Deposition of intact tetrairon(III) single molecule magnet monolayers on gold: an STM, XPS, and ToF-SIMS investigation

Abstract

The ability of a tetranuclear iron(III) single-molecule magnet functionalized with thioacetyl-terminated ligands to form monolayers on gold has been investigated by a multitechnique approach based on STM, XPS and ToF-SIMS. We discuss in detail several aspects, which are relevant to the reported observation of a memory effect on the monolayer prepared from dichloromethane (Mannini et al., Nat. Mater., 2009, 8, 194). In particular we show that the adsorbate comprises intact surface-bound Fe₄ clusters as opposed to microcrystals or multilayers. The influence of the solvent used for the self-assembly process on the morphology, composition and structure of the adsorbate is also studied for four different solvents (dichloromethane, n-hexane, toluene, 1,4-dioxane).

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1 Introduction

The realization of basic functional units using molecular building blocks represents significantly the stride of fundamental research toward near-future technological applications. The need for completely new paradigms appears most clearly in the field of electronics, when considering that traditional top-down nanofabrication techniques are approaching their physical limits and will soon be unable to satisfy the quest for ever-smaller electronic devices. For this reason, several types of functional molecular materials have been studied to replace classical electronic units such as mass memory storage and logic gates.¹ Among these, single-molecule magnets (SMMs) seem highly promising candidates in the field of magnetic storage and molecular spintronics² because they show magnetic hysteresis³ at the molecular level in addition to rich quantum behaviour.⁴ Since many proposed applications require the addressing of individual molecules,⁵ our group and others have undertaken the organization of derivatives of the archetypal Mn₁₂ SMM on gold^6–8 and silicon surfaces.^9–11 Although the morphological and compositional data obtained using STM and XPS, respectively, suggested the presence of intact SMMs at the gold surface, a number of subsequent experiments using MCD,¹² XAS and XMCD¹³ have proven that Mn₁₂ complexes are chemically unstable when bound to gold^14,15 and are structurally too weak to maintain SMM behavior even at the surface of bulk samples.¹⁶ This case highlights the difficulties inherent to the investigation of polynuclear metal complexes at surfaces and the need for a multi-technique approach.

A different class of iron(III)-based SMMs, namely Fe₄ complexes, has been recently shown to exhibit the chemical and structural robustness required for deposition on surfaces. Their propeller-like structure^17–19 (see Fig. 1) stabilizes a S = 5 ground spin state and generates an anisotropy barrier as high as 17 K.²⁰ Furthermore, they can be functionalized at will to promote interaction with different substrates,²¹ such as gold surfaces²² and silicon substrates²³ or carbon nanotubes.^24,25

Fig. 1 Top: molecular structure of the Fe₄C₉SAc complex with iron atoms drawn as large grey spheres, sulfur atoms as pale grey spheres, carbon as light grey sticks and oxygen as black sticks. Bottom: scheme of the sulfur-functionalized tripodal ligand H₃L used in this work.

Experiments at large-scale facilities using low-temperature XMCD²² have demonstrated that the electronic structure and magnetic properties of Fe₄ molecules deposited on gold from dichloromethane (DCM) solution closely resemble those of bulk phases (antiferromagnetic intramolecular interactions, high-spin ground state, hysteresis loop). This important result confirms the intactness of Fe₄ complexes at surfaces and, at the same time, provides a crucial proof-of-concept, i.e. that SMMs can be linked to metallic surfaces while not altering their characteristic functionality.

The Fe₄ derivative specifically designed for grafting to gold surfaces, hereafter denoted as Fe₄C₉SAc, has formula Fe₄(L)₂(dpm)₆, where Hdpm is dipivaloylmethane and H₃L is the sulfur-functionalised triol ligand 11-(acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol (Fig. 1).²¹ Such ligand design is motivated by the strong affinity of sulfur toward gold; notice that acetyl-protected sulfur atoms are used instead of free thiols, because of the redox instability of iron(III) ions in the presence of –SH groups; notice also that acetyl-protected thiols have been used without deprotection by an exogenous base to generate self-assembled monolayers (SAMs).^26–28 However it is still debated whether the acetyl group is retained when the Au–S bond is formed.^27,28 The long alkyl chain was chosen as an insulating spacer, in order to minimise the interaction of free electrons of gold with the magnetic Fe₄ core.

In this work we present a complete morphological, chemical and structural investigation of Fe₄C₉SAc adsorbates on gold. Four different solvents including DCM have been employed, as strong solvent effects have been already observed in monolayers of Mn₁₂ complexes²⁹ and simpler molecules.^30,31 The solvents examined in this study are DCM (low viscosity, moderately polar), n-hexane (low-viscosity, apolar), toluene (moderately viscous, apolar and aromatic) and 1,4-dioxane (very viscous, polar). Information on the morphology of the surface, as revealed by STM imaging, has been complemented by semi-quantitative chemical composition data obtained using XPS spectroscopy and by structural information provided by ToF-SIMS. Monolayers obtained from these solvents, called Fe₄C₉SAc-exDCM, -exHex, -exTol and -exDiox, respectively, were characterised using the above-mentioned techniques, which demonstrated the intactness of Fe₄ units in all samples but Fe₄C₉SAc-exDiox. Information was gained on the nature of the deposits, which are shown to largely consist of an array of surface-bound complexes rather than aggregates, microcrystals or multilayers. In addition, the arrangement of Fe₄ units on the gold surface and the fate of acetyl groups were inferred.

2 Experimental

Sample preparation

Complex Fe₄C₉SAc was prepared by reacting the precursor [Fe₄(OMe)₆(dpm)₆] and the tripodal ligand H₃L, as detailed elsewhere.^20,21 Anhydrous dichloromethane, 1,4-dioxane, n-hexane and toluene were purchased from Sigma-Aldrich and used as received. The cluster was found to be stable in the four solvents investigated for at least two days, see ESI† for the details on the aging experiments. Gold 5 mm × 5 mm substrates (Agilent Technologies) consisted of 150 nm thick layers of polycrystalline gold evaporated on mica; substrates were annealed with a hydrogen flame immediately before use in order to promote Au(111) reconstruction of the surface. To prepare monolayers of Fe₄C₉SAc, the gold substrates were immersed for 20 hours in a 0.3 mM DCM solution of the complex in the investigated solvents and were subsequently rinsed by immersion in the same pure solvent for 10 minutes. The sample of bulk Fe₄C₉SAc was prepared by drop casting 50 µl of a 0.3 mM solution of the complex on either gold or silicon surfaces, to afford reasonably flat layers with a thickness of a few hundreds of nanometers. All handling operations were carried out under the dry nitrogen atmosphere of a glove box.

STM measurements

In-air STM characterisation was performed on an NT-MDT P47-pro instrument working with a custom made low current head and using 90:10 Pt/Ir tips obtained by mechanical sharpening. Tunneling parameters are reported for each scan in the text.

XPS measurements

XPS measurements were performed on a Perkin-Elmer Φ 5600-ci spectrometer using a monochromatised (1486.6 eV) AlK_α radiation (15 kV, 300 W). The sample analysis area was 800 µm in diameter, and the working pressure was lower than 10⁻⁹ mbar. The spectrometer was calibrated assuming the binding energy (BE) of the Au4f_7/2 line at 83.9 eV with respect to the Fermi level. The standard deviation for the BEs was ±0.2 eV. Samples were mounted on steel holders under dry nitrogen environment in a portable glove bag which was then connected to the fast-entry lock system of the XPS analytical chamber, in order to minimise air exposure and atmospheric contamination. Survey scans were run in the 0–1300 eV range and detailed scans were recorded for the C1s, O1s, S2s, S2p, Fe2p and Au4f photopeaks. All samples were sufficiently electrically conductive at room temperature that no compensation for charging effects was required. The residual BE shifts (<0.5 eV) were corrected by assigning to the C_1s peak associated with aliphatic and adventitious hydrocarbons a BE of 284.8 eV.³² The analysis involved Shirley-type background subtraction³³ and, whenever necessary, spectral deconvolution, which was carried out by non-linear least-squares curve fitting, adopting a Gaussian-Lorentzian sum function. The atomic composition of the samples was calculated by peak integration, using sensitivity factors provided by the spectrometer manufacturer (Φ V5.4A software) and taking into account the geometric configuration of the apparatus. The experimental uncertainty on the reported atomic composition values does not exceed 5%.

ToF-SIMS measurements

ToF-SIMS analysis was carried out with a TRIFT III time-of-flight secondary ion mass spectrometer (Physical Electronics, Chanhassen, MN, USA) equipped with a gold liquid-metal primary ion source. Positive and negative ion spectra were acquired with a pulsed, bunched 22 keV Au⁺ (SAMs) and Au₃⁺ (drop cast samples) primary ion beam, by rastering the ion beam over a 100 µm × 100 µm sample area. The primary ion dose was kept below 10¹¹ ions/cm² to maintain static SIMS conditions.³⁴ Positive mass spectra were calibrated to CH₃⁺ (m/z 15.023), C₂H₃⁺ (m/z 27.023), C₃H₅⁺ (m/z 41.039), and C₁₁H₁₉O₂Fe⁺ (m/z 239.073, [Fe + dpm]⁺); negative data were calibrated to CH⁻ (m/z 13.008), OH⁻ (m/z 17.003), C₂H⁻ (m/z 25.008), and C₁₁H₁₉O₂⁻ (m/z 183.138, [dpm]⁻), in the low mass region. A number of peaks of increasing mass were assigned and added to the calibration set for an accurate mass calibration. Mass accuracy was better than 10 ppm. The mass resolution (m/Δm) was up to 8000 depending on the sample. Also for these experiments a portable glove bag was used to transfer the sample to the UHV chamber.

3 Results and discussion

STM measurements

In the case of monolayers of Fe₄C₉SAc, STM studies were rather difficult due to the presence of a compact layer of molecules, which could be only partly resolved. A representative image of Fe₄C₉SAc-exDCM samples is shown in Fig. 2a. The topography image clearly shows a dense coverage throughout the gold surface. Image analysis reveals that the layer is made up of round objects whose average diameter is 2.5 ± 0.2 nm (see statistics in Fig. 2b), in good agreement with the size estimated for Fe₄ from the crystal structure (1.8 × 1.8 × 3.4 nm).²⁰ The best imaging conditions were found to correspond to an average tunneling interaction regime, with bias voltages around 0.3–0.4 V and tunneling currents of the order of 10 pA; no significant dependence was found on the tunneling conditions for what concerns the ability of the tip to displace Fe₄ units from the surface. Molecular resolution, however, could be achieved only with interactions not stronger than those indicated above.

Fig. 2 STM topography images of typical Fe₄C₉SAc-exDCM (a), -exHex (c) and -exTol (d) monolayers (tunneling conditions: U = 0.35 V, I = 9 pA). Diameter statistics extracted from four images of Fe₄C₉SAc-exDCM samples are shown in (b).

This is probably due to the fact that the tip, while not being able to actually remove the molecules from the surface, as seen instead on Mn₁₂ adsorbates from DCM,²⁹ scans through them producing noisy and unresolved images.

The absence of any long-range ordering in the adsorbate is an expected consequence of the bulky cluster core, which prevents lateral interactions between aliphatic chains and thus disfavours the closely-packed arrangement typical of alkanethiol SAMs.³⁵ Very similar morphologies, i.e. a densely-packed layer of molecules, were found in samples Fe₄C₉SAc-exHex and -exTol (see Fig. 2c and d); on the other hand, STM imaging of Fe₄C₉SAc-exDiox samples failed to detect molecular adsorbates at the surface.

STM imaging is an important tool to detect monolayer formation and to optimize deposition protocols, although topographic information alone can be deceptive and can lead to erroneous conclusions. A clearer view on the compositional and structural properties of the adsorbates was provided by XPS and ToF-SIMS data, which are presented in the following sections.

XPS measurements

In order to have dependable reference data to study monolayers, we carried out a preliminary XPS characterisation on a massive sample of Fe₄C₉SAc obtained by the drop-cast method on gold. The survey scan (reported in ESI†) shows no strong signals from the underlying gold substrate, thus confirming that the technique is sampling bulk Fe₄C₉SAc. Detailed spectra of the S_2p and C_1s peaks are shown in Fig. 3a and b, respectively (Fe_2p and O_1s peaks are reported in ESI†). Atomic percentages are gathered in Table 1 and compared with calculated values, which agree very closely with the observed composition.

Table 1 Comparison between nominal and experimental atomic compositions (% at.) as evaluated by XPS for drop cast and monolayer Fe₄C₉SAc samples. The corrected BE values (eV) of the different photopeaks are also indicated. Data related to Fe₄C₉SAc-exDiox monolayer are not reported

Fe4C9SAc samples	Fe	S	C	O
Calcd for Fe4O20C96S2	3.3	1.6	78.7	16.4
Drop cast	2.9	1.5	78.5	17.1
BE	711.1	163.9	284.8	531.2
-exDCM Monolayer	3.7	2.0	71.5	22.8
BE	711.2	162.7	284.8	532.2
-exHex Monolayer	4.6	2.4	69.5	23.5
BE	710.9	162.1, 163.6	284.8	531.9
-exTol Monolayer	3.7	2.3	73.3	20.7
BE	711.2	162.2	284.8	531.9

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Fig. 3 Detailed XPS spectra of S_2p (a) and C_1s (b) peaks carried out on a drop cast and monolayer samples of Fe₄C₉SAc (labelling scheme in the legend of panel b). In panel a) SI, SII, and SIII indicate the BE of surface-bound, surface-free, and oxidized sulphur components, respectively. In panels (c) and (d) we show the deconvolution for the C_1s peaks of the drop cast and the -exDCM absorbate, respectively. In (e), the grey scale coding on the ligands describes the various components used to deconvolute the C_1s signal. Fe_2p and O_1s peaks are reported in ESI.†

A deeper analysis of the C_1s signal was attempted by peak deconvolution, and three components were found (see Fig. 3c): a dominant one at 284.8 eV that can be ascribed to aliphatic and adventitious carbon; a second component at higher BE (286.3 eV) that is attributed to oxygen-bound carbon in the trimethyolol group and in dpm ligands (see Fig. 3e); a very weak component at even higher BE (288.4 eV) that can be coherently assigned to the electron-depleted thioacetyl carbon. Relative integral values of the three components, which are reported in Table 2 along with their BEs and FWHMs, are in close agreement with the expected values. The O_1s peak could not be resolved into the three components originating from trimethylol, dpm and acetyl groups. Indeed, a limited peak shift is expected for these types of oxygen atoms.^32,36 The sulfur S_2p photopeak is centered at 163.9 eV, i.e. close to the BE exhibited by unbound thioacetyls in acetyl-protected dithiols²⁷ or in derivatives of calix[4]arene³⁷ on gold (163.4–163.6 eV for S_2p(3/2)).

Table 2 Component details, energy position, full width at half maximum, and relative abundance for the deconvolution of the C_1s peak in a Fe₄C₉SAc drop cast deposit and Fe₄C₉SAc-exDCM monolayer

	BE (eV)	FWHM (eV)	% Calc	% Found
Drop cast (Fig. 3c)
CI	284.8	1.8	79.2	81.1
CII	286.3	1.7	18.7	17.0
CIII	288.4	1.8	2.1	1.9
-exDCM monolayer (Fig 3d)
CI	284.8	1.7	79.2	78.6
CII	286.2	2.1	18.7	19.0
CIII	288.8	1.9	2.1	2.4

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XPS studies on monolayer samples Fe₄C₉SAc-exDCM, -exHex and -exTol required very long acquisition times to improve the signal-to-noise ratio. A survey scan, taken in the energy range of 0–1300 eV (see ESI†), is dominated by the strong peaks originating from gold photoelectrons and shows additional signals from iron, sulfur, carbon and oxygen. Detailed scans of the S_2p and C_1s peaks are shown in Fig. 3a and b respectively, while Fe_2p and O_1s peaks are reported in ESI.† The spectral features are very similar to those observed in the drop-cast sample, all photopeaks showing relatively low broadening and the expected BEs. No significant charging effects are evident, indicating that the neutrality of the single layer of molecules is easily restored through the conducting gold surface.

The atomic percentages obtained by peak integration (Table 1) are in reasonable agreement with the calculated ones. In particular, the Fe:S ratio is in the range 1.6–1.9 (vs. 1.9 in the drop-cast sample and an expected value of 2.0), suggesting that the clusters are largely intact at the gold surface. On the other hand, carbon and oxygen integrals do not perfectly agree with the expected values, the former being less than expected, and the latter in slight excess.

The analysis of the S_2p photopeaks provides important information on the arrangement of the Fe₄ units on the gold surface. According to the molecular structure revealed by X-ray diffraction, the alkyl chains are long enough to allow the simultaneous binding of the two sulfur-based “alligator clips” to the gold surface, giving rise to a surface-parallel (or lying down) phase, using classical SAM terminology.²⁷

Alternatively, Fe₄ units could interact asymmetrically with the surface via only one sulfur atom, affording an upright (or standing up) phase as schematized in Fig. 4.

Fig. 4 Schematic view of the standing-up and lying-down grafting modes of the Fe₄C₉SAc SMM, with the relative orientation of the easy axis of magnetization indicated by the black arrow. The sulfur atom not bound to gold of the standing-up configuration is depicted as a dark sphere.

Indeed, the S_2p peak in Fe₄C₉SAc-exDCM and Fe₄C₉SAc-exTol samples is observed at 162.7–162.2 eV vs. 163.9 eV in the bulk reference (Fig. 3). The shift toward lower BE is taken as evidence of an interaction with gold^27,36–38 (a similar shift is observed in the S_2s photopeaks reported in ESI†). Since the peak is slightly broader as compared with bulk Fe₄C₉SAc, we cannot completely rule out the presence of a fraction of surface-free S atoms in these samples. The contribution from unbound sulfur is clearly detectable in the Fe₄C₉SAc-exHex sample, whose S_2p photopeak can be fitted with two components centered at 162.1 (surface-bound sulfur, component S_I in Fig. 3a) and 163.6 eV (surface-free sulfur, component S_II). From the integrated areas, the two sulfur species occur in comparable amounts in the -exHex sample (see ESI†). We attribute these surface-free sulfur atoms to molecules wired to the surface via a single alligator clip or, less likely, forming an overlayer. The simultaneous presence of both surface-parallel and upright configurations of the SMM, with a prevalence of the first, would be consistent with the powder-like magnetic response of the Fe₄C₉SAc-exDCM sample observed within the resolution of XMCD measurements.²² In fact, the lying-down and standing-up molecules are expected to have their easy axes of magnetization in the plane of the substrate and roughly orthogonal to it, respectively (see Fig. 4). A 2:1 proportion of surface-perpendicular and parallel configurations would closely mimic a random distribution, while the ratio between surface-bound and surface-free sulfur atoms (5:1) would still be consistent with XPS data.

Deconvolution of the C_1s signal³⁹ (Fig. 3d) afforded the results reported in Table 2 for the Fe₄C₉SAc-exDCM sample. Similar results were obtained for Fe₄C₉SAc-exHex and Fe₄C₉SAc-exTol, the C_1s spectra being superimposable. The excellent agreement with the calculated percentages provides a strong indication that acetyl groups are still present in the monolayers.⁴⁰ From XPS data alone, however, it is not possible to ascertain whether acetyl groups are still bound to sulfur or remain quantitatively entrapped in the monolayer.²⁸ On the other hand, Fe₄C₉SAc-exDiox samples showed peculiar features, with no detectable XPS signal from sulfur, a little amount of carbon with a shoulder at higher BE and anomalous amounts of iron and oxygen (see Fig. 3a,b and ESI† for details). This finding is coherent with the results of STM imaging (see previous section), that was not able to evidence morphological features comparable to those observed in the other samples.

ToF-SIMS measurements

Secondary ion mass spectrometry proved to be an extremely powerful tool for the inspection of monolayers of Fe₄ on gold. Its soft ionisation mechanism is particularly suited for the desorption of fragile molecules, such as the metal ion polynuclear systems under study. Moreover, the technique is completely surface sensitive, since only the first molecular layers of the sample are involved in the ionic bombardment and subsequent desorption.

First, positive and negative ToF-SIMS spectra of Fe₄C₉SAc drop cast on Si were acquired as a bulk reference (Fig. 5).

Fig. 5 Positive (a) and negative (b) ToF-SIMS spectra of Fe₄C₉SAc drop cast on Si (* signals from silicon grease contamination).

The characteristic peaks detected and their assignments are listed in Table 3. In the high mass region (m/z 800–2200) of the positive ion spectrum (Fig. 5a), peaks are detected which are assigned to ion fragments generated by loss of one, two, and three dpm moieties from the molecular ion. Similar ion fragments have been previously observed on thin films of non-functionalized Fe₄ derivatives obtained by thermal evaporation.⁴¹

Table 3 Characteristic peaks detected in the positive and negative ToF-SIMS spectra of Fe₄C₉SAc drop cast on Si and self-assembled on Au and their assignments

Drop cast	Monolayers on Au				Fragment
	-exDCM	-exHex	-exTol	-exDiox
Positive ions
56	56				Fe^+
57	57				^57Fe^+/FeH^+
—	197				Au^+
239	239				[Fe + dpm]^+
422	422				[Fe + 2dpm]^+
733	733				[3Fe + 3dpm + O]^+
1036	1036				[3Fe + 3dpm + L + O]^+
1380	very weak (vw)				[M − 3dpm]^+
—	1488				[M − 2dpm − SAc]^+
—	1520	vw	1520	—	[M − 2dpm − Ac]^+
1563	1563	vw	1563	—	[M − 2dpm]^+
—	—	—	—	1568	[M* − dpm]^+
weak	1671				[M − dpm − SAc]^+
—	—	—	1686	—	[M − dpm − SAc − H + O]^+
—	1702	1702	1702	—	[M − dpm − Ac]^+
1746	1746				[M − dpm]^+
—	1900				[M − dpm − Ac + Au]^+
—	2051	vw	2051	—	[M − SAc + Au]^+
—	—	—	2066	—	[M − SAc − H + O + Au]^+
—	2083	vw	2083	—	[M − Ac + Au]^+
—	2126				[M + Au]^+
Negative ions
32	32				S^−
75	75				[SAc]^−
183	183				[dpm]^−
—	197				Au^−
902	—				[2Fe + dpm + 2L]^−
1085	—				[2Fe + 2dpm + 2L]^−
1507	—				[M − Fe − 2dpm]^−
1929	—	1929	—	1929	[M]^−
—	1934				[M*]^− ≡ [M − Ac + 3O]^−
—	vw	vw	vw	2131	[M* + Au]^−

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The intense peak at m/z 1036 is attributed to the ion fragment with formula [3Fe + 3dpm + L + O]⁺ originated by rearrangement of the Fe₄ complex after loss of one Fe atom, three dpm groups and one tripodal ligand. In the low mass region (m/z 0–800, Fig. 5a), Fe⁺ and other characteristic ion peaks are observed. The high mass region of the negative ion spectrum (m/z 800–2200, Fig. 5b) is characterised by the molecular ion peak at m/z 1929 ([M]⁻), and other signals assigned to molecular ion fragments generated by loss of Fe atoms and/or dpm moieties.

Positive and negative ToF-SIMS spectra were then recorded on Fe₄C₉SAc monolayers prepared by dissolving the Fe₄ complex in the four solvents. The recorded spectra present a rich variety of molecular secondary ions and show peaks assigned to molecular ions ([M]⁻), to key molecular ion clusters ([M + Au]⁺) and fragments (e.g. [M − dpm]⁺, [M − 2dpm]⁺, [M − 3dpm]⁺), demonstrating the presence of Fe₄C₉SAc complexes on Au.

In Fig. 6, we gather the high mass regions (m/z 800–2200) of the positive ion spectra recorded on the four samples. Besides the distinctive peaks of the Fe₄C₉SAc complex detected in the drop-cast sample, new characteristic signals are observed in the spectra of monolayers (Table 3). The Fe₄C₉SAc-exDCM sample (Fig. 6a) shows peaks at m/z 1702 ([M − dpm − Ac]⁺) and m/z 1520 ([M − 2dpm − Ac]⁺) assigned to molecular ion fragments generated by loss of the acetyl group, as well as ion–Au and molecular fragments–Au clusters at m/z 1900 ([M − dpm − Ac + Au]⁺), m/z 2083 ([M − Ac + Au]⁺), and m/z 2126 ([M + Au]⁺). These last peaks are not observed in the drop-cast sample, thus excluding that they are originated by the combination with the Au⁺ of the beam. The positive ion spectra acquired on Fe₄C₉SAc-exHex, -exTol, and -exDiox show some differences and similarities in the fragmentation patterns.

Fig. 6 Positive ToF-SIMS spectra of Fe₄C₉SAc-exDCM (a) and (b), -exHex (c), -exTol (d), and -exDiox (e) SAMs on Au. The mass regions m/z 800–2200 (a) and m/z 1500–2200 (b–e) are shown.

The -exDCM and -exHex (Fig. 6c) spectra are identical, except for differences in the peak relative intensities, whereas the -exTol (Fig. 6d) spectrum shows additional features. Finally, the -exDiox spectrum (Fig. 6e) differs from the previous three spectra by the absence of the peaks at m/z 1900, 1702, and 1520 and the presence of a new peak at m/z 1568 tentatively attributed to an oxidized molecular ion fragment [M* − dpm]⁺ (Table 3), as discussed later.

The high mass regions (m/z 800–2200) of the negative ion spectra are characterised by very intense signals from the Au substrate (Fig. 7). All monolayer spectra show an intense peak at m/z 1934, slightly above the molecular ion. A more detailed investigation of the isotopic distribution (see ESI†) allows us to assign it to a species [M*]⁻ with formula [Fe₄(LL*)(dpm)₆]⁻, where L* is an oxidized tripodal ligand containing a terminal sulfonate group (R–SO₃⁻). Such a partially-oxidized species is absent in the drop-cast sample and is accompanied by the molecular ion peak in -exHex and -exDiox samples.

Fig. 7 Negative ToF-SIMS spectra of Fe₄C₉SAc drop cast (a) -exDCM (b), -exHex (c), -exTol (d), and -exDiox (e) SAMs on Au.

The oxidation of sulfur to sulfonate has been previously observed in non-tightly packed layers of thiolated molecules upon exposure to atmospheric oxygen.^42–44 The extent of oxidation was closely related to the surface coverage and to the layer thickness, which determine the permeability of the layer to atmospheric oxidants.⁴⁵

A similar situation is expected to hold in the monolayers under study, where a tight-packing arrangement of alkylsulfanyl chains cannot be achieved due to the bulky cluster core. Notice that a hint of a S_2p component around 168–169 eV (S_III component in Fig. 3a), attributable to oxidised sulfur,^27,37 is only visible in the XPS spectra of the -exHex and -exDiox samples. The failure to clearly resolve such oxidized sulfur species in XPS spectra may be related to the different sensitivities and detection mechanisms of the two techniques. In fact, whereas XPS provides a quantitative detection of all S-containing species, the ToF-SIMS spectra are influenced by the interaction strength of the analyte with the substrate, as well as by ionization yields. In this respect, sulfonates are expected to be very easily detected by the ToF-SIMS technique because of their weak interaction with the surface and their negative charge. As a confirmation of our hypothesis, the ozone-induced oxidation of alkanethiol monolayers on gold was reported to become detectable at a much earlier stage by ToF-SIMS than by XPS.⁴⁶

The absence of the [M*]⁻ peak in the bulk reference as well as other peculiarities observed in the fragmentation patterns of -exDCM, -exHex and -exTol monolayers, like the extensive clustering with Au, support the occurrence in these samples of a large fraction of intact Fe₄ complexes interacting with the Au surface via their terminal alligator clips. In addition, fragments are detected in all monolayer samples which still contain one ([M − 2dpm − SAc]⁺, [M − dpm − SAc]⁺, [M − dpm − Ac]⁺, [M − dpm − Ac + Au]⁺, [M*]⁻, [M* + Au]⁻) or two ([M + Au]⁺, [M − dpm]⁺, [M − 3dpm]⁺) acetyl groups. Combined with the XPS signature of an extensive S–Au interaction (especially in sample Fe₄C₉SAc-exDCM), the ToF-SIMS data suggest that absorption can be largely (though not exclusively) promoted by unprotected thioacetyls.

4 Conclusions

We have studied the morphology, composition and structure of a tetrairon(III) SMM functionalized with thioacetyl alligator clips and deposited on gold from four different solvents. Using DCM and toluene, the features of the S_2p peak observed in XPS spectra at around 162 eV established an extensive S–Au interaction, thus ruling out the presence of a dominant fraction of aggregated or multilayered material on the surface. ToF-SIMS analysis confirmed the predominantly monolayer nature of the adsorbate by showing the presence of characteristic ion patterns and of extensive clustering with Au. In particular, we detected a partially-oxidized molecular ion featuring one terminal sulfonate group, as previously observed in alkanethiol monolayers deposited on gold. According to STM imaging, the adsorbates comprise disordered but densely-packed arrays of molecules.

Deposition from n-hexane afforded similar results, though with a broader S_2p photopeak, taken as evidence of comparable amounts of surface-bound (162.1 eV) and surface-free (163.6 eV) sulfur. By contrast, use of 1,4-dioxane failed to give adsorbates showing the expected morphology and XPS signatures.

The work presented here not only addressed the questions concerning the specific system under study, but also suggests some general methodological guidelines for the study and characterisation of monolayers of complex systems. The deposition of intact SMM layers is directly proved by their XMCD-detected magnetism.²² However, laboratory-scale instrumentation can be employed with success to individuate the most effective deposition protocols, provided that several techniques are used in parallel. In this respect, combined STM, XPS and ToF-SIMS studies contributed to gain considerable insight into the nature of SMM adsorbates and must be regarded as a valuable tool for the investigation of nanostructured molecular materials.

Acknowledgements

This work was supported by the EC projects: FP6-NMP3-CT-2005-515767 NE-MAGMANet, FP7-ICT-2007-211284 MolSpinQIP, ERANET “NanoSci-ERA: NanoScience in the European Research Area” (SMMTRANS – Three terminal transport through Single Molecule Magnets). Financial support by Italian projects: CNR-INSTM Promo, MIUR FIRB RBNE033KMA, MSE Industria 2015, ALADIN, and CARIPARO 2006 (Multi-layer optical devices based on inorganic and hybrid materials by innovative synthetic strategies) is also acknowledged.

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Footnote

† Electronic supplementary information (ESI) available: Details of ligand and samples preparation, ²H-NMR stability experiments, XPS and ToF-SIMS additional spectra. See DOI: 10.1039/b916895h

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