The nonlinear optical (NLO) response of three π-conjugated azobenzene (AB) derivatives was investigated under picosecond laser excitation by means of the Z-scan technique to evaluate the effect of an ethynyl-based conjugated spacer on the NLO properties of ABs. All modules possessed large third-order nonlinearity, but unexpectedly it was the less extended AB derivative that exhibited the largest NLO response. This finding has been confirmed by means of DFT calculations and was attributed to a higher cis/trans ratio of the particular AB derivative in its investigated photoequilibrated state. Furthermore, the influence of the amount of cis isomer on the third-order nonlinear susceptibility [χ((3))] of the less extended AB derivative has been thoroughly investigated. Specifically, modulation of the NLO response has been successfully achieved by tuning the isomeric composition of the investigated photostationary state. These results highlighted the cis-dependent increase of the NLO response to support the general idea that such compounds can be used for multistep switching NLO materials.

A closely related series of three ferrocene derivatives, bearing a short carbon-carbon tethering arm for covalent attachment to H-terminated Si(1 0 0). has been used to produce electroactive hybrid materials. Three different grafting procedures have been applied for the first time on this type of molecules in order to obtain the full series of possible unsaturations in the C-C lateral chain bound to Si: ethyl, vinyl and ethynyl group, respectively, associated with the surface species (eta(5)-C5H5)Fe2+(eta(5)-C5H4)-CH2-CH2-Si (EtFC/Si), (eta(5)-C5H5)Fe2+(eta(5)-C5H4)-CH=CH-Si (VFC/Si), and (eta(5)-C5H5)Fe2+(eta(5)-C5H4)-C C-Si (EFC/Si).The surface reactions here applied involve Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetylide anion. The redox potentials of the three molecule/surface hybrids, measured by cyclic voltammetry, are 0.059, 0.136 and 0.251 V vs. Ag/Ag+, respectively, from EtFC/Si to EFC/Si and are supposedly following the increasing trend of the extent of unsaturation in the tethering arm. The above trend in redox potentials parallels the one experimentally found and theoretically reproduced for the corresponding isolated molecules, which shows that the adopted functionalization procedures may be used to control the number of residual unsaturations in the C-C arm. The presence and chemical nature of the redox species covalently attached to Si are strongly supported after evaluation of XPS spectra and electrochemical data. The formation of electroactive hybrids on Si with tuneable well-defined switching potentials can be highly beneficial to the development of Si-based memory devices. (C) 2010 Elsevier Ltd. All rights reserved.

In the frame of our research activity on covalent anchoring of functional molecules on Si oriented surfaces, we report here on the hybrid species resulting from the reaction of ethynylferrocene with H-Si(100) either in a single, direct step, or after production of a self-assembled alpha,omega-bifunctional alkyl monolayer. In the former approach, a short Si-C covalent bond is established through visible-light activation of the unsaturated side arm of ethynylferrocene via an extra-mild approach. In the latter case, an azide-alkyne Huisgen cycloaddition reaction ("click" chemistry) is induced between the C[triple bond]C group of the molecule and the -N3 terminations of undecyl hydrocarbon chains pre-assembled onto H-Si(100). These two routes lead to electroactive monolayers wherein the ferrocene head groups are respectively located close to or far from the Si surface. This allows for a fine tuning of the properties of the resulting hybrids on Si, in terms of distinct redox potentials, electron transfer rate constants, oxidation resistance. The different reaction steps have been monitored with X-ray Photoelectron Spectroscopy (XPS), in order to evaluate the presence and the chemical nature of the anchored species, while electrochemical measurements have evidenced reversible redox responses of the resulting SAMs.

New or rarely reported synthetic routes have been applied to produce electroactive hybrid materials from ferrocene (Fc) derivatives with a known and pre-established variation in the tethering arm bound to Si. The series is characterized for having the shortest molecular link for a direct covalent attachment to H-terminated Si(100). Grignard derivatization, Lewis acid catalysis and nucleophilic substitution by an acetilyde anion have been applied for the first time in order to obtain the full series of possible unsaturations in the C-C lateral chain bound to Si: ethyl (EtFC/Si), vinyl (VFC/Si) and ethynyl group (EFC/Si). The redox potentials of these three molecule/surface hybrids, measured by cyclic voltammetry, are respectively 0.059, 0.136 and 0.251 V vs. Ag/Ag(+), and increase with the extent of unsaturation in the tethering arm. XPS and electrochemical measurements have been used to assess the chemical nature and electronic properties of the hybrids. (c) 2008 Elsevier Ltd. All rights reserved.

Various [60]fullerene derivatives were covalently anchored via a Si-C bond to hydrogen-terminated Si(100) following wet-chemistry recipes. The applied reactions require a two-step approach consisting of a pre-modification of the Si(100) surface with an organic monolayer bearing a terminal functionality that undergoes a bond-forming reaction with a [60]fullerene synthon. The resulting Si-C bound hybrids were characterized by a combination of XPS, AFM, contact angle, and electrochemical analysis. The hybrid surfaces containing [60]fullerene-ferrocene fragments are exceptionally robust towards repeated reduction oxidation cycles. Moreover, several surface-confined redox couples were observed in CH3CN solution. The surface coverage was measured to be ca. 2.5 x 10(-11) mol cm(-2).

Herein, we report the covalent immobilisation, through Si-C bonds, of various [60]fullerene derivatives on flat silicon surfaces following three different preparative protocols. Each synthetic strategy comprises a two-step approach that includes a pre-modification step of the Si(100) surface with an organic monolayer bearing a terminal functionality that undergoes a bond-forming reaction with a [60]fullerene synthon as characterized by X-ray photoelectron spectroscopy (XPS) measurements. Water contact angle measurements clearly showed a characteristic change of the surface hydrophobicity upon covalent immobilisation of the carbon functions. The hybrid [4b-Si(100)] surfaces, containing [60]fullerene-ferrocene fragments, were also investigated by means of cyclic voltammetry (CV), and were revealed to be exceptionally robust towards repeated reduction-oxidation cycles. Moreover, several surface-confined redox couples were observed in CH3CN solution. The surface coverage was measured to be ca. 2.5 x 10(-11) mol cm(-2).

We prepared ferrocene-modified silicon surfaces through a three-step procedure consisting of the photochemical anchoring of 11-bromo-1-undecene on H-Si(1 0 0), followed by treatment with NaN3 and by a reaction with ethynylferrocene via azide-alkyne Huisgen cycloaddition reaction, also known as "click" chemistry. The advantages of this approach are multiple: the synthetic approach is flexible, provided a CC tethering arm is present on the molecule of interest; a self-assembled hydrocarbon chain can guarantee a good coverage and resistance to the further synthetic steps; the redox centers are located at the outer surface, where a good contact with the electrolyte becomes possible. We have monitored the progression of the reaction steps by XPS, and characterized the resulting new hybrid on Si by electrochemical methods. The presence and chemical nature of the redox species covalently attached to the SAM on Si has been evaluated by XPS, while the overall coverage has been calculated by CV measurements. A reversible electrochemical response has been evidenced for the hybrids and the progressive ageing followed at thousands of oxidation–reduction cycles.

The spatial self-organization of molecular species on an Si oriented surface can be less ideal than that of an SAM on a metal, likely affecting the electronic structure of the resulting hybrids and their electrochemical response as electrodes in solution. In order to investigate such effects, a series of molecular precursors was investigated, consisting of three substituted ferrocenes with a lateral C–C group fully saturated (ethylferrocene) or with a single (vinylferrocene) or double unsaturation (ethynylferrocene). The corresponding functionalized Si(100) wafers were produced following new or literature recipes, starting from hydrogenated Si surfaces. The relationship between the degree of unsaturation in the anchored arm of each adduct and its electronic structure and electrochemical behaviour was investigated by comparing experimental (XPS, electrochemical) and ab initio results of the redox potentials in the series. Density functional theory (DFT) was applied, with inclusion of solute–solvent interactions. Different bond arrangements of the C–C arm with Si surface dimer atoms have been produced theoretically within the series of ferrocenes. Distinct values of redox potentials were displayed by the hybrids, which can be consistently related to the structural differences presented. In fact, measured and computed potentials showed a very satisfactory match only for specific adduct isomers, providing strong indications that the carbon–carbon unsaturation initially present in the anchoring arm is preserved upon addition reaction, an unprecedented result. The demonstrated tunability of a well-defined switching potential in these molecules on silicon can be beneficial to the development of Si-based memory devices.

We report the synthesis and characterization of several hybrid [60]fullerene–SWCNT materials that combine [60]fullerenes with appended photoactive ferrocenyl or porphyrinyl functionalities and SWCNTs into a single multifunctional structure, where the dyads are covalently attached to the exo-surface of SWCNTs. The structural properties of all hybrids have been characterized using a large variety of spectroscopic and HR-TEM techniques. Raman spectra showed how all SWCNTs were functionalized and the presence of functional groups in the nanotube derivatives. Furthermore, these spectra reveal a new electronic activity of the compounds due to the interaction of the functional groups with the SWCNT frameworks. XPS investigations have documented the presence of [60]fullerene derivatives around the exo-surface of the oxidized SWCNT walls, exhibiting a characteristic photoelectron N 1s emission peak at 400.3eV. Very importantly, by means of HR-TEM investigations we have also observed the presence of the [60]fullerene functions on the SWCNT outer surface by imaging spherical structures. The presence of the porphyrinyl and ferrocenyl fragments, which can act as effective chromophores and electroactive species, makes this class of materials very interesting for applications in optoelectronics and photovoltaics, and bio-applications, for example in the field of diagnosis and treatment.

Metalloporphyrins bound to silicon wafers exhibit a redox behaviour useful for information storage and resist the conditions of temperature required for processing real devices. An XPS and AFM study has been undertaken to evidence and understand the electronic states of different free bases and metalloporphyrins on Si(100) and (111). Both carbosilane (RC–Si) and alkoxysilane (RO–Si) bonds have been produced via thermal and electrochemical routes. In the resulting hybrids XPS has revealed, for the first time, that porphyrins exist as two distinct species on silicon. The behaviour of Co, Cu and Zn(II) porphyrinates has been evidenced, and a strategy to improve the molecular coverage by use of an organic spacer has been also proposed. The results will assist in the choice of suitable candidates for molecular electronics.

Recently, we have presented a novel in situ simultaneous formation-functionalization (FORFUN) method of porous silicon (PS) layers in which the molecules to be chemisorbed are present in the HF ethanoic solution used for the electrochemical preparation of the PS. In this work we demonstrate that is possible to extend this method to a wider group of organic molecules, in principle even to molecules not compatible with the preparation solution. First a PS single layer functionalized with methyl-6-heptynoate was prepared by the FORFUN method (I step) and then, after a second ex situ chemical process (II step), we obtained the correspondent tertiary alcohol bonded to the surface, as proved by the IR reflection spectra. This two-steps process maintains all the features of the FORFUN method: simplicity, effectiveness and the possibility of confining the chemisorbed species in selected layers in a PS stack (“spatially selective functionalization”). These findings can be relevant for many applications based on PS systems and devices. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Three distinct wet chemistry recipes were applied to hydrogen-terminated n- and p-Si(100) surfaces in a comparative study of the covalent grafting of two differently substituted 2,2'-bipyridines. The applied reactions require the use of heat, or visible light under a controlled atmosphere, or a suitable potential in an electrochemical cell. In this last case, hydrogen-terminated silicon is the working electrode in a cathodic electrografting (CEG) reaction, in which it is kept under reduction conditions. The resulting Si--C bound hybrids were characterized by a combination of AFM, dynamic contact-angle, and XPS analysis, with the help of theoretical calculations. The three distinct approaches were found to be suitable for obtaining ligand-functionalized Si surfaces. CEG resulted in the most satisfactory anchoring procedure, because of its better correlation between high coverage and preservation of the Si surface from both oxidation and contamination. The corresponding Si-bipyridine hybrid was reacted in a solution of CH3CN containing CuI ions coordinatively bound to the anchored ligands, as evidenced from the XPS binding-energy shift of the N atom donor functions. The reaction gave a 1:2 Cu-bipyridine surface complex, in which two ligands couple to a single CuI ion. The surface complex was characterized by the Cu Auger parameter and Cu/N XPS atomic-ratio values coincident with those for pure, unsupported CuI complex with the same 2,2'-bipyridine. Further support for such a specific metal-ligand interaction at the functionalized Si surface came from the distinct values of Cu2p binding energy and the Cu Auger parameter, which were obtained for the species resulting from CuI ion uptake on hydrogen-terminated Si(100).

Unoxidized crystalline silicon, characterized by high purity, high homogeneity, sturdiness and an atomically flat surface, offers many advantages for the construction of electronic miniaturized biosensor arrays upon attachment of biomolecules (DNA, proteins or small organic compounds). This allows to study the incidence of molecular interactions through the simultaneous analysis, within a single experiment, of a number of samples containing small quantities of potential targets, in the presence of thousands of variables. A simple, accurate and robust methodology was established and is here presented, for the assembling of DNA sensors on the unoxidized, crystalline Si(100) surface, by loading controlled amounts of a monolayer DNA-probe through a two-step procedure. At first a monolayer of a spacer molecule, such as 10-undecynoic acid, was deposited, under optimized conditions, via controlled cathodic electrografting, then a synthetic DNA-probe was anchored to it, through amidation in aqueous solution. The surface coverage of several DNA-probes and the control of their efficiency in recognizing a complementary target-DNA upon hybridization were evaluated by fluorescence measurements. The whole process was also monitored in parallel by Atomic Force Microscopy (AFM).

In this study we modified and optimized a technique using an ordinary spin-coater device to deposit lipids on chemically etched silicon substrates. Tapping-mode atomic force microscopy is used for sample characterization, thereby realizing non destructive characterization with nanometrical resolution.

A reversible electrochemical behavior is demonstrated on a specially prepared redox-functionalized H-Si(100) surface, obtained via an extra-mild grafting procedure from vinylferrocene. The results of a detailed XPS and electrochemical characterization of the resulting hybrid are reported and discussed to propose it as a reference system for high-quality electroactive monolayers on Si. The investigated ferrocene derivative bears a functional group suitable for a mild route to covalent anchoring on Si, which is based on a photoinduced reaction with visible light under an inert atmosphere. Electrochemical reversibility is shown by sharp symmetric voltammograms on freshly prepared p-Si electrodes. Anodic oxide growth is responsible for the progressive degradation of the electrochemical response. Still, fast electron transfer to the surface redox species is maintained during several thousands cycles.

Carboxylic terminated monolayers have been covalently attached on phosphorous doped crystalline (100) silicon surfaces using a cathodic electro grafting technique. The functionalization concentration and efficiency have been evaluated with different techniques. In particular, topographic images, performed with an atomic force microscope, were used to optimize the protocol in order to obtain a surface whose characteristics of uniformity and reproducibility are ideal for a bio-electronic device. Phase lag images of the functionalized surfaces were also performed, and show non-topographic structures that have been interpreted as areas of different molecule self-orientation. Poly-thymine oligonucleotides have been anchored on such a surface to form a nano-biosensing device capable to react selectively with a specific target molecule, a poly-adenine oligonucleotide. AFM images of high density (approximately 3x10(12) mol/cm2) single strand and double strand covered samples show toroidal shaped structures formed by the self-assembly of the oligonucleotides on the silicon surface.

Two enantiomeric pairs of menthol derivatives, the menthol ester with 10-undecynoic acid and the 2-tricyanovinyi-pyrrole derivative of the methoxy-menthol, have been synthesized and their pure enantiomers obtained. The X-ray crystal and molecular structure of the (+)-enantiomer of the latter compound is reported. (c) 2006 Elsevier Ltd. All rights reserved.

Magnetite nanoparticles of 5 nm mean diameter, coated with 10-undecynoic acid, have been anchored to crystalline Si(100) surfaces via the hydrosilylation reaction at 180 degrees C. The iron content in the sample (0.54 +/- 0.05 mu g/cm(2)) has been determined by atomic absorption analysis. The sample has been further characterized by X-ray photoelectron spectroscopy, field emission scanning, scanning-tunneling, and atomic force (AFM) microscopies. The thermal anchoring does not alter the morphology of the nanoparticles, causing only a slight oxidation of their surfaces. However, a second layer of nanoparticles was also formed. covering about 50% of the surface. The magnetic properties were studied using a SQUID magnetometer. ZFC (zero-field cooled) and FC (field cooled) curves were obtained in the 5-300 K temperature range. The ZFC curve shows a rounded maximum at T-max similar to 20 K. A hysteretic magnetization cycle was also observed at 5 K with associated magnetization saturation and coercitivity values of 40 emu/g and 160 Oe, respectively. The magnetic behavior of the sample was found to be typical of an assembly of noninteracting (or very weakly interacting) super-paramagnetic particles.

Light-assisted surface anchoring to H-terminated n- and p-Si(100) wafers has resulted in the production of molecular electroactive monolayers from Si–C bound vinylferrocene (VFC). The resulting hybrids have been characterized by means of X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM) and electrochemically. White-light photoactivated anchoring has resulted in a mild route. The functionalized Si surface results negligibly oxidized, and the C/Fe atomic ratio is close to the value for the precursor. Electrochemical methods have been applied to investigate the role played by a covalent Si–C anchoring mode towards substrate–molecule electronic communication, a crucial issue for future molecular electronics devices. The response from cyclic voltammograms (cv's) for p-Si(100) functionalized electrodes, run in the dark and under illumination, has shown that the electron transfer is not limited by the number of charge carriers, confirming the occurrence of electron transfer via the Si valence band. The hybrids have shown a noticeable electrochemical stability and reversibility under cyclic voltammetry, and the trend in peak current intensity vs. the scan rate was linear. The molecule–Si bond is preserved for thousands voltammetric cycles, although both the coverage, evaluated from cv and XPS, and the electron transfer rate constant decrease with electrode ageing. VFC/p-Si resulted in the best-to-date large-area charge storage hybrid device responding to AC with no dissipation up to 100 Hz.