Unsupported Films Research Articles

Unsupported Pt Nanoparticles: Synthesis, Deactivation, and Hydrogen Electrocatalysis in Unpurified Electrolytes

Platinum is ubiquitous in electrochemical catalysis owing to its ability to accelerate redox reactions involving surface-bound hydrogen and oxygen. Accordingly, Pt is used as a calibration standard and activity benchmark against which novel electrocatalysts are compared. These measurements are often executed in unpurified, reagent grade electrolytes where Pt is also susceptible to deactivation by several routes. This constitutes a challenge where the ease of making measurements with Pt-based catalysts must be balanced against the difficulty of obtaining accurate and consistent results. We report herein a synthetic procedure for unsupported Pt nanoparticles that uses readily available reagents and laboratory apparatus, with the goal of making high-quality control experiments in electrocatalysis as accessible as possible. We also identified conditions under which these particles deactivate in unpurified aqueous acid and base and differentiated between mechanisms involving catalyst poisoning, which dominates at more negative applied potentials, and particle growth, which dominates at positive potentials where Pt-oxide species are produced. Finally, we demonstrated that unsupported Pt nanoparticle films can be used to good effect for reference electrode calibration and benchmarking of hydrogen evolution/oxidation electrocatalysts, even in unpurified electrolytes, provided steps are taken to minimize the impact of deactivation.

Quantification of Unsupported Thin-Film X-ray Spectra Using Bulk Standards.

A quantification model which uses standard X-ray spectra collected from bulk materials to determine the composition and mass thickness of single-layer and multilayer unsupported thin films is presented. The multivariate model can be iteratively solved for single layers in which each element produces at least one visible characteristic X-ray line. The model can be extended to multilayer thin films in which each element is associated with only one layer. The model may sometimes be solved when an element is present in multiple layers if additional information is added in the form of independent k-ratios or model assumptions. While the algorithm is suitable for any measured k-ratios, it is particularly well suited to energy-dispersive X-ray spectrometry where the bulk standard spectra can be used to deconvolve peak interferences in the thin-film spectra. The algorithm has been implemented and made available in the Open Source application National Institute of Standards and Technology DTSA-II. We present experimental data and Monte Carlo simulations supporting the quantification model.

Metal-induced crystallization of amorphous semiconductor films: Nucleation phenomena in Ag-Ge films

The initial stages of interfacial interactions between Ag nanoparticles and amorphous Ge (a-Ge) films, in particular the mechanism and kinetics of metal-induced crystallization (MIC) of a-Ge films are investigated herein. Unsupported Ag-Ge films formed by PVD were used as a model. In situ high-resolution (S)TEM and low-loss monochromated STEM-EELS techniques were used for real-time observation of the nucleation and growth of crystalline Ge and intermediate phases, as well as mapping of chemical elements at the reaction zone. We show that crystallization is activated at the metal–semiconductor interface around ≈200 °C, followed by lateral crystallization of a-Ge film at temperatures above ≈400 °C. It was revealed that the MIC process occurs through intermediate phases. Metastable Ag0.8Ge0.2 hcp phase was identified at the reaction front at elevated temperatures, thus revealing the non-equilibrium melting of a eutectic alloy as the most probable mechanism of Ag-induced crystallization of a-Ge film. We show that the MIC process is activated only if the size of Ag nanoparticles exceeds a critical value, which is ≈8 nm in our case and equal to the thickness of the a-Ge film.

Devitrification reduces beam-induced movement in cryo-EM.

As cryo-EM approaches the physical resolution limits imposed by electron optics and radiation damage, it becomes increasingly urgent to address the issues that impede high-resolution structure determination of biological specimens. One of the persistent problems has been beam-induced movement, which occurs when the specimen is irradiated with high-energy electrons. Beam-induced movement results in image blurring and loss of high-resolution information. It is particularly severe for biological samples in unsupported thin films of vitreous water. By controlled devitrification of conventionally plunge-frozen samples, the suspended film of vitrified water was converted into cubic ice, a polycrystalline, mechanically stable solid. It is shown that compared with vitrified samples, devitrification reduces beam-induced movement in the first 5 e Å-2 of an exposure by a factor of ∼4, substantially enhancing the contribution of the initial, minimally damaged frames to a structure. A 3D apoferritin map reconstructed from the first frames of 20 000 particle images of devitrified samples resolved undamaged side chains. Devitrification of frozen-hydrated specimens helps to overcome beam-induced specimen motion in single-particle cryo-EM, as a further step towards realizing the full potential of cryo-EM for high-resolution structure determination.

Carbon Supported Ni-Mo Catalysts for Reversible Alkaline Hydrogen Electrochemistry

Alkaline anion exchange membrane (AAEM) electrolyzers are attractive for cost-effective production of hydrogen using carbon-free electricity as the energy input. These devices facilitate the use of non-precious metal catalysts and ancillary components, which together offer significant reduction in capital cost. When combined with the low and still rapidly decreasing cost of renewable electricity, AAEM electrolyzers have the potential to meet the cost target of $2/kg for hydrogen production set by the US Department of Energy. Despite this promise, considerable work remains to develop efficient and stable catalysts and membranes for use under alkaline operating conditions. In this context, non-precious Ni-based catalysts are among the most promising for use as hydrogen-evolving cathodes. Our focus is on Ni-Mo composites, which have been studied and deployed for decades as alkaline hydrogen evolution catalysts, but the associated reaction mechanism remains relatively less explored.We recently showed that Ni-Mo catalysts comprise a Ni-rich metallic core surrounded by Mo-rich oxide shell, where the electrically resistive shell limits the overall catalytic activity in unsupported thin films. By contrast, incorporation of conductive carbon black supports significantly enhances catalyst mass activity toward the alkaline hydrogen evolution reaction (HER) without measurably altering the composition and structure of the active Ni-Mo composite. More recently, we have directly observed the evolution of the core-shell architecture in Ni-Mo catalysts using in situ environmental transmission electron microscopy (ETEM). These measurements suggest that thermal reduction of NiMoO4 precursors proceeds in at least two sequential events. When combined with HER catalytic activity measurements, the ETEM data show that the most competent Ni-Mo catalyst requires intimate contact between a metallic, Ni-rich face-centered cubic alloy phase and a Mo-rich suboxide phase. Carbon supported Ni-Mo catalyst composites were also found to be active for the alkaline hydrogen oxidation reaction (HOR), which lends to their potential use in unitized reversible fuel cell systems. Preliminary results suggest that the Ni-Mo mass loading relative to the carbon support has a stronger influence of its HOR activity than for the HER, suggesting that mass transfer plays a significant role in the practical HOR activity.

Cassava-based biocomposites as fertilizer controlled-release systems for plant growth improvement

One of the main problems caused by intensive agriculture, is the consequent water pollution resulting from fertilizer leaching, especially as a result of fertilizers overuse. Therefore, in the present study an evaluation of the efficiency of different urea controlled-release systems, based on cassava starch and bagasse, over tomato plants growth was carried out. All fertilized systems showed a tendency towards a greater general development, photosynthetic pigments content and nitrogen status of the plant. However, significant differences were observed regarding macronutrient dosage, being unsupported biocomposite films with 25 wt.% urea content the systems with a better performance. Thus, an ecofriendly, efficient and easy to implement urea dosage alternative system for greenhouse seedling has been developed.

Long Lifetime and Coupling of Acoustic Vibrations of Gold Nanoplates on Unsupported Thin Films.

Radiation losses due to energy transfer to acoustic waves in the surroundings lead to short vibrational lifetimes of plasmonic nanoresonators and severely limits their applications in quantum technologies and mass sensing. Here we demonstrate a general method for improving the acoustic vibration lifetimes of Au nanoplates (flat structures with edge lengths of several microns and thicknesses of 10-40 nm) by using unsupported thin films instead of thick substrates, which blocks the outgoing acoustic waves. We experimentally confirm the vibrational quality factor improvement of Au nanoplates on Si3N4 and Al2O3/Si3N4 thin films with thicknesses less than 100 nm. Through this substrate phonon engineering, we successfully observe strong vibrational coupling between two Au nanoplates. Furthermore, the increased vibrational lifetimes allow us to study the vibrations of the nanoplates relative to the substrate and, for stacked nanoplates, relative to each other. The frequencies of these relative vibrational modes are well described by a simple coupled mass model, which provides the effective spring constants between interfaces separated by surface bonded molecules. We also demonstrate that the continuous thin films can be used for small plasmonic nanostructures (dimensions of several hundred nm), with similar increases in the vibrational lifetimes.

Protein denaturation at the air-water interface and how to prevent it.

Electron cryo-microscopy analyzes the structure of proteins and protein complexes in vitrified solution. Proteins tend to adsorb to the air-water interface in unsupported films of aqueous solution, which can result in partial or complete denaturation. We investigated the structure of yeast fatty acid synthase at the air-water interface by electron cryo-tomography and single-particle image processing. Around 90% of complexes adsorbed to the air-water interface are partly denatured. We show that the unfolded regions face the air-water interface. Denaturation by contact with air may happen at any stage of specimen preparation. Denaturation at the air-water interface is completely avoided when the complex is plunge-frozen on a substrate of hydrophilized graphene.

Electronic structure of Al, Ga, In and Cu doped ZnO/Cu(111) bilayer films.

The effect of doping with group-III metals (Al, Ga and In) and Cu free standing and Cu(111) supported ZnO bilayer films has been investigated computationally by using the DFT+U method including dispersion contributions. The changes in the electronic properties of doped ZnO and ZnO/Cu(111) films have been tested by adsorbing CO probe molecules. The replacement of a lattice Zn ion in a free-standing ZnO bilayer by a group-III element generates an extra electron whose distribution depends on the dopant. In particular, while the excess electron is delocalized over the conduction band for Al or Ga doping, it is localized on the dopant in the case of In. The situation is different on the supported ZnO/Cu(111) film, where the extra electron is transferred to the underlying Cu support. While the CO adsorption energy at the doped sites in the ZnO bilayer is the same as in the ZnO/Cu(111) ultrathin films, CO exhibits a larger red-shift in the unsupported ZnO bilayer. The oxidation state of Cu replacing Zn in the unsupported ZnO films is 2+, Cu(3d9) state, while it is 1+, Cu(3d10) state, in the ZnO/Cu(111) supported films where a charge transfer from the supporting Cu metal to the Cu impurity occurs. Cu doping results in a stronger interaction with CO and a large red-shift of the CO stretching frequency. In this respect, Cu doping of ZnO/Cu(111) bilayer films could have interesting consequences in gas adsorption while doping with group-III elements does not lead to major changes of the adsorption properties when the free-standing ZnO films are compared to the supported ZnO/Cu(111) counterparts.

Theory of linear sweep voltammetry with diffuse charge: Unsupported electrolytes, thin films, and leaky membranes.

Linear sweep and cyclic voltammetry techniques are important tools for electrochemists and have a variety of applications in engineering. Voltammetry has classically been treated with the Randles-Sevcik equation, which assumes an electroneutral supported electrolyte. In this paper, we provide a comprehensive mathematical theory of voltammetry in electrochemical cells with unsupported electrolytes and for other situations where diffuse charge effects play a role, and present analytical and simulated solutions of the time-dependent Poisson-Nernst-Planck equations with generalized Frumkin-Butler-Volmer boundary conditions for a 1:1 electrolyte and a simple reaction. Using these solutions, we construct theoretical and simulated current-voltage curves for liquid and solid thin films, membranes with fixed background charge, and cells with blocking electrodes. The full range of dimensionless parameters is considered, including the dimensionless Debye screening length (scaled to the electrode separation), Damkohler number (ratio of characteristic diffusion and reaction times), and dimensionless sweep rate (scaled to the thermal voltage per diffusion time). The analysis focuses on the coupling of Faradaic reactions and diffuse charge dynamics, although capacitive charging of the electrical double layers is also studied, for early time transients at reactive electrodes and for nonreactive blocking electrodes. Our work highlights cases where diffuse charge effects are important in the context of voltammetry, and illustrates which regimes can be approximated using simple analytical expressions and which require more careful consideration.

Water splitting as a tool for obtaining insight into metal–support interactions in catalysis

Abstract This review is focused on the use of the water splitting reaction for characterizing oxygen vacancies in supported metal catalysts and more generally to get insight into the high-temperature modifications of metal–support interactions. Three supports widely used in catalysis are considered, namely alumina, silica and ceria. The catalysts were reduced at temperatures TR ranging from 200 to 1000 °C. The reaction with water was carried out at temperatures TOX ranging from 100 to 1000 °C. In every case, the metal (Rh or Pt) was chosen among those which are not oxidizable by water. Extensive investigations of the reactivity of water with unsupported metals and films confirmed this choice. The reaction is then selective for the titration of O vacancies, generally associated with reduced cations of the support. On alumina-supported catalysts, reduction at TR > 600 °C leads to the formation of oxygen vacancies strictly confined to the periphery of metal particles. The amount of hydrogen produced QH is coherent with the peripheral oxygen density. Reduction of silica-supported catalysts at TR > 600 °C generates metal silicides that can be selectively destroyed by water with reformation of silica and metal nanoparticles. Oxygen vacancies are formed on ceria catalysts at 200 °C. These oxygen vacancies are confined to the surface up to 600 °C. At higher temperatures, oxygen vacancies are formed in the bulk: about 50% of CeO2 would be reduced at 900 °C. The amount of H2 produced by reaction with water is thus very high on metal-ceria catalysts. At TR > 900 °C, metal cerides start to form. Remarkably, a significant reactivity of H2O on a Rh/CeO2 catalyst reduced at 850 °C is recorded as of 100 °C. However, the quantitative titration of oxygen vacancies required temperatures TOX > 500 °C. As a rule, the technique of water splitting allows the detection of 1 μmol g−1 of oxygen vacancies, i.e. a few 0.1% of the surface in the case of reducible oxides of 10–20 m2 g−1.

Green approach in the sol–gel synthesis of defect free unsupported mesoporous alumina films

Sol–gel synthesis of defect free alumina films by the hydrolysis of metal alkoxide precursors require proper drying and calcination procedures. In the present paper an attempt has been made to gain control over the difficult process of sol–gel synthesis of plain, porous, defect free alumina films through proper design and selection of parameters like contact angle (120°), viscosity (1–2 Pa s), drying and calcination temperature. SANS, AFM, TEM probes were used to investigate porous alumina film (α alumina) made up of parallel channels with cylindrical shape. Nitrogen adsorption studies (BET method) were carried out to determine porosity and surface area of the film. The SEM and X-ray diffraction studies were done to characterize the alumina film. The kind of technique used during synthesis of alumina film is not only cost affective but has an added advantage of controlling the shape and geometry of alumina film. The novel technique of disc furnace introduced here has overcome the difficulties of cracking and deformation in the sol–gel synthesis of alumina film during calcination. It has been shown that various sizes of porous, plain unsupported alumina films with a diameter of 30–50 mm and thickness of 0.5–5 mm were prepared by using sol–gel method.

Preparation and characterization of epitaxially grown unsupported yttria-stabilized zirconia (YSZ) thin films

Epitaxially grown, chemically homogeneous yttria-stabilized zirconia thin films (“YSZ”, 8mol% Y2O3) are prepared by direct-current sputtering onto a single-crystalline NaCl(001) template at substrate temperatures ≥493K, resulting in unsupported YSZ films after floating off NaCl in water.A combined methodological approach by dedicated (surface science) analytical characterization tools (transmission electron microscopy and diffraction, atomic force microscopy, angle-resolved X-ray photoelectron spectroscopy) reveals that the film grows mainly in a [001] zone axis and no Y-enrichment in surface or bulk regions takes place. In fact, the Y-content of the sputter target is preserved in the thin films. Analysis of the plasmon region in EEL spectra indicates a defective nature of the as-deposited films, which can be suppressed by post-deposition oxidation at 1073K. This, however, induces considerable sintering, as deduced from surface morphology measurements by AFM.In due course, the so-prepared unsupported YSZ films might act as well-defined model systems also for technological applications.

Adsorption of Li, Na, K, and Mg Atoms on Amorphous and Crystalline Silica Bilayers on Ru(0001): A DFT Study

The interaction of Li, Na, K, and Mg atoms with crystalline and amorphous silica bilayers grown on Ru(0001) supports has been studied by performing DFT calculations. (Si4O6H4)n clusters have been chosen to represent different ring sizes (n = 4–8) of amorphous silica bilayers. We found that, on unsupported silica films, the alkali metal atoms adsorb as neutral entities with weak adsorption energies, whereas the diffusion into the cage is not thermodynamically favorable. Interaction of Na with Al-doped silica cages has also been investigated. The presence of this Al dopant enhances the strength of the metal–framework interaction, changing completely the bonding mechanism that is dominated by charge transfer contributions. Na and Mg interacting with a crystalline silica bilayer on a 3O(2 × 2)/Ru(0001) surface adsorb preferentially at the interface between the support and the silica film with high adsorption energies and transfer their valence electrons to the Ru metal.

Boehmite-phenolic resin carbon molecular sieve membranes—Permeation and adsorption studies

Composite carbon molecular sieve membranes (c-CMSM) were prepared in a single dipping–drying–carbonization step from phenolic resin solutions (12.5–15wt.%) loaded with boehmite nanoparticles (0.5–1.2wt.%). A carbon matrix with well-dispersed Al2O3 nanowires was formed from the decomposition of the resin and dehydroxylation of boehmite. The effect of the carbon/Al2O3 ratio on the porous structure of the c-CMSM was accessed based on the pore size distribution and gas permeation toward N2, O2, CO2, He, H2, C3H6 and C3H8. c-CMSM with higher carbon/Al2O3 ratios had a more open porous structure, exhibiting higher permeabilities and lower permselectivities. c-CMSM performance was above the upper bound curves for polymeric membranes for several gas pairs, particularly for C3H6/C3H8 (permeability toward C3H6 of 420 barrer and permselectivity of 18.1 for a c-CMSM with carbon/Al2O3 ratio of 4.4).Unsupported films were also prepared (carbon/Al2O3 ratio 7.3) and crushed into small flakes. Equilibrium isotherms of H2, N2, O2, CO2, C3H8 and C3H6 at 293K were determined on these flakes to obtain the kinetic and adsorption selectivities toward gas pairs of interest; obtained adsorption and diffusion coefficients accurately predicted the permeabilities of all studied gases except CO2 (experimental and predicted permeabilities of 1148 and 154 barrer, respectively).

Viscoelasticity of nanobubble‐inflated ultrathin polymer films: Justification by the coupling model

AbstractThe nanobubble inflation method is the only experimental technique that can measure the viscoelastic creep compliance of unsupported ultrathin films of polymers over the glass–rubber transition zone as well as the dependence of the glass transition temperature (Tg) on film thickness. Sizeable reduction of Tg was observed in polystyrene (PS) and bisphenol A polycarbonate by the shift of the creep compliance to shorter times. The dependence of Tg on film thickness is consistent with the published data of free‐standing PS ultrathin films. However, accompanying the shift of the compliance to shorter times, a decrease in the rubbery plateau compliance is observed. The decrease becomes more dramatic in thinner films and at lower temperatures. This anomalous viscoelastic behavior was also observed in poly(vinyl acetate) and poly (n‐butyl methacrylate), but with large variation in the change of either the Tg or the plateau compliance. By now, well established in bulk polymers is the presence of three different viscoelastic mechanisms in the glass–rubber transition zone, namely, the Rouse modes, the sub‐Rouse modes, and the segmental α‐relaxation. Based on the thermorheological complexity of the three mechanisms, the viscoelastic anomaly observed in ultrathin polymer films and its dependence on chemical structure are explained in the framework of the Coupling Model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013

Influence of temperature on crack-bridging ability of coatings for concrete

The crack-bridging properties of different organic coatings have been studied over a range of thicknesses and applied temperatures. In each case the data allowed to determine linear relationships between crack-bridging ability (CBA) and coating thickness. In addition, a direct correlation between crack bridging ability and elongation at break of the unsupported films has been found, once verified that adhesion properties do not change over temperature.

Properties of Wheat Gluten/Poly(lactic acid) Laminates

Laminates of compression-molded glycerol-plasticized wheat gluten (WG) films surrounded and supported by poly(lactic acid) (PLA) films have been produced and characterized. The objective was to obtain a fully renewable high gas barrier film with sufficient mechanical integrity to function in, for example, extrusion-coating paper/board applications. It was shown that the lamination made it possible to make films with a broad range of glycerol contents (0-30 wt %) with greater strength than single unsupported WG films. The low plasticizer contents yielded laminates with very good oxygen barrier properties. In addition, whereas the unsupported WG films had an immeasurably high water vapor transmission rate (WVTR), the laminate showed values that were finite and surprisingly, in several cases, also lower than that of PLA. Besides being a mechanical support (as evidenced by bending and tensile data) and a shield between the WG and surrounding moisture, the PLA layer also prevented the loss of the glycerol plasticizer from the WG layer. This was observed after the laminate had been aged on an "absorbing" blotting paper for up to 17 weeks. The interlayer adhesion (peel strength) decreased with decreasing glycerol content and increasing WG film molding temperature (130 degrees C instead of 110 degrees C). The latter effect was probably due to a higher protein aggregation, as revealed by infrared spectroscopy. The lamination temperature (110-140 degrees C) did not, however, have a major effect on the final peel strength.

Behavior of Thin Films of Poly(oxyethylene)−Poly(oxybutylene) Copolymers Studied by Brewster Angle Microscopy and Atomic Force Microscopy

Surface films of two copolymers of ethylene oxide (E) and butylene oxide (B), namely E23B8 and E87B18, have been examined by Brewster angle microscopy (BAM) and atomic force microscopy (AFM). Isotherms taken on unsupported films of these copolymers at the air-water interface showed a clear gas to liquid phase transition for E57B18 and a barely discernible phase transition for E23B8. The BAM studies showed a gradual brightening of the films as the surface pressure was increased, which was associated with a film thickening and/or a film densification. Several bright spots were also observed within the films, with the number of spots increasing gradually as the film surface pressure was increased. AFM studies of these films did not show any localized ordering, which fits in with the results from our previous X-ray study of these copolymers [Hodges, C. S.; Neville, F.; Konovalov, O.; Gidalevitz, D.; Hamley, I. W.; Langmuir 2006, 22 (21), 8821-8825], where no long-range ordering was observed. AFM imaging showed two sizes of particulates that were irregularly spaced across the film. The larger particulates were associated with silica contaminants from the copolymer synthesis, whereas the smaller particulates were assumed to be aggregated copolymer. An analysis of the semidilute region of the isotherm showed that while both copolymers had intermixed ethylene oxide and butylene oxide units, the lower molecular weight E23B8 copolymer manifested significantly more intermixing than E87B18.

Exchange interactions and critical temperature of bulk and thin films of MnSi: A density functional theory study

Recent theoretical work [H. Wu et al., Phys. Rev. Lett. 92, 237202 (2004); M. Hortamani et al., Phys. Rev. B 74, 205305 (2006); M. Hortamani, Ph.D. thesis, Freie Universit\at, Berlin, 2006] predicted ferromagnetism at zero temperature in thin MnSi films of B2-type crystal structure on Si(100). The relevance of this finding for finite-temperature experiments needs to be clarified by further investigations, since bulk MnSi is a weak ferromagnet with an experimentally measured Curie temperature of only ${T}_{c}=30\text{ }\text{K}$, and ${T}_{\text{c}}$ is generally expected to be lower in thin films than in bulk materials. Here, we estimate ${T}_{c}$ of such MnSi films using a multiple-sublattice Heisenberg model with first- and second-nearest-neighbor interactions determined from density functional theory calculations for various collinear spin configurations. The Curie temperature is calculated either in the mean-field approximation (MFA) or in the random-phase approximation (RPA). In the latter case we find a weak logarithmic dependence of ${T}_{\text{c}}$ on the magnetic anisotropy parameter, which was calculated to be 0.4 meV for this system. In stark contrast to the above mentioned rule, large Curie temperatures of above 200 K for a monolayer (ML) MnSi film and above 300 K for a two ML MnSi film with B2-type structure on Si(100) are obtained within the RPA, and even higher values in MFA. Complementary calculations of MnSi bulk structures and thin unsupported MnSi films are performed in order to analyze these findings. We find that bulk MnSi in the cubic B2 structure is paramagnetic, in contrast to MnSi in the B20 ground-state structure in agreement with the Stoner criterion. In a tetragonally distorted B2 structure, the Mn atoms gradually develop a spin magnetic moment, passing through a low-spin and a high-spin state. However, the ferromagnetism of the MnSi/Si(100) films cannot be explained by tetragonal distortions alone, since the distorted B2 bulk structure is found to order antiferromagnetically. Comparison of the calculations of supported and unsupported films suggests that the reduced coordination of Mn atoms near surfaces and interfaces is crucial for the ferromagnetic ground state of the films. The coordination number of the Mn atoms in B2-type MnSi films on Si(100) constitutes a borderline case, where the spin magnetic moments of Mn are still large despite their sixfold coordination to Si, but the $sp\text{\ensuremath{-}}d$ hybridization with Si states gives rise to a sizable ferromagnetic coupling of the Mn spins. We conclude that the Curie temperatures predicted from the Heisenberg Hamiltonian make thin MnSi films an interesting subject for further experimental investigation of spintronics materials.