Surface-Grafted Paramagnetic Macrocyclic ... - ACS Publications

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STM Study of Au(111) Surface-Grafted Paramagnetic Macrocyclic Complexes [Ni2L(Hmba)]+ via Ambidentate Coligands Christian Salazar,*,† Jochen Lach,‡ Florian Rückerl,† Danny Baumann,† Sebastian Schimmel,† Martin Knupfer,† Berthold Kersting,‡ Bernd Büchner,†,§ and Christian Hess†,§ †

Leibniz Institute for Solid State and Materials Research (IFW-Dresden), Helmholtzstrasse 20, 01171 Dresden, Germany Institut für Anorganische Chemie, Universität Leipzig, 04103 Leipzig, Germany § Center for Transport and Devices, Technische Universität Dresden, 01069 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: Molecular anchoring and electronic properties of macrocyclic complexes fixed on gold surfaces have been investigated mainly by using scanning tunnelling microscopy (STM) and complemented with X-ray photoelectron spectroscopy (XPS). Exchange-coupled macrocyclic complexes [Ni2L(Hmba)]+ were deposited via 4-mercaptobenzoate ligands on the surface of a Au(111) single crystal from a mM solution of the perchlorate salt [Ni2L(Hmba)]ClO4 in dichloromethane. The combined results from STM and XPS show the formation of large monolayers anchored via Au−S bonds with a height of about 1.5 nm. Two apparent granular structures are visible: one related to the dinickel molecular complexes (cationic structures) and a second one related to the counterions ClO4− which stabilize the monolayer. No type of short and long-range order is observed. STM tip-interaction with the monolayer reveals higher degradation after 8 h of measurement. Spectroscopy measurements suggest a gap of about 2.5 eV between HOMO and LUMO of the cationic structures and smaller gap in the areas related to the anionic structures.

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INTRODUCTION Organic magnetic materials are nowadays intensively investigated due to their potentiality in the development of a new generation of devices based in electron spin currents. The latter grounds in two aspects: (i) organic molecules could have the capability to build up circuits in a bottom-up approach reaching smaller sizes than the actual electronic circuits; and (ii) organic molecules could preserve spin-coherence over longer times and distances in contrast with conventional metals or semiconductors.1,2 In that sense, the development of chemical systems that contain magnetically bistable transition metal complexes arranged on planar surfaces could contribute to the architecture of materials with controllable magnetic or spintronic properties,3−6 leading to potential applications on information storage at the molecular level,7,8 in molecular electronics,2 and molecular spintronics.1,9−11 Research lines have been mainly focused on the physisorption or chemisorption of planar organometallic complexes like phthalocyanines,12−14 porphyrines,15−18 and spin-crossover compounds. Recently, single molecule magnets (SMMs) bearing several coupled transition metal ions have received increasing attention,19,20 and several strategies have been developed to obtain these materials as rows, 21 thin films,11,22−24 or multilayers.25−31 Unfortunately, polynuclear transition metal complexes suffer from limited thermal and © 2016 American Chemical Society

kinetic stability, which does not allow their thermal evaporation on surfaces.32 Therefore, alternative methods have been considered for surface functionalization. Among them all, chemisorption of open-shell transition metal complexes via Au−S bonds in the form of self-assembled monolayers (SAM) has turned out to be an attractive and suitable method.33,34 For the deposition of polynuclear complexes with single molecular magnet (SMM) behavior two major strategies have been established. The first approach consists in the growth of a SAM of a bifunctional thiol HS-X-L, where X represents a spacer and L usually a carboxylate function, and afterward the adsorption of a SMM. The second method, which we address here, utilizes an exchange-coupled mixed-ligand complex with an exposed thiol function that is directly deposited onto the gold surface.20,35−37 [The SMM behavior of carboxylatebridged complexes of the type [Ni2L(O2CR)]+ (CR, carbonradicals) were previously substantiated by HF-EPR measurements. However, the magnetic anisotropy barrier is too small for getting sufficient magnetization at finite temperature.57] Monolayers produced by adsorption from solution are not Received: February 28, 2016 Revised: April 18, 2016 Published: April 19, 2016 4464

DOI: 10.1021/acs.langmuir.6b00772 Langmuir 2016, 32, 4464−4471

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a saturated atmosphere of evaporated dichloromethane, preventing a quick evaporation of the 2ClO4 solution as well as contamination through contact with air. Finally, the substrate was washed with dichloromethane and ethanol, dried under vacuum, and transferred to the STM. Prior to the measurements the sample was subjected to an annealing process at 70 °C for 3 h in order to exclude the presence of organic solvents on the sample. XPS Measurements. X-ray photoemission spectroscopy (XPS) experiments have been carried out using a two-chamber ultrahigh vacuum system. The measurement chamber with a base pressure of about 2 × 10−10 mbar is equipped with an electron-energy analyzer PHOIBOS-150 purchased from SPECS and an X-ray tube providing monochromatic Al K α radiation with a photon energy of 1486.6 eV. The total energy resolution of the spectrometer was 0.35 eV. More detailed information concerning photoemission spectroscopy can be found in ref 41. STM Measurements. The measurements were carried out at room temperature in a home-built STM attached to the preparation chamber in high vacuum conditions (pSTM ∼ 10−9 mbar). On the other hand, low temperature (70 K) measurements were conducted in an Omicron variable temperature STM under ultrahigh vacuum conditions (pSTM ∼ 10−10 mbar). Regarding tip preparation, Pt−Ir wires were mechanically prepared in a standard manner and subsequently treated by applying voltage pulses on the surface of a clean tungsten crystal (W(110)). Topographic images were acquired in constant current mode, and single point I(U) curves were measured with an open feedback loop at the given stabilization parameters. The spectroscopy curves were averaged out of 30 measurements.

restricted to gold and can be applied to other substrates like silicon, silicon oxides, or metal oxides.38,39 Previously, the synthesis of the perchlorate salt [Ni2L(Hmba)]ClO4 (2ClO4) has been reported.40 This compound is composed of a paramagnetic dinickel macrocyclic complex ([Ni2L(Hmba)]2+) with an S = 2 ground state and negative axial anisotropy and a perchlorate ion (ClO4−). In the cation, L represents a 24-membered macrocyclic hexaazadithiophenolate ligand and Hmba an ambidentate 4-mercaptobenzoate ligand (HO2CC6H4SH, H2mba). The ambidentate ligand has a hard carboxylate function that serves to bind to the NiII ions in [Ni2L]2+ and a soft thiol group for the chemisorption to the gold surface (see Figure 1). Here, we report the evaluation of

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RESULTS AND DISCUSSION Monolayer Characterization via Core Level Spectroscopy (XPS). Figure 2 depicts the obtained spectrum for the Figure 1. Chemisorption of dinickel macrocyclic complexes [Ni2L(Hmba)]ClO4 (2ClO4) on Au(111).

the molecular anchoring and electrical conductance of these complexes on a Au(111)-surface via ambidentate mercaptobenzoate ligands by using STM and XPS. We found anchored monolayers via Au−S bonds with a height of about 1.5 nm apparently showing a granular structure, with the dinickel complexes appearing as roundish structures and the ClO4− counterions as the surrounding environment. No type of long or short-range order was observed. Tip-interaction reveals higher degradation rate after 8 h of measurement. Spectroscopy measurements suggest a HOMO−LUMO gap of about 2.5 eV in the cationic structures of dinickel complexes and a smaller gap in the surrounding environment.

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Figure 2. XPS S 2p core level data for a monolayer of [Ni2L(Hmba)]ClO4 chemisorbed on Au(111). The experimental spectrum (gray) could be fitted (red) by the use of the contributions of sulfonate groups (orange), gold-bound thiolate (green), and bridging sulfur functions (blue).

EXPERIMENTAL SECTION

Preparation of the Substrate. Two different Au(111) single crystals were first sputtered with argon ions at 2 kV using a filament current of 10 mA for 8 h. After the sputtering process the crystals were annealed at 550 °C at a chamber pressure of 5 × 10−9 mbar for 12 h. A posterior STM topographic constant-current measurement revealed for the crystal used in the experiments at room temperature a typical Au(111) surface reconstruction with atomic steps shown in Figure S1a. The crystal used in the experiments at low temperature additionally showed the well-known herringbone surface reconstruction (Figure S1b,c). Preparation of Monolayers. A few drops of a 1.4 mM solution of 2ClO440 in dichloromethane were deposited on the gold single crystal until the surface was fully covered. Then the sample was stored at room temperature under a dichloromethane atmosphere for 15 h (see schematic of the setup in Figure S2). This setup assured a selective contact of the 2ClO4 solution only with the gold crystal, avoiding possible contaminations coming from the sample holder made of tungsten. The liquid dichloromethane in the setup contributes to form

sulfur (S) 2p core level. The main photoemission feature is located in the region between 161 and 167 eV photon energy. A second broad feature can be found around 170 eV. The data could be fitted very well by using three different contributions. The broad feature at higher energies can be assigned to a sulfonate group (RSO3−), which stems from an X-ray-induced decomposition of the ClO4−.40,42 Note that there is only minor decomposition due to a preferably short measurement time, which resulted in a lower sulfonate percentage compared to former studies.40 The main feature can be divided into two spin−orbit split contributions:43 one at 165.3 and 164.1 eV and the other at 163.4 and 162.6 eV binding energy. The ratio between both contributions was found to be 2:1. From the spectroscopic 4465


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low number of much brighter and much darker objects), whereas in Figure 3b the contrast mainly arises from a compact arrangement of roundish structures where the Au(111) surface steps appear less clear. This result evidence the presence of a monolayer on the surface of a gold crystal which is, however, detected only at relatively high bias voltages. The latter fact suggests discrete electronic states in the molecular structures and located relatively far away from the Fermi energy in contrast with the continuous electronic states presented in bulk materials. Therefore, the bias voltage of 1 V achieves the electronic tunnelling current mainly involving occupied states of the tip and unoccupied states of the gold. However, in the case of 2.5 V, the tunnelling current is established between the occupied states of the tip and unoccupied states of the monolayer (see Figure S3). Looking at different areas of the sample, we could determine that the monolayer covers a large surface of the gold crystal (Figure 4a is another example of the surface topography). Figure 4b shows a digital zoom-in of the marked area (dashed white line) in Figure 4a, and Figure 4c depicts a line profile along the arrow in Figure 4b. As it can be seen in these data, the characteristic minimum distance between two neighboring roundish bright structures is between 3−4 nm with a typical height of about 1.5−2.0 nm (for more details, see Figure S4). We checked for evidence of long-range and shortrange order of these structures by investigating both the Fourier transformed and autocorrelated data. However, no such evidence was observed. Monolayer Characterization via STM at Low Temperature. Figure 5a shows a topographic image of a sample with a roughness of 1.958 ± 0.002 nm. I(U) curves taken at different positions of the surface in Figure 5a result in an average curve shown in Figure 5b. Such averaged-curve indicates an insulating sample with a gap that is larger than 3 eV. Interestingly, after six measurement hours we can observe in Figure 5c that the surface in panel a modifies its appearance reducing the roughness to 1.933 ± 0.002 nm. Furthermore, some fissures on the surface start to show up. After 18 measurement hours, in Figure 5d, it is possible to clearly identify dark tracks corresponding to the gold surface and other bright tracks corresponding to the remaining monolayer. A line profile taken along the dark-arrow in Figure 5d probes a height of approximately 1.5 nm shown in Figure 5e. Figure 5f is a broad topographic image (1.2 μm × 1.2 μm) taken in a

point of view, the feature at lower binding energy distinguishes itself by a higher electron density and a resulting different screening in comparison to the feature at higher energies. The ratio of 2:1 gives an additional hint on the bonding situation of the sulfur atoms in [Ni2L(Hmba)]ClO4. According to the literature, the first contribution at higher binding energies can be addressed to the bridging sulfur functions while the other at lower energies to the gold-bound thiolate group,40,44−46 whereby two out of three sulfur atoms of the compound are bridged while the remaining one is bonded to the gold substrate. We note that the shape of the Ni 2p3/2 core level (not shown) with its well-known shakeup satellites is characteristic for divalent, six-coordinate nickel complexes and hence implies that there are no redox state changes upon surface fixation.40 Thus, the XPS data confirm the bonding of the molecules to the gold surface. Monolayer Characterization via STM at Room Temperature. Figure 3a,b shows typical topographic results

Figure 3. (a) Backward topographic image after the 2ClO4 deposition acquired with Ubias = 1 V, I = 10 pA, and T = RT. (b) Forward topographic image after the 2ClO4 deposition acquired with Ubias = 2.5 V, I = 10 pA, and T = RT.

obtained after ex situ deposition of 2ClO4. The two panels represent the bias-selective backward and forward images of the same surface area, simultaneously obtained. The topographic data shown in Figure 3a was obtained with Ubias = 1 V during the backward-moving direction of the tip, and Figure 3b was obtained with Ubias = 2.5 V during the forward-moving direction of the tip. The former seems to be less sensitive to the molecular electronic states than the latter, as in Figure 3a the contrast that apparently predominantly arises from the steps of the underlying Au(111) surface (apart from a relatively

Figure 4. (a) Constant-current topographic image after 2ClO4 deposition (I = 10 pA, Ubias = 2.5 V, T = RT). (b) Digital zoom-in of square area (dashed white line) in (a). (c) Line profile along arrow in panel b. 4466


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(Figure 6 bottom-left, black areas = holes) and afterward, by subtracting from the total area, the area of the monolayer

Figure 6. Monolayer (ML) area versus time.

(Figure 6 bottom-left, green areas = ML). The degradation tendency is shown in the curve of Figure 6. The monolayer is slightly modified in the first 8 h, covering the surface in more than 96%. A linear fit for the data in the first 8 h (not shown) gives a degradation rate of approximately 0.3% per hour. After 8 h of continuous measurement, the film is faster removed with a rate of approximately 1.7% per hour as is evidenced in the slope of the curve. A further zoom-in, marked with a dashed white line rectangle in Figure 5d, is shown in Figure 7a. It reveals some protrusions on the surface, which are qualitatively comparable with the bright structures plotted in Figure 4b,c. These topographic features corroborate our earlier notion that the surface-bonded dinickel complexes can be directly observed in STM topography measurements. The observed inhomogeneity in size and aspect is reasonable considering a likely random orientation of the molecules on the surface (see further below). Furthermore, the formation of agglomerates of several molecules is likely to play a crucial role here, particularly in view of the molecules’ propensity to stack via π···π or CH···π interactions.47 It is worth mentioning in this regard that the behavior of benzene rings to aggregate via CH···π bonds is wellknown48 and such interactions are often observed in dinuclear complexes.49 In order to analyze the conductance of the sample, spectroscopic curves were taken in different points indicated as G, M, X1, and X2. The results are shown in Figures 7b,c,d. The I(U) curve taken at G (orange curve in Figure 7b) shows no gap and linear behavior around zero bias in good agreement with the band structure of metals and therefore, in this case, corresponds to the gold surface. The I(U) curve taken at G is slightly different from pertinent data in pristine Au(111),50 most likely due to the close proximity of the molecules. On the other hand, the curve taken at M (black curve in Figure 7b) indicates a gap of approximately 2.5 V, which is not strictly symmetric with zero bias. In Figure 7c, we plot I(U) curves recorded at different points within the monolayer (M, X1, X2). The differential conductance (dI/dU) (Figure 7d) was calculated out of the curves in Figure 7c by numerical differentiation. A clear reduction of the gap in the surrounding environment (X1, X2) of the roundish structures marked with M is observed. Discussion. The relatively large initial surface roughness of the low-temperature experiment (Figure 5a) is likely due to the ex situ conditions of the deposition process.

Figure 5. Topographic constant-current image of complex 2ClO4 on Au(111) (I = 30 pA, Ubias = 2 V, T = 70 K) revealing tip interaction with the monolayer. (a) Topography after 36 min, (b) characteristic I(U) and dI/dU spectra in (a) (stabilization conditions: I = 30 pA, Ubias = 2.5 V, T = 70 K), (c) topography after 6 h, (d) topography after 18 h, (e) line profile indicated by the black arrow in (d), (f) zones of monolayer before and after tip interaction.

different spot of the monolayer after similar measurements described in Figure 5a−e. In Figure 5f the color scale was changed in order to see clearly different zones before and after tip-interaction. Zone 1 corresponds to the area where the tip was approached. In zone 1, the tip scanned for approximately 1 h, after that, the scan area was changed to zone 2 (dashed black line, 500 nm x 500 nm) where similar measurements were conducted as described for Figure 5a−e and approximately for the same time (18 h). The blue-dark lines within zone 2 correspond to the gold surface, and the red features at its rim correspond to removed parts during scanning from the monolayer. Zones 3 and 4 are areas where no previous measurements were carried out, whereas zone 4 represents the monolayer in a higher step of the gold crystal. In order to determine how much the tip interaction modifies the sample, the area corresponding to the monolayer was measured in a series of topographic images of 500 nm x 500 nm taken systematically during 18 h (some images correspond to Figure 5a,c,d). The calculation of the area was carried out identifying first the area of gold surface by selecting the points on the topography with values of height lower than 0.75 nm 4467


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Figure 7. (a) Zoom-in image marked with dashed white line rectangle in Figure 5d and digital zoom-in (30 nm x 15 nm) marked with dashed black line rectangle. (b) Tunnelling current versus bias voltage taken in the point G (orange line) and in M (black line). (c) Comparison of the I(U) curves taken in M, X1, and X2. (d) First derivative of I(U) curves in (c) (stabilization conditions: I = 30 pA, Ubias = 2.5 V, T = 70 K).

Figure 8. Schematic of the tip-interaction with the monolayer.

that several factors could be playing a role in the film modification, it is difficult to clearly establish which one has stronger influence in our experiments. In view of all this, the reason for the apparent two-stage degradation remains unknown. A possible explanation is the critical damage that is reached after ∼8 h of measurement, where uncompensated electrostatic forces due to previously created defects in the monolayer facilitate the degradation. The length scale over which the film is damaged, in a wavy track manner, may be related with the herringbone surface reconstruction of Au(111), which is known to exhibit different surface reactivity associated with different surface coordination.53 One thus may speculate that the bond strength between the sulfur atoms and the Au(111) varies spatially, tracking the herringbone structure. It is interesting to note in this context that the longer periodicity of the herringbone structure in Au(111)54 is on the order of ∼30 nm, which is comparable with the observed width of the molecular stripes in Figure 5d. Furthermore, one cannot exclude the possibility that inhomogeneous electrostatic forces, likely originated from defects in the molecular arrangement, may also contribute to the mentioned wavy degradation pattern. The monolayers shown in this work exhibit roundish structures surrounded by an environment that in this particular case (bias voltage of 2.5 and 2 V) have a darker contrast (e.g., Figures 4b and 7a). Considering that the chemical precursor is a salt, these structures surrounding the molecular complexes should correspond to the counterions ClO4−. It lets us propose in Figure 8 a growth granular model where two structures are involved. One structure depicted as brown-round entities, representing the cationic dinickel molecular complexes and a

As can be seen in Figure 5c,d,e, the tip-interaction modifies the surface of the film basically reducing the surface roughness and after long scanning time removing parts of the film leaving evidence of the gold surface. The removed parts could be found on the remaining monolayer (red contrast) and also aside from the scan area as is visible in Figure 5f. However, it is also possible that some of them are desorbed to the vacuum. Fieldinduced and current-induced mechanisms are likely to be responsible of the film modification. On one hand, strong electric fields provided by high bias voltages (higher than 2 V in this case) may yield field-induced molecular desorption. Alternatively, a chemical degradation mechanism, promoted by high electric fields, is likely to be related to electronic reduction processes in the layers: Although the macrocyclic complexes are resistant toward reduction in solution, in a solid phase some reduction processes and thus the destruction of the [Ni2L(Hmba)]+ molecules due to the negatively charged tunnelling tip appear possible. Furthermore, it could be possible that reduction of the counterions (ClO4−) occurs, which has already been observed in XPS experiments.40 On the other hand, relatively high tunnelling currents also contribute to molecular desorption.33,51,52 We observed in this work that the sample analyzed using a current of 30 pA and a bias voltage of 2 V (Figure 5) suffered higher modification in contrast to the sample analyzed using 10 pA and a bias voltage of 2.5 V (Figures 3 and 4). For both mechanisms described above, it is clear that long-term measurements enhance the removing process. Finally, a nonproper adjustment of the bias voltage, in order to tunnel within molecular electronic states, leads to a possible mechanical interaction between the tip and the sample, which also could remove the monolayer. Taking into account 4468


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Figure 9. (a) Proposed binding model of a single 2ClO4 to gold. (b) Aggregation model for molecular pairs on gold (top view). Dashed lines indicate the CH···π interactions that potentially lead to molecular binding.

second one composed by anions of ClO4−, depicted in orange. In this model, the dinickel complexes do not have the chance for establishing any kind of order as they are surrounded by the anionic structures, which does not let them interact properly through any of the ordering forces. The spectroscopic data taken at the roundish structures marked with M suggest a gap of 2.5 eV, which is in good agreement with the electronic structure of organic layers with a gap in the order of eV, as it has been theoretically shown in related structures.55 On the other hand, the spectroscopic curves taken in the surrounding structures (X1 and X2) show a reduction of the gap. The origin of this relatively small gap is unclear, but may be attributed, in part, to the potential barrier between the tip and sample becoming similar to that of the tip−gold configuration. Surface Binding Model. Figure 9a shows a likely orientation of a single 2ClO4 on the gold surface. Such a single molecule can be understood as the elementary building block for the observed structures on the surface, either the single molecules or agglomerates of several molecules. The height of the [Ni2L(Hmba)]+ complex on the surface in this case is given by the distance between two opposing N−Me substituents, approximately being 12.5 Å. In an alternate orientation (not shown in Figure 9a) the [Ni2L]2+ fragment is rotated by 90° around the O2CC6H4−S bond. In this case, the height is given by two opposing tert-butyl CH3 groups, being 15.5 Å (H32a···H37b); the latter value is calculated by adding the van der Waals radii of two H atoms (2.4 Å) to the atom, resulting in a distance of 13.123 Å. This value is in reasonable agreement with the thickness determined by STM measurements (see Figures 4c and 5e). Regardless of the actual orientation of the [Ni2L]2+ fragment, the surface complex is presumably also stabilized by van der Waals interactions between the Au surface and the methylene or methyl groups of the supporting macrocycle. However, the thiolate sulfur atoms are out of range of bonding interactions with the Au surface. A further stabilization is likely to occur and compatible with the lateral size of the observed structures through molecule··· molecule attraction by noncovalent CH···π interactions and lead to molecule pairs (see schematic drawing in Figure 9b).

the monolayer due to field and current-induced desorption until the point of damage in some regions. Single point spectroscopy indicates a gap for the cationic dinickel structures of about 2.5 eV and a gap reduction in the anionic environment becoming close to the tip-gold configuration. The height of the monolayers suggests that the molecular complexes are not perpendicularly anchored to the gold surface, but they are canted.

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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/acs.langmuir.6b00772. Characterization of clean Au(111) surfaces, ex situ deposition of the dinickel molecular complexes, illustration of the tunnelling process and corrugation analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Phone: (++49)(0)351-4659501. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft DFG-FOR 1154 “Towards molecular spintronics”. Christian Salazar thanks the German Academic Exchange Service (DAAD) for the scholarship for doctoral studies. The authors kindly acknowledge WSxM Solutions for the free software WSxM.56

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ABBREVIATIONS STM, scanning tunnelling microscopy; L, 24-membered macrocyclic hexaazadithiophenolate ligand; Hmba, HO2CC6H4SH; CR, carbon-radicals; SMM, single molecule magnet

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CONCLUSIONS This work successfully demonstrated the possibility of anchoring paramagnetic molecular complexes to gold surfaces via ambidentate ligands. The molecular complexes form monolayers with a height of about 1.5 nm. The monolayers contain two apparently granular structures: one cationic structure corresponding to the dinickel molecular complexes, and another anionic one corresponding to the counterions. No evidence of long or short-range order of these structures was found. Further characterization reveals the tip interaction with

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