Thiol-functionalization of Mn 5 Ge 3 thin films

Thiol-functionalization of Mn5Ge3 thin films Marta Schütz, Matthieu Petit, Lisa Michez, Alain Ranguis, Guillaume Monier, Christine Robert-Goumet, Jean-Manuel Raimundo To cite this version: Marta Schütz, Matthieu Petit, Lisa Michez, Alain Ranguis, Guillaume Monier, et al.. Thiolfunctionalization of Mn5Ge3 thin films. Applied Surface Science, Elsevier, 2018, 451, pp.191-197. ฀10.1016/j.apsusc.2018.04.231฀. ฀hal-01784119฀ HAL Id: hal-01784119 https://hal-amu.archives-ouvertes.fr/hal-01784119 Submitted on 3 May 2018 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Published in Applied Surface Science, available online 04/30/2018 DOI: 10.1016/j.apsusc.2018.04.231 Article + supplementary informations Thiol-functionalization of Mn5Ge3 thin films 5 Marta K. Schütz1+ Matthieu Petit1* Lisa Michez1 Alain Ranguis1 Guillaume Monier2 Christine RobertGoumet2 Jean-Manuel Raimundo1* 1 Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), Aix Marseille Université, CNRS, CINaM UMR CNRS 7325, 13288 Marseille, France 10 2 Institut Pascal, Université Clermont Auvergne, CNRS, SIGMA, F-63000 Clermont-Ferrand, France Present address: + School of Chemistry, Pontifical Catholic University of Rio Grande do Sul (PUCRS), Av. Ipiranga, 6681, Porto Alegre, RS, Brazil. * Corresponding authors: [email protected], [email protected] 15 HIGHLIGHTS The surface state of Mn5Ge3 exposed to air and common colvents, is analyzed by XPS. The surface free energy of Mn5Ge3 is calculated with the OWRK method. The grafting of Mn5Ge3 films by octane- and perfluorodecane-thiol SAMs is proposed. 20 KEYWORDS thiol; passivation; self-assembled monolayers; manganese germanide; Mn5Ge3; wettability; spintronics; surface free energy. 25 ABSTRACT Hybrid organic/inorganic interfaces could pave the way to chemically designed or new multifunctional electronic devices, in particular in the spintronics field where, for instance, the interfacial spin polarization can be tuned through chemical interactions and surface modifications. We report herein, for the very first time, the assays of self-assembled monolayers (SAMs) formation on Mn5Ge3 surface. 30 Interestingly, Mn5Ge3 is a ferromagnetic metal possessing interesting features for spintronics such as a high Curie temperature, a capability to grow epitaxially on Ge germanium that is a key point for integration in the mainstream Si technology. Mn5Ge3 thin films are synthesized by molecular beam epitaxy under ultra-vacuum condition. We studied the Mn5Ge3 surface prior to deposition of SAMs, meaning the surface exposed to air and different solvents, by XPS and contact angle measurements 35 leading to the value of the surface tension of this surface. Then SAMs of octanethiol and perfluorodecanethiol are formed on Mn5Ge3 surface at room temperature. The best experimental conditions to form the SAMs are found for an immersion time of 36 h and a concentration of 4 mM. 1 INTRODUCTION 40 Over the last decade the manganese germanide Mn5Ge3 has appeared to be a potential candidate for spin injection into group-IV semiconductors due to its room-temperature ferromagnetism properties (Curie temperature TC= 297 K), high spin polarization, metallic character, ability to epitaxially grow on Ge(111) substrates as well as compatibility with the already-existing Si-Ge-based technology [1–3]. Moreover, several studies have shown that its TC can be enhanced by carbon doping into the interstitial 45 sites of the Mn5Ge3 lattice and that the Schottky contact between Ge and Mn5Ge3 may be suitable for electrical spin injection [4–7]. Furthermore, hybrid structures of organic molecules and inorganic materials have been recently in the spintronic community's crossroad. The first purpose was to use the molecules as spin transport media because of the rather weak coupling between the spin and orbital momentum of the electron that allows keeping the spin orientation over long distances. It has been 50 rapidly anticipated that molecules could also have strong impacts on the magnetic properties, for instance, several teams dealing with spin valves reported negative magnetoresistance [8,9]. These phenomena confirmed the growing interest on the developing molecular “spinterface” which could lead to single multi-functional devices thanks to the “LEGO” capability of molecular engineering [10–13]. Molecules and biomolecules possessing a magnetic core are potential candidates to be grafted on the 55 magnetic substrate Mn5Ge3. One of the most promising approaches to modify surface involves the deposition of organic self-assembled monolayers (SAMs). SAMs constitute a method of choice to achieve highly dense and ordered thin films, robustly anchored to large surface areas either on clean metallic (e.g. Au, Ag), oxidized (e.g. SiO2, ITO) or semiconducting (e.g. Si, Ge) substrates. This method does not require either sophisticated and expensive instrumentation, or intricate sample 60 preparations, expert manpower and allows the opportunity to vary both the length of the chains and functional groups to manipulate macroscopic properties of the surface, such as wettability, biocompatibility and protein/cellular adhesion [14–17]. Furthermore, magnetic properties of metals and diluted magnetic semiconductors (DMS) such as (Ga, Mn)As have been tailored by SAMs surface modifications leading to induced magnetic moments or modification of ferromagnetic properties [18– 65 20]. Alkanethiols are so far the most studied self-assembled molecules because of the covalent polarized bond formed between thiol headgroups and appropriate substrates (e.g. Au, Ge, Ag, Cu, Pt, Hg, GaAs). Regarding Ge surface, HBr and HCl are often used to halogenate the surface of this semiconductor prior to the SAMs formation, mainly to avoid the process of reoxidation of the surface. Indeed, the 70 classical hydrogenation, which is used for silicon surface passivation, is less stable in the case of the Ge surface [21–25]. The inherent ability of adsorbed monolayers to modify physicochemical properties of surfaces makes this approach attractive to implement the organic functionalization of the surface of the Mn5Ge3/Ge system. To the best of our knowledge, functionalization of Mn5Ge3/Ge with alkylthiols has never been done before. The Mn5Ge3 thin films being elaborated under ultra high vacuum conditions by 75 molecular beam epitaxy (MBE), we present herein a study of the surface state of the Mn5Ge3 thin films under air and common solvents exposure and secondly the first insights on the functionalization of Mn5Ge3 with thiol molecules. 2 MATERIALS AND METHODS 80 2.1 Substrates. Sb-doped Ge(111) wafers (n-type, 600 µm thickness, 12-16 Ω cm) were purchased from Umicore (Belgium). Mn5Ge3/Ge(111) heterostructures were grown by molecular beam epitaxy (MBE) technique according to the method described by Petit & al [26]. This method is based on the co-deposition at room temperature of Mn and Ge. It allows growing high crystalline quality films of Mn5Ge3 with an 85 abrupt interface on Ge(111) and with a small RMS roughness (ca. 2 nm). The Mn5Ge3 thin films were 30 nm thick. 2.2 Grafting procedures. 2.2.1 Cleaning. Mn5Ge3/Ge(111) substrates were first successively sonicated in acetone, deionized water and ethanol. Each cleaning step was repeated three times with a fresh solvent for 5 min, followed 90 by blow-drying with argon gas. 2.2.2 Thiol-functionalization. Cleaned Mn5Ge3/Ge(111) substrates were immersed into octanethiol (OT) (CH3(CH2)7SH, Alfa Aesar) or perfluorodecanethiol (PFDT) (CF3(CF2)7CH2CH2SH, SigmaAldrich) solutions for surface functionalization. The experiments were carried out at room temperature to avoid any interdiffusion of Ge and Mn at the Mn5Ge3/Ge interface that could be detrimental for 95 further spintronic applications. Freshly thiol solutions were prepared in ethanol prior to be used. The vials were backfilled with argon gas in order to minimize O2 in the headspace. After experiments, the substrates were rinsed three times with ethanol to remove physisorbed molecules on the surface and dried over 24 h in a desiccator at room temperature. We have mainly investigated two parameters: i) the thiol concentration (from 1 to 5 mM) and ii) the immersion reaction time (from 12 to 24, 36, 48 and 100 72 h). First, a range of concentration of thiol was tested, at fixed immersion time of 24h, in order to determine the optimal concentration needed. Optimum condition of concentration was selected (4 mM, see the Results and Discussion) and experiments were conducted by varying the immersion times. 2.3 Characterizations. Foremost, a chemical analysis of the untreated Mn5Ge3 substrates have been done by X-ray 105 photoelectrons spectroscopy (XPS) in an ultrahigh vacuum system equipped with an Omicron DAR 400 X-ray source. The Mg anode was used to perform the analysis. The hemispherical analyser (HSA) is an Omicron EA 125 (normal detection was used). The angle between the X-ray source and the HSA was equal to 55°. The topographies of the substrates were imaged by atomic force microscopy (AFM) with a Nanoscope IIIA Multimode (Digital instruments) equipped with a 10×10×2.5 µm scanner. The 110 images were recorded in the tapping mode at room temperature using silicon nitride probes (HQ: NSC15/AL BS, Mikromasch). The curvature radius of the silicon tips was about 8 nm (from the supplier specifications). Contact angle measurements were performed for the assessment of the hydrophobic-hydrophilic character of the substrates. Static contact angles (CA) were measured with an OCA 15 apparatus (DataPhysics) at room temperature using the sessile drop method and image 115 analysis of the drop profile (SCA20 software). Deionized water droplet volume was 5 μL, and the contact angle was measured 10 s after the drop was deposited onto the surface. 3 RESULTS AND DISCUSSION 3.1 Characterization of the Mn5Ge3/Ge(111) substrates before the grafting process. 120 3.1.1 XPS: The Mn5Ge3 samples were grown in a MBE chamber and taken out from the UHV chamber for the grafting process. The different samples were exposed successively to atmosphere and chemical cleaning prior undergoing the SAMs grafting procedures. To our knowledge this is the first report describing both aging changes of the Mn5Ge3 surface under air exposure, stability under chemical cleaning/etching with polar aprotic or protic solvents such as acetone and ethanol respectively. 125 After air exposure, Mn5Ge3 surface has been analyzed by XPS in order to determine the chemical state of the surface atoms and then the surface free energy has been measured. The Ge3d, O1s and Mn2p core levels were recorded and the parameters for the XPS fits are given in the electronic supplementary information (ESI). The Figure 1 shows the electronic state of the Mn5Ge3 surface after air exposure. Figures 1a and 1b exhibit the evolution of the Ge3d core level before (cleaned) and after air exposure 130 whereas fig. 1c concerns the evolution of the Mn2p core level. The “cleaned” Ge3d core level exhibits one peak at 29.1 eV, related to Ge atoms in the bulk state in Mn5Ge3 (blue contributions in Figure 1a and 1b respectively). On Fig.1b, the peak around 31.5 eV is typical of germanium oxide. It can be deconvoluted into two main contributions corresponding to the Ge atoms in the oxidized states Ge2+ and Ge3+ [27]. Thus, the oxidized state of the Ge atoms differs from a cleaned (non oxidized) Mn5Ge3 135 surface and to a Ge(111) substrate in which the Ge4+ state, corresponding to the GeO2 oxide (see ESI), is prevailing. This difference is also confirmed by the analysis of the O1s core levels (see ESI). Regarding the Mn2p core level (Fig. 1c), the oxidation by air leads to a strong shift of about 2.5 eV at higher energy that could be attributed to the Mn atoms in oxidized states around 2-2.7+ similar to those found in MnO or Mn3O4 [28]. These features confirm a clear difference for the native oxide structures 140 of Ge(111) and Mn5Ge3 suggesting that the latter may correspond to a ternary oxide of Mn-Ge-O. 145 150 155 Figure 1: XPS spectra related to the effect of air exposure on the Mn5Ge3 surfaces. a) Reference: Ge3d core level of a cleaned (non oxidized) Ge(111) substrate. b) Deconvolution of the Ge3d core levels from an oxidized Mn5Ge3 surfaces. c) Superposition of Mn2p3/2 core levels of a cleaned and oxidized Mn5Ge3 layers. d) Evolution of the contributions of the Ge3d core level according to a depth profile of oxidized Mn5Ge3 thin films done by ionic Ar+ bombardment. 160 Moreover the Fig. 1d exhibits the evolution of the Ge0 as well as the oxide contributions Ge2+ and Ge3+ for the Ge3d core level along a depth profile in the oxidized Mn5Ge3 layer. The depth profile has been made using an Ar+ ionic sputtering. We have evidenced that the oxidation of the Mn5Ge3 thin film took place in the first 4 nm, after one week of air exposure. This indicates that the oxidation of the Mn5Ge3 165 film is not limited to the very surface and oxygen diffuses into the layer. Prior the SAMs formation, the Mn5Ge3 film surface is successively cleaned with acetone then ethanol and carefully analysed each step by XPS (Figure 2). The Ge3d core level is followed after each solvent rinse highlighting specific behaviours (Fig. 2a, 2b). For instance, after acetone cleaning, the surface exhibits a Ge4+ contribution unlike the ethanol case. The deconvolution profile after ethanol treatment is very similar to the one of 170 the air exposed sample and also the intensity of the overall oxide component is smaller than in that case. This last observation could be explained by the fact that GeO2 is soluble in water and that we used a 96% ethanol. This is consistent with several studies regarding the influence of wet surface treatments and SAM deposition processes which have demonstrated that water or a mixture of water and ethanol can dissolve germanium oxide [24,29,30]. Surprisingly, concerning the Mn2p3/2 and O1s core levels (Fig. 175 2c and 2d), no significant discrepancy can be noticed. The O1s shapes slightly differ due to the stoichiometry changes of the oxide accordingly to the solvent used and to the water sensitivity of the germanium oxide component of the overall oxide. 180 185 Figure 2: XPS analysis of the Mn5Ge3 surface after exposure to acetone and ethanol. a) and b) Ge3d core level. c) Mn2p3/2 and d) O1s core levels. (For c) and d), the air exposed case is included as a comparison). 190 These XPS results clearly show that the oxide layer does not consist only on a germanium oxide layer but contains also a manganide part. Consequently the nature of the oxide layer may be close to that of a ternary oxide Mn-O-Ge. The oxidation state of the Mn5Ge3 oxide layer slightly changes depending on environmental conditions. This observation was also carried out on Ge surfaces, which are known to exhibit Ge2+ and/or Ge4+ oxidation states depending on the surface treatments [31,32]. 195 3.1.2 Surface free energy (SFE): SFE determinations have been achieved using the Owens, Wendt, Rabel and Kaelble (OWRK) method with the measure of the contact angles (CAs), with different liquids (deionized water, formamide, glycerol and dichloromethane), on the air exposed Mn5Ge3 surface [33–35]. Based on the Fowkes' method, the surface free energy is divided into a polar part (non-dispersive, P) and a dispersive part (D) (Eq.1) [36]: 200 σ sl =σ s +σ l −2 ( √( σ D D s σl )+ √ ( σ Ps σ lP )) (Eq.1) The interfacial tension σsl between liquid and solid is calculated with the two surface tensions σs and σl of the solid and the liquid and the similar interactions between the phases. These interactions are interpreted as the geometric mean of a dispersive component σD and a polar one σP of the surface tension. The Figure 3 displays the plot of the OWRK method. 205 210 Figure 3: Plot of the Owens, Wendt, Rabel and Kaelble method for calculating the surface free energy of the Mn5Ge3 thin film exposed to air. The corresponding linear fit affords a dispersive contribution of σDMn5Ge3 = 14.7 mJ.m-2 and a polar part 215 of σPMn5Ge3 = 31.8 mJ.m-2, which corresponds to a total SFE of σMn5Ge3 = 46.5 mJ.m-2. As for comparison, the SFE of SiO2, an extensively studied surface for SAM deposition, is within the range of 40-77 mJ.m2 with a predominance of the polar component fostering the interactions with polar head-groups such as thiol derivatives [37–39]. In addition, it is known that under vacuum conditions, the surface of Mn5Ge3 is terminated by Mn atoms therefore the SFE value of an unchanged surface might be close to that of 220 the Mn metal, around 1350-1600 mJ.m-2 [40,41]. Moreover, the reported SFE value for a pure manganese oxide surface (Mn3O4, 72 mJ.m-2 ) supports the XPS data, evidencing that the surface of a Mn5Ge3 thin film is strongly affected both by air exposition and solvents treatments [42]. 3.2 Grafting of the Mn5Ge3/Ge(111) substrates. 225 The Mn5Ge3/Ge(111) substrates have been subjected to SAMs surface modifications with two different alkyl thiols namely OT and PFDT (1-octanethiol and perfluorodecanethiol, respectively) by varying the thiol concentrations and then the immersion reaction times. The morphology and wettability of the newly formed SAMs were investigated with AFM and water CA measurements, respectively, for a given immersion time of 24h. The surfaces roughness obtained from the AFM images and the CA 230 values are presented on the Figures 4a and 4b (the AFM images are available in the ESI). 235 240 245 Figure 4: a)-b) RMS Roughness (from 2×2 μm2 AFM images) and water CA versus thiol molecule concentrations for Mn5Ge3 surfaces grafted with a) OT and b) PFDT SAMs. (Immersion time = 24h). c)-f) AFM images 500×500nm2 of OT SAMs for concentration of 0 (image c) (i.e. Mn5Ge3 bare surface), 3 (image d), 4 (image e) to 5 mM (image f) respectively, along with the corresponding profile cross-sections. 250 It was observed that OT molecules could easily chemisorb onto the Mn5Ge3 surface without any significant changes in the surface roughness, except for concentrations ≥ 5 mM. Increasing the solutions concentration (≥ 5 mM) ensue the formation of multilayers that affect dramatically the roughness (fig. 4f). Noteworthy, if the concentration is lower than 4 mM (Figures 4d: 3mM, 4e: 4mM) 255 small separated aggregates of about 30-50 nm in diameter and 2-2,5 nm in height are present within the monolayer. This height is in the range of the length of the grafted molecules (~2,5 nm). A different behaviour was obtained when PFDT was used to passivate the surface of the substrate. Results obtained with PFDT molecules exhibited higher RMS roughness on average but with a greater dispersion of the values. Water CA measurements for each experiment have revealed that the hydrophobicity of the 260 Mn5Ge3/Ge SAM-modified substrate does not significantly change between both thiol molecules regardless the used concentration for an immersion reaction time of 24 hours, reaching values around 85°. However, PFDT molecules provided the highest values of CA at lower concentrations than OT grafting which is normally expected due to higher hydrophobic character of the perfluoroalkyl chains compared to the alkyl chains. 265 In order to evaluate the effect of the immersion time on the Mn5Ge3/Ge functionalization, we have performed the experiments with OT solutions at a concentration of [C] = 4 mM as, both the dispersion of the RMS roughness and the CA values are lower than those measured with the PFDT. The immersion reaction time was varied from 12 to 72 hours. RMS roughness and the water CA are displayed in Figure 5. 270 275 Figure 5: RMS roughness and water CA of Mn5Ge3/Ge substrates after functionalization with 4 mM OT as a function of immersion time. 280 No significant difference on the hydrophobicity nor the roughness of the surface was observed by varying the immersion time. The maximum value of the CA was obtained for 36h of immersion and reached 95°, before decreasing slightly. However, the grafting process occurs before 12h of immersion. We observed by AFM (not shown here) that a more homogeneously ordered and dense layer is formed after 36 hours of immersion whereas prolonged immersion time (> 48 hours) conducts to the formation 285 of aggregates and less ordered layers. This behaviour is similar to the kinetic observed for nalkylthiolates in 1:1 water-ethanol solvents mixture deposited on Ge surfaces [21,24]. Nevertheless, the measured CAs remain lower than the ones reported on Ge surfaces, around 100-110°. This feature could be ascribed to an uncompleted coverage of the Mn5Ge3 surface by the OT SAM or a deterioration of the SAM upon extended immersion time that could facilitate the diffusion of ethanol till the film 290 surface and leading to a possible dissolution of the oxide layer (attested by a decrease of the CA after 36h). 4 CONCLUSIONS We presented herein a simple and effective method to deposit SAMs on Mn5Ge3 thin films. We have 295 evidenced that thiol derivatives (i.e. OT and PFDT) can easily chemisorb onto Mn5Ge3 surface as selfassembled monolayers. Our results showed that changes in surface roughness are more pronounced with PFDT than with OT molecules. The optimal conditions, based on the concentrations and immersion reaction time, to ensure well-ordered, dense and homogeneous SAMs were found to be a thiol concentration of [C] = 4 mM and an immersion time of 36 hours. Higher values of these 300 parameters resulted mainly in an increase of the surface roughness due to the formation of aggregates on the surface and less ordered SAM layers. Comparing the SAMs from OT and PFDT we did not observe any significant changes in the wettability properties of the film surface even it was expected that perfluorinated alkylthiols could render the Mn5Ge3 surfaces more hydrophobic (C–F bonds are more hydrophobic than the C–H bonds). Further improvements on the deposition methodologies still 305 need to be matured for Mn5Ge3 substrate and could lean for instance towards the use of a solvent mixture of 1:1 ethanol-water and a process temperature around 60-80°C which has been proved to be efficient for the deposition of SAMs onto Ge substrates [21,24]. Moreover, further XPS analysis should be performed on the grafted surfaces so as to assess the formation of the covalent polarized bonds between the molecules and the Mn5Ge3 substrate. 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Schütz,1 Matthieu Petit,1* Lisa Michez,1 Alain Ranguis,1Guillaume Monier,2 Christine Robert-Goumet,2 5 Jean-Manuel Raimundo1* 1 Centre Interdisciplinaire de Nanoscience de Marseille (CINaM), Aix Marseille Université, CNRS, CINaM UMR CNRS 7325, 13288 Marseille, France 2 Clermont Université, Université Blaise Pascal, Institut Pascal, BP 10448, F-63000, Clermont-Fd, France 10 * Corresponding authors: [email protected], [email protected] 15 1- XPS : Survey spectrum of the Mn5Ge3 surface after air exposure 2- Table related to the fitting parameters used to deconvolute the O1s and Ge3d core S1 S2 levels (data in eV: accuracy = +/- 1 eV) 3- Ge3d core levels recorded on (left) a cleaned Ge(111) substrate cleaned Mn 5Ge3 S3 thin film and (right) an oxidized Ge(111) substrate after air exposure. The Ge 4+ contribution related to the GeO2 oxide, is prominent 4- Comparison of the O1s core levels of an oxidized Ge(111) substrate and an S4 oxidized Mn5Ge3 thin film after air exposure. 5- 2x2 μm2 AFM topography images of Mn5Ge3/Ge substrate after functionalization S5 with octanethiol (OT) for 24 hours as a function of thiol concentration: a) Mn 5Ge3 surface prior to grafting, b)-f) OT concentrations 1, 2, 3, 4, 5 mM. 6- 2x2 μm2 AFM topography images of Mn5Ge3/Ge substrate after functionalization S6 with perfluorodecanethiol (PFDT) for 24 hours as a function of thiol concentration: a)-e) PFDT concentrations 1, 2, 3, 4, 5 mM. S0 20 1- XPS: Survey spectrum of the Mn5Ge3 surface after air exposure S1 2- Table related to the fitting parameters used to deconvolute the O1s and Ge3d core levels (data in 25 eV: accuracy = +/- 1 eV): After air exposure O1s Ge3d O1s Ge3d FWHM (eV) 1.5 1.1 1.5 1.1 %LG 20 36 20 36 Position of contribution 1 (eV) 529.8 29.1 529.6 29.1 A1/Apeak 0.67 0.43 0.65 0.51 Position of contribution 2 (eV) 531.0 30.9 531.1 30.8 A2/Apeak 0.25 0.20 0.30 0.19 Position of contribution 2 (eV) 532.1 31.7 532.3 31.7 A3/Apeak 0.07 0.37 0.05 0.30 30 35 40 S2 45 After chemical rinsing 3- Ge3d core levels recorded on (top) a cleaned Ge(111) substrate cleaned Mn 5Ge3 thin film and (bottom) an oxidized Ge(111) substrate after air exposure. The Ge 4+ contribution related to the GeO2 oxide, is prominent 50 55 S3 4- Comparison of the O1s core levels of an oxidized Ge(111) substrate and an oxidized Mn 5Ge3 thin film after air exposure. 60 S4 5 65 5- 2x2 μm2 AFM topography images of Mn5Ge3/Ge substrate after functionalization with octanethiol (OT) for 24 hours as a function of thiol concentration: a) Mn 5Ge3 surface prior to grafting, b)-f) OT concentrations 1, 2, 3, 4, 5 mM. 70 S5 75 6- 2x2 μm2 AFM topography images of Mn5Ge3/Ge substrate after functionalization with perfluorodecanethiol (PFDT) for 24 hours as a function of thiol concentration: a)-e) PFDT concentrations 1, 2, 3, 4, 5 mM. 80 85 S6