can be extended by means of annealing, as reported in one of the author communications.23 We have found that preparing mixed solutions of 1,10-phenanthroline with DPPH in dichloromethane results in the intercalation of DPPH molecules into ordered and extended arrays of 1,10-phenantroline. Because annealing can reduce the number of paramagnetic molecules in solution and therefore the number of DPPH molecules that will retain their paramagnetic character at the surface, the annealing temperature and the sample time exposition need to be evaluated. To set the last two parameters to acceptable values, we have studied the electron spin resonance (ESR) spectra of various solutions. Figure 1c illustrates the ESR spectra of a 5 mM methylene chloride (MC) solution of 1,10-phenanthroline with the addition of a few drops of a 1 mM MC solution of DPPH. The spectra show the typical five lines due to the hyperfine interaction of the electron spin with the two nonequivalent protons on the NR and Nβ positions.45 This mixed solution was pipetted into a quartz tube that was immersed in water bath at 80 °C for intervals of 10, 20, and 30 min (for a total of 1 h). ESR spectra were taken at the end of each interval, and solutions were cooled at room temperature prior to acquire the spectrum.46 It is possible to recognize from Figure 1c that the intensity of the ESR signal diminishes upon increasing the heating time. The line integration of each spectra was obtained. We measured the maximum of the integrated line to asses the percentage of DPPH molecules that lose their paramagnetic character. We found 10, 66, and 75% ESR signal loss per time interval. To further explore this, we also obtained ESR spectra of the 1,10-phenanthroline/DPPH mixtures at different temperatures. (See Supporting Information for a discussion of the factors determining the accuracy of these measurements.) We found the annealing time to be a controlled parameter for the loss of ESR signal. Finally, we studied the ESR spectra from a solution of DPPH in MC held at the temperature of 90 °C for 1, 2, and 3 h. We found that 50% of the signal was lost during the first hour of annealing. This finding suggests that the temperature effect is to suppress part of the paramagnetic molecules possibly as a result of the interaction of the DPPH molecules with the solvent. As the DPPH spin density is delocalized over the aromatic rings,45 it is possible that both interaction with the solvent and the 1,10-phenanthroline results in a quenching of the paramagnetic character. However, after 1 h of annealing, 25% of the DPPH molecules preserve their paramagnetic character. This is an acceptable threshold for designing ESN-STM experiments aimed at probing locally the g factor of paramagnetic molecules. We added 0.5 mL of a 0.01 mM MC solution of DPPH to a 10 mL solution of 1 mM 1,10-phenanthroline (DPPH is 0.05% of the molecular species in solution). The Au(111) surface was immersed in the solution for 1 h, and the temperature was kept constant at 80 °C. The sample was then rinsed in MC and dried under nitrogen. (45) Dalal, N. S.; Kennedy, D. E.; McDowell, C. A. J. Chem. Phys. 1973, 59, 3403–3410. (46) The tube was cooled at room temperature prior to recording ESR spectra.

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Figure 3a shows an STM image of the surface prepared as described before. The size of the ordered arrays has significantly increased with respect to that of the previous arrays obtained by overnight incubation. The inset in Figure 3a shows the arrangement of two adjacent 1,10-phenanthroline molecules in a stripe composing the ordered arrays. The arrangement is such that the positively charged portions of one molecule are in proximity to the electronegative center of the adjacent phenyl ring. The apparent corrugation of the adjacent stripes is less than 1 Å as highlighted in Figures 2e and 3c. As illustrated in Figure 3b, a number of protruding features appear in the STM image. These features were not observed in the experiments in which only 1,10-phenanthroline was deposited on Au(111). We attribute these features to DPPH molecules and dimers. Before explaining why we believe the protruding features are DPPH molecules, we briefly revise some of the criteria used in previous studies to distinguish inserted molecules into host layers. Donhauser and co-workers showed that the presence of pits in the host layer is important in accommodating guest molecules.29,30 However, the lateral size of the pit does not necessarily need to match the size of the inserted molecule and can actually be larger. Cunha and co-workers showed there are two types of pits in the 1,10-phenantroline layer self-assembled on (22 × 3)Au(111) at the solid-liquid interface.41,42 One type is found at the boundaries and the second type is found inside the ordered stripes region. We actually find both types of pits. A pit inside the stripes region can be easily seen in Figure 3b, and a line profile is illustrated in Figure 3c; the pit depth is typically 1 to 1.5 Å. Also, Figure 3b shows a pit at the boundaries of the striped region. The pit accommodates a protrusion. (DPPH molecules are seen, and a line profile is shown in Figure 3c.) In addition, Donhauser and co-workers utilize the difference in apparent height between the inserted molecules and the nearby alkanethiol monolayer as a distinguishing proof of the guest molecule presence.29,30 This is an important element as the height measured in STM imaging is rather the result of a convolution of the geometry and density of states of the molecule adsorbed at the surface. Thus, it is accepted to use the apparent height of several protrusions to identify the guest molecule in the layer. In the following text, we will provide similar evidence. Another aspect in which the current work differs from Donhauser and co-workers’s is that the hosting layer is formed in the presence of the guest molecule whereas Donhauser and co-workers deposit the guest molecule after the host alkanethiol monolayer has been grown. The guest molecule can actually alter the self-assembly process in the formation of the hosting layer.47 We focus now on determining a criterion to asses the expected later size for a DPPH molecule deposited on Au(111) in order to make a parallel conclusion with the experimental findings (Introduction). The structure of DPPH has been extensively studied both in solution45,48,49 (electron double-resonance, ELDOR; electron nuclear double resonance, ENDOR; Fourier transform nuclear magnetic resonance, FT NMR) and in frozen glassed solutions50 (ESR). These studies have shown that the structure of the solvated molecule is nearly planar (e.g., the NRNβC bond angle is 180°). However, there is a possibility that the two phenyl rings can (47) We have also attempted to deposit the guest molecule after the hosting layer was formed. However, initial results were not satisfactory because the host layer tended to dissasemble. For this reason, we abandoned this avenue. (48) Dalal, N. S.; Ripmeester, J. A.; Reddoch, A. H. J. Am. Chem. Soc. 1982, 104, 3241–3242. (49) Dinse, K. P.; Biehl, R.; Mobius, K. J. Chem. Phys. 1974, 61, 4335–4341. (50) Dikanov, S. A.; Astashkin, A. V.; Tsvetkov, Y. D. J. Struct. Chem. 1984, 25, 200–204.

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Figure 3. (a) 100 × 85 nm2 STM image of ordered assemblies of 1,10phenanthroline on Au(111) intercalated with agglomerates of DPPH. BV ) 0.3 V and It ) 150 pA. The inset shows the arrangement of two adjacent 1,10-phenanthroline molecules within a stripe. (b) 50 × 50 nm2 STM image of a spot with several ordered stripes. It is possible to recognize two features intercalated into the regular structure of 1,10phenanthroline (see inside circles). We associate these structures with DPPH molecules and agglomerates. (c) Four line profiles of the STM image form. (b) Each profile corresponds to a colored line in b. The red and pink profiles show the difference in height between the 1,10phenantroline stripes and the molecule (about 3 Å ) 300 pm for these features). The green profile shows a pit in the 1,10-phenantroline layer 1 Å deep.

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undergo rotation along the C-NR direction. Similarly, the picryl ring is also expected to undergo a rotation along the C-Nβ because the two meta protons are not equivalent. The results from frozensolution ESR studies show that Nβ has sp2 hybridization and that the NRNβC angle is 118.5°. In addition, the pycril ring is twisted 33° relative to the NRNβC plane. Furthermore, this arrangement is also confirmed by the study of the modulation effects in electron spin echo (ESE) experiments.50 We use the structural data for DPPH inserted in solid matrixes as a reference for the structure of DPPH on Au(111). We have inherently built a geometrical model of the DPPH molecule (Figure 1a) where the pycril ring is out of plane and slightly tilted. Several possibilities for twisting angles of the pycril ring and the phenyl rings have been explored by changing the geometrical model. However, we found that the length of the molecule along the phenyl-pycril axis is 1.2 ( 0.1 nm whereas the DPPH lateral length along the phenyl-phenyl axis is 0.7 ( 0.1 nm. To the best of our knowledge, a detailed STM study combined with the electronic structure calculations36 for DPPH molecules deposited on Au(111) or other surfaces is not available. Therefore, we find that the lateral size assessed by utilizing spectroscopic data from molecules inserted into a solid host matrix helps to define an upper and lower limit for the DPPH length and breath. The (0.1 nm uncertainty quoted above is within our current experimental accuracy and therefore represents no arbitrary choice. We use these data to interpret the STM images described in the following text (obtained from more than 10 monolayers and by using 20 different STM tips). Figure 4a,b illustrates detailed STM images of 30 × 30 nm2 and 15 × 15 nm2 1,10-phenantroline layers including three protruding features. The noise in these features is presumably due to a combination of factors. Wakamatsu and co-workers have studied both the thermal motion51 and diffusion52 of guest molecules in alkanethiols layers. They conclude that both motion forms are present at room temperature for guest molecules. This is particularly possible in the case studied here because the DPPH molecule is not chemically bonded to the Au(111) surface. In addition, STM imaging of DPPH molecules on bare Au(111) is prohibitive when the set point is higher than 30 pA. It is possible that under the tunneling barrier parameters set in the images (in Figure 4) the STM tip can collide against the DPPH molecules while scanning. Because some molecules are diffusing over the host layer, we observe such noise also on some molecules when the tunneling current is set to 1 pA. We find that three different molecular species are present on the surface. One has a diameter of 2 ( 0.2 nm, and the other two have diameters of 1.1 ( 0.1 nm but different heights, as shown in Figure 4c. The heights of the three different molecular species relatives to the host layer base lane are 0.5, 0.4, and 0.20 nm. A comparison of our molecular model and previous data from DPPH molecules deposited directly on Au(111) suggests that the 1.1 ( 0.1 nm molecules can be assigned to individual DPPH molecules. We attribute the difference in apparent height to the orientation of the molecule relative to the surface. The molecules with the greatest height are on top of the 1,10-phenanthroline layer whereas those with the shortest apparent height are inserted into the 1,10-phenanthroline layer. The comparison between the lateral size of the 2 ( 0.2 nm molecule with previously obtained images of DPPH molecules on Au(111) suggests that this molecule is actually a dimer of two individual DPPH molecules. (51) Wakamatsu, S.; Fujii, S.; Akiba, U.; Fujihira, M. Nanotechnology 2003, 14, 258–263. (52) Wakamatsu, S.; Fujii, S.; Akiba, U.; Fujihira, M. Nanotechnology 2004, 15, S137–S141.

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Figure 4. (a) 30 × 30 nm2 STM image of the 1,10-phenantroline layer. BV ) 0.3 V and It ) 150 pA. The blue circle highlights an individual DPPH molecule embedded into the 1,10-phenanthroline layer. The red circle highlights a DPPH dimer positioned on top of the 1,10phenanthroline layer. (b) 15 × 15 nm2 STM image of the 1,10phenantroline layer. BV ) 0.3 V and It ) 150 pA. The black circle highlights an individual DPPH molecule positioned on top of the 1,10phenanthroline monolayer. (c) Line scan profile of the molecules and dimer from a and b. The red line refers to the dimer from a, the blue line refers to the molecule in a, and the black line refers to the molecule in b. Note that the line cuts are highlighted in a and b.

Three controlling experiments were also carried out to further support the evidence that DPPH molecules are inserted into the 1,10-phenantroline layer. In one case, the amount of DPPH solution added to the 1,10-phenantroline solution was progressively increased. STM images of the resulting layers showed a progressive increase in the number of DPPH clusters up to a point where DPPH islands are formed within the 1,10phenantroline layer. This shows a direct dependence of the emergence of protruding features in the 1,10-phenantroline layer with the increased number of DPPH molecules in the incubating solution. Figure 5 shows STM images taken on large areas of two different combinations of DPPH and 1,10-phenantroline relative percentage in the depositing solution. For the a-b sequence, the DPPH concentration in the mother solution was 1.5% whereas in the d-e sequence it was 0.05%. A detailed investigation of several line scans (Figure 5c,f) reveals that the pit depth in the first sequence can be as high as 4 Å whereas in the second sequence the depth is below 1.5 Å. There is therefore tangible evidence that the system guest-host structure varies while the relative concentration of the two components in the mother solution is varied. In the second experiment, we obtained laser desorption mass spectra from monolayers obtained from concentrated solutions

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Figure 5. (a) 400 × 400 nm2 STM of mixtures of 1,10-phenanthroline and DPPH on Au(111) intercalated with agglomerates of DPPH. The concentration of DPPH in the mother solution is 1.5%. BV ) 0.6 V and It ) 1 pA. (b) 200 × 200 nm2 STM enlarged image of part a. It is possible to recognize several islands on top of the Au(111) layer. (c) Height profile for three different line scans taken from b. The islands’ apparent height is 3 to 4 Å, which is significantly higher than in f. Also, the pits are deeper than for the 1,10-phenantroline layer reported in Figure 2. (See the text for additional comments.) (d) 400 × 400 nm2 STM of the 1,10-phenanthroline monolayer with intercalated DPPH on Au(111). The concentration of DPPH in the mother solution is 0.5%. BV ) 0.6 V and It ) 1 pA. (e) 200 × 200 nm2 STM image of 1,10-phenanthroline monolayer with intercalated DPPH on Au(111). Deposition and scanning parameters are the same as in part d. (f) Apparent height profile for three different line cuts highlighted in part d with lines of different colors. The apparent island height (or pit depth) is 1 to 1.5 Å (150 pm) The blue and green profiles also contain a protruding features signaled with a filled circle next to it. These are DPPH molecules as described in the text.

of DPPH annealed at 80 °C. We obtained a peak for DPPH53 (Supporting Information). In a third experiment, we have compared the height range of DPPH molecules deposited directly onto Au(111) to that of the protrusions imaged on the host layer of 1,10-phenantroline. Figure 6 shows a sequence of STM images taken on Au(111) covered (53) Mass spectra from layers prepared from very dilute DPPH solution could not be obtained with our mass spectrometer sensitivity.

with DPPH at two different concentrations. (See the figure caption for details.) A new sequence of STM images of large areas was also acquired for DPPH deposited into and on the host 1,10phenantroline monolayer. In both cases, the bias voltage (+0.6 V) and the tunneling current (1 pA) were kept the same so that the apparent height could be used for comparison.29,30 Figure 7 shows the apparent height histograms for three different set of experiments. For high-coverage DPPH molecules

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Figure 6. Sequence of STM images of Au(111) surfaces covered with DPPH molecules and agglomerates. No 1,10-phenantroline is present in the mother solution from which the DPPH molecules are deposited on Au(111). The sequence from a to c is obtained by deepening the Au(111) into the 0.1 mM solution of DPPH in MC for 45 min. The series from d to f is obtained by exposing the Au(111) surface to DPPH solution in MC 0.01 mM for 20 s. See the text for further details. The images reflect the difference in coverage and DPPH cluster size in the two cases. (a) 400 × 400 nm2 STM of DPPH on Au(111). BV ) 0.6 V and It ) 1 pA for all images a-f. (b) 200 × 200 nm2 STM of DPPH on Au(111). (c) 100 × 100 nm2 STM of DPPH on Au(111). (d) 400 × 400 nm2 STM of DPPH on Au(111). (e) 200 × 200 nm2 STM of DPPH on Au(111). (f) 100 × 100 nm2 STM of DPPH on Au(111).

on Au(111), the best Gaussian fit yields a height of 1.21 ( 0.55 Å. For low-coverage DPPH molecules on Au(111), the best Gaussian fit yields 1.40 ( 0.50 Å. Finally, Figure 7c shows the height histogram for the DPPH molecule and cluster guests in the 1,10-phenantroline matrix. The best Gaussian fit yields two distinct peaks and a third broader peak. The relative heights for the three peaks are h1) 1.49 ( 0.5 Å, h2) 3.40 ( 0.65 Å, and h3) 6.2 ( 0.1.5 Å. The reason that the third set of heights is broadened may be acribed to the presence of different DPPH agglomerates in the host matrix. Some of them may stay on top of the 1,10-phenantroline layer, and some may be inserted into the layer pits. Overall, the number of agglomerates decreases as their height increases, and the majority of molecules seem to be present in the monomer or dimer form. The measured heights seem to be insensitive to the variation of bias voltage and tunneling current, and this suggests that they may actually reflect a true variation in geometrical height. It is clear than that there is a net difference between depositing DPPH directly on Au(111) and in the 1,10-phenantroline layer.

Figure 7. Sequence of apparent heights as digitally measured from STM images. Here we report the height range, meaning the difference between the highest height and the smallest height within the features analyzed. This is obtained by utilizing a feature in the image processing software (SPIP). This feature allows the selection of an elliptical or circular perimeter and returns several measurands. All of the STM images are taken with BV ) 0.6 V and It ) 1pA. (a) Apparent height for DPPH molecules and cluster deposited on bare Au(111) in the absence of 1,10-phenantroline. The deposition conditions are Au(111) immersed in a DPPH solution that is 0.1 mM in MC for 20 s. The best Gaussian fit returns an average apparent height of 1.21 ( 0.55 Å. (b) Apparent height for DPPH molecules and cluster deposited on bare Au(111) in the absence of 1,10-phenantroline. The deposition conditions are Au(111) immersed in a DPPH solution that is 0.01 mM in MC for 20 s. The best Gaussian fit returns an average apparent height of 1.4 ( 0.5 Å. (See the text for comments on this particular value.) (c) Apparent height for DPPH molecules and a cluster intercalated or deposited on top of a 1,10-phenantroline layer grown on Au(111). Deposition conditions are the same as in Figures 3 and 4. The best Gaussian fit returns three peaks with apparent heights distributed as follows: h1) 1.49 ( 0.5 Å, h2) 3.40 ( 0.65 Å, and h3) 6.2 ( 0.1.5 Å.

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The net advantage of the guest-host matrix approach is that DPPH lateral diffusion under the application of tunneling currents as high as 250 pA is reduced with respect to the approach used in the experiments in Figure 6. These results also agree well with other experiments carried in our laboratory. For instance, we obtain the formation of aggregates on top of the 1,10-phenantroline layer when the organic layer is prepared by starting from a mixture containing 1,10phenantroline and fullerenes. Once again, the protruding features in the STM imaging of 1,10-phenantroline appears only when the organic layer is deposited from a mixture of 1,10-phenantroline and a second molecule. In summary, we have deposited single spin probes and dimers into a diamagnetic conducting layer at the solid-air interface and have imaged them with STM. This result opens up intriguing possibilities for studying the local dynamics of spin probes in various configurations. In addition, local experiments on small

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spin ensembles54 may also be possible by creating clusters of DPPH inserted into 1,10-phenantroline. We believe that this protocol is not necessarily restricted to DPPH molecules but can be of more general application. Acknowledgment. This work was supported by the US DOEBES under contract no. DE-AC02-06CH11357. Supporting Information Available: Mass spectrometry and ESR spectroscopy. This material is available free of charge via the Internet at http://pubs.acs.org. LA8039863 (54) Budakian, R.; Mamin, H. J.; Chui, B. W.; Rugar, D. Science 2005, 307, 408–411. (55) Messina, P.; Mannini, M.; Caneschi, A.; Gatteschi, D.; Sorace, L.; Sigalotti, P.; Sandrin, C.; Prato, S.; Pittana, P.; Manassen, Y. J. Appl. Phys. 2007, 101, 053916.