The modification of surfaces by the deposition of a robust overlayer provides an excellent handle with which to tune the properties of a bulk substrate to those of interest. Such control over the surface properties becomes increasingly important with the continuing efforts at down-sizing the active components in optoelectronic devices, and the corresponding increase in the surface area/volume ratio. Relevant properties to tune include the degree to which a surface is wetted by water or oil. Analogously, for biosensing applications there is an increasing interest in so-called "romantic surfaces": surfaces that repel all biological entities, apart from one, to which it binds strongly. Such systems require both long lasting and highly specific tuning of the surface properties. This Review presents one approach to obtain robust surface modifications of the surface of oxides, namely the covalent attachment of monolayers.

On H-Si(111), monolayer assembly with 1-alkenes results in alkyl monolayers with a Si-C-C linkage to the silicon substrate, while 1-alkynes yield alkenyl monolayers with a Si-C=C linkage. To investigate the influence of the different linkage groups on the final monolayer structure, organic monolayers were prepared from 1-alkenes and 1-alkynes with chain lengths from C(12) to C(18), and the final monolayer structures were studied in detail by static water contact angles measurements, ellipsometry, attenuated total reflectance infrared (ATR-IR) spectroscopy, and X-ray photoelectron spectroscopy (XPS). The thicknesses, tilt angles, and packing densities of the alkyl monolayers are in good agreement with literature values, whereas increased thicknesses, reduced tilt angles, and improved packing densities were observed for the alkenyl monolayers. Finally, the surface coverages for alkyl monolayers were determined to be 50-55% (in line with literature values), while those for the alkenyl monolayers increased with the chain length from 55% for C(12) to as high as 65% for C(18)! The latter value is very close to the theoretical maximum of 69% obtainable on H-Si(111). Such enhanced monolayer quality and increased surface coverage of the alkenyl monolayers, in combination with the oxidation-inhibiting nature of the Si-C=C linkage, significantly increases the chance of successful implementation of organic monolayers on oxide-free silicon in molecular electronic and biosensor devices, especially in view of the importance of a defect-free monolayer structure and the corresponding stability of the monolayer-silicon interface.

A very mild method has been developed to obtain covalently attached alkyl monolayers from the attachment of 1-alkynes onto hydrogen-terminated silicon surfaces at room temperature in the dark. Apart from being the mildest method reported so far for the preparation of such monolayers, their quality, as indicated by water contact angles, XPS, and infrared spectroscopy, equals within experimental error that of the best reported alkyl monolayers on silicon.

Electronic transport across n-Si-alkyl monolayer/Hg junctions is, at reverse and low forward bias, independent of alkyl chain-length from 18 down to 1 or 2 carbons! This and further recent results indicate that electron transport is minority, rather than majority carrier-dominated, occurs via generation and recombination, rather than (the earlier assumed) thermionic emission and, as such is rather insensitive to interface properties. The (m)ethyl results show that binding organic molecules directly to semiconductors provides semiconductor/metal interface control options, not accessible otherwise.

Metal-organic molecule-semiconductor junctions are controlled not only by the molecular properties, as in metal-organic molecule-metal junctions, but also by effects of the molecular dipole, the dipolar molecule-semiconductor link, and molecule-semiconductor charge transfer, and by the effects of all these on the semiconductor depletion layer (i.e., on the internal semiconductor barrier to charge transport). Here, we report on and compare the electrical properties (current-voltage, capacitance-voltage, and work function) of large area Hg/organic monolayer-Si junctions with alkyl and alkenyl monolayers on moderately and highly doped n-Si, and combine the experimental data with simulations of charge transport and electronic structure calculations. We show that, for moderately doped Si, the internal semiconductor barrier completely controls transport and the attached molecules influence the transport of such junctions only in that they drive the Si into inversion. The resulting minority carrier-controlled junction is not sensitive to molecular changes in the organic monolayer at reverse and low forward bias and is controlled by series resistance at higher forward bias. However, in the case of highly doped Si, the internal barrier is smaller, and as a result, the charge transport properties of the junction are affected by changing from an alkyl to an alkenyl monolayer. We propose that the double bond near the surface primarily increases the coupling between the organic monolayer and the Si, which increases the current density at a given bias by increasing the contact conductance.

We compare the charge transport characteristics of heavy doped p ++ -and n ++ -Si-alkyl chain/Hg junctions. Photoelectron spectroscopy (UPS, IPES and XPS) results for the molecule-Si band alignment at equilibrium show the Fermi level to LUMO energy difference to be much smaller than the corresponding Fermi level to HOMO one. This result supports the conclusion we reach, based on negative differential resistance in an analogous semiconductor-inorganic insulator/metal junction, that for both p ++ -and n ++ -type junctions the energy difference between the Fermi level and LUMO, i.e., electron tunneling, controls charge transport. The Fermi level-LUMO energy difference, experimentally determined by IPES, agrees with the non-resonant tunneling barrier height deduced from the exponential length-attenuation of the current.

Recently, a new method for the preparation of high-quality organic monolayers with 1-alkynes at room temperature in the dark (i.e., without any external activation) was reported. To pinpoint the precise origin of this self-assembly process and to compare the reactivity of 1-alkenes and 1-alkynes toward hydrogen-terminated Si(111) [H-Si(111)], we followed the gradual formation of both monolayers at room temperature by static water contact angle measurements. Subsequently, attenuated total reflection infrared spectroscopy (ATR-IR) and X-ray photoelectron spectroscopy (XPS) were used to obtain detailed information about the structure and quality of the resulting monolayers. Our data clearly demonstrate that 1-alkynes are considerably more reactive toward H-Si(111) than 1-alkenes. 1-Alkynes are able to self-assemble into densely packed hydrophobic monolayers without any external activation (i.e., at room temperature under ambient light and even in the dark) whereas for 1-alkenes under the same conditions hardly any reactivity toward H-Si(111) was observed. The self-assembly of 1-alkynes on H-Si(111) at room temperature is explained by three factors: the higher nucleophilicity of 1-alkynes, which results in a facile attack at the electron-hole pairs at the H-Si surface and easy Si-C bond formation, the stabilization of the beta radical by delocalization over the double bond, and the lower-energy barrier encountered for H abstractions.

Self‐healing antifouling materials have gained rapidly increasing interest over the past decade and have been studied and used in a rapidly increasing range of applications. Recent developments and challenges in self‐healing antifouling materials are summarized in four sections: first, the different mechanisms for both antifouling and self‐healing are briefly discussed. Second, three main categories of self‐healing antifouling materials based on surface replenishing and dynamic covalent and noncovalent interactions are discussed, with a focus on the preparation, characterization, and central characteristics of different self‐healing antifouling materials. Third, different types of potential applications of self‐healing antifouling materials are summarized, such as injectable hydrogels and oil/water separations. Finally, a summary of future development of the field is provided, and a number of critical limitations that are still outstanding are highlighted.