Structure of Mixed Acid/Decyl Monolayers Grafted on Oxide-Free Si(111) Surfaces.

The structure of mixed acid/decyl monolayers (MLs) grafted on oxide-free Si(111) surfaces by photochemical hydrosilylation in a mixture of neat undecylenic acid and 1-decene is studied in detail. After appropriate surface cleaning of the as-grafted surfaces, atomic force microscopy (AFM) (topography and phase imaging) and calibrated FTIR analysis demonstrate that a mixed monolayer is formed, free of residue. When the acid-molecule fraction (ΓSOL) is >0.1, mixed MLs are homogeneous on the scale of observations and they are only slightly enriched in acid chains with respect to the solution. Conversely, when ΓSOL < 0.1, the acid chain fraction within the ML becomes quasi-independent of the solution composition and may become much larger than ΓSOL. In addition, dark domains are observed in AFM phase images. Correlations between the characteristic parameters of νCO IR bands and AFM phase images suggest a strong phase separation of acid and alkyl chains at various length scales. A model involving a structuration of the grafting solution is proposed to explain observations.

Growth of Fe-BDC Metal-Organic Frameworks onto Functionalized Si (111) Surfaces.

The realization of metal-organic framework (MOF) layers onto solid surfaces is a prerequisite for their integration into devices. This work reports the direct growth of Fe3+ /benzene di-carboxylate MOFs onto functionalized silicon surfaces, compatible with a wide range of MOF synthesis conditions. The co-nucleation and growth of different crystalline phases are evidenced, whose coverage depends on the surface chemistry and/or the solution composition. Three structural phases - the cubic MIL-101(Fe), a hexagonal phase with a structure close to MOF-235 and a MIL-53(Fe) with a monoclinic symmetry - are identified through characteristic crystal shapes and their structural parameters inferred from X-Ray Diffraction. In addition to the oriented growth of 3D crystallites, the formation of two-dimensional MIL-101 nano-crystallites or thin layers/islands exhibiting extended monocrystalline domains with (111) texture is also demonstrated through high-resolution atomic force microscopy. Post-synthesis treatments reveal a weak adhesion of the hexagonal phase, indicating a different surface anchoring.

ToF-SIMS Li Depth Profiling of Pure and Methylated Amorphous Silicon Electrodes After Their Partial Lithiation.

Pure (a-Si:H) and methylated [a-Si0.95(CH3)0.05:H] amorphous silicon thin films were analyzed by time-of-flight secondary ion mass spectrometry after partial lithiation. Depth profiling gives insights into the lithiation mechanism of the material, enabling us to study the detailed biphasic process in the first lithiation process. Lithiation induces swelling and roughening of the active layer. In both a-Si:H and a-Si0.95(CH3)0.05:H, no measurable Li diffusion was observed after stopping current-induced lithiation. After applying the same lithiation charges, distinct Li profiles were observed for these two materials. Unlike a-Si:H, the Li concentration drops slowly from the heavily lithiated region to the non-lithiated region in a-Si0.95(CH3)0.05:H. This apparent progressive transition between the lithiated and lithium-free regions is attributed to the presence of nanovoids inside the material. When their concentration is high enough, these nanovoids constitute favorable quasi-percolating paths for lithium during the first lithiation. A specific model was developed to simulate the Li depth profiles, fully supporting this hypothesis.

Novel and Innovative Interface as Potential Active Layer in Chem-FET Sensor Devices for the Specific Sensing of Cs.

An innovative novel interface has been designed and developed to be used as a potential active layer in chemically sensitive field-effect transistor (Chem-FET) sensor devices for the specific sensing of Cs+. In this study, the synthesis of a specific Cs+ probe based on calix[4]arene benzocrown ether, its photophysical properties, and its grafting onto a single lipid monolayer (SLM) recently used as an efficient ultrathin organic dielectric in Chem-FETs are reported simultaneously. On the basis of both optical and NMR titration experiments, the probe has shown high selectivity and specificity for Cs+ compared to interfering cations, even if an admixture is used. Additionally, Attenuated Total Reflectance Fourier Transform Infra Red (ATR-FTIR) spectroscopy was successfully used to characterize and prove the efficient grafting of the probe onto a SLM and the formation of the innovative novel sensing layer.

Well-defined carboxyl-terminated alkyl monolayers grafted onto H-Si(111): packing density from a combined AFM and quantitative IR study.

This work demonstrates that well-defined mixed carboxyl-terminated/methyl-terminated alkyl monolayers can be prepared in one step on H-terminated Si(111) via direct photochemical hydrosilylation of undecylenic acid and 1-decene mixtures. As evidenced by AFM imaging and IR spectroscopy, a final rinse in hot acetic acid leaves the functionalized surface atomically smooth and perfectly free of physisorbed contaminants while unwanted material remains atop the monolayer with most other common solvents. The compositional surface chemistry was determined from a truly quantitative IR (ATR geometry) study in the range of 900-4000 cm(-)(1). Results prove that neither surface oxidation nor grafting through the carboxyl end groups occurs. Monolayers are fairly dense for such bulky end groups, with a total molecular surface density of approximately 2.7 10(14) cm(-)(2) corresponding to a surface coverage of 0.35 (maximum theoretical value approximately 0.5). Careful analysis of the CH- and COOH-related IR bands reveals that the composition of the grafted layers is richer in acid chains than the starting grafting mixture. A simple model is presented that shows that the grafting kinetics is about twice as fast for undecylenic acid as for 1-decene. Complementary electrochemical impedance measurements indicate the excellent electronic properties of the interface with a very low density of gap states. They also show that the acid terminal groups promote the penetration of water in the outer part of the organic film.

Truly quantitative XPS characterization of organic monolayers on silicon: study of alkyl and alkoxy monolayers on H-Si(111).

The quantitative characterization of the chemical composition (bonding at grafted and ungrafted sites, surface coverage) is a key issue for the application of silicon-organic monolayer hybrid interfaces. The primary purpose of this article is to demonstrate that X-ray photoelectron spectroscopy (XPS) requires to be truly quantitative to deal with two main questions. The first one is accounting for X-ray photodiffraction (XPD), a well-known phenomenon that is responsible for azimuthal variations of the XPS signal intensity. A simple procedure is proposed to account for XPD in angle-resolved measurements. The second critical point concerns the choice of photoelectron attenuation lengths (AL). This article demonstrates that n-alkanethiol self-assembled monolayers on Au(111) can be used as a reference system to derive the effective monolayer thickness on silicon substrates and that one may use the empirical relationship established by Laibinis and co-workers to calculate the relevant ALs (Laibinis, P. E.; Bain, C. D.; Whitesides, G. M. J. Phys. Chem. 1991, 95, 7017). A self-consistent approach is presented to justify the above assertions and to give a complete compositional description of alkyl and alkoxy monolayers directly grafted on atomically flat H-Si(111) surfaces. Direct evidences are provided that a Si-C and a Si-O-C linkage is formed, respectively, after reaction with decene and decanol and that the ungrafted sites remain saturated with H atoms. Moreover, the quantitative spectra analysis of satellite peaks at fixed polar angle and three independent angle-resolved Si2p and C1s spectra all give the same surface coverage very close to its theoretical limit.