Orthogonal functionalization of silicon substrates using self-assembled monolayers

Facile synthesis of an aminopropylsilane layer on Si/SiO2 substrates using ethanol as APTES solvent

(3-aminopropyl)triethoxysilane (APTES) is a commonly used organosilane on surface functionalization of silicon oxide surfaces. However, its deposition process from solution-phase usually involves the use of toluene, which has often been identified as crucial for the formation of an aminopropylsilane monolayer. Toluene is ranked as a problematic solvent in the guide developed by a group referred to as the solvent sub-team of CHEM21. In this work, we propose a facile synthetic route for functionalizing a silicon substrate with APTES via solution-phase approach using only solvents that are classified as recommended. The influence of the APTES concentration, reaction times and different post-deposition conditions using acetic acid and methanol were studied in order to evaluate the quality and thickness of the organosilane layers.•

The method uses ethanol as APTES solvent for functionalizing silicon dioxide surfaces and only uses solvents classified as recommended.

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The method uses a solution phase approach, does not require complicated equipment and can be prepared at room temperature.

Maskless Spatioselective Functionalization of Silicon Nanowires

Spatioselective functionalization of silicon nanowires was achieved without using a masking material. The designed process combines metal-assisted chemical etching (MACE) to fabricate silicon nanowires and hydrosilylation to form molecular monolayers. After MACE, a monolayer was formed on the exposed nanowire surfaces. A second MACE step was expected to elongate the nanowires, thus creating two different segments. When monolayers of 1-undecene or 1-tetradecyne were formed on the upper segment, however, the second MACE step did not extend the nanowires. In contrast, nanowires functionalized with 1,8-nonadiyne were elongated, but at an approximately 8 times slower etching rate. The elongation resulted in a contrast difference in high-resolution scanning electron microscopy (HR-SEM) images, which indicated the formation of nanowires that were covered with a monolayer only at the top parts. Click chemistry was successfully used for secondary functionalization of the monolayer with azide-functionalized nanoparticles. The spatioselective presence of 1,8-nonadiyne gave a threefold higher particle density on the upper segment functionalized with 1,8-nonadiyne than on the lower segment without monolayer. These results indicate the successful spatioselective functionalization of silicon nanowires fabricated by MACE.

Design and Fabrication of Wettability Gradients with Tunable Profiles through Degrafting Organosilane Layers from Silica Surfaces by Tetrabutylammonium Fluoride

Surface-bound wettability gradients allow for a high-throughput approach to evaluate surface interactions for many biological and chemical processes. Here we describe the fabrication of surface wettability gradients on flat surfaces by a simple, two-step procedure that permits precise tuning of the gradient profile. This process involves the deposition of homogeneous silane SAMs followed by the formation of a surface coverage gradient through the selective removal of silanes from the substrate. Removal of silanes from the surface is achieved by using tetrabutylammonium fluoride which selectively cleaves the Si-O bonds at the headgroup of the silane. The kinetics of degrafting has been modeled by using a series of first order rate equations, based on the number of attachment points broken to remove a silane from the surface. Degrafting of monofunctional silanes exhibits a single exponential decay in surface coverage; however, there is a delay in degrafting of trifunctional silanes due to the presence of multiple attachment points. The effects of degrafting temperature and time are examined in detail and demonstrate the ability to reliably and precisely control the gradient profile on the surface. We observe a relatively homogeneous coverage of silane (i.e., without the presence of islands or holes) throughout the degrafting process, providing a much more uniform surface when compared to additive approaches of gradient formation. Linear gradients were formed on the substrates to demonstrate the reproducibility and tuneability of this subtractive approach.

A Close Look at the Structure of the TiO2-APTES Interface in Hybrid Nanomaterials and Its Degradation Pathway: An Experimental and Theoretical Study

The surface functionalization of TiO₂-based materials with alkylsilanes is attractive in several cutting-edge applications, such as photovoltaics, sensors, and nanocarriers for the controlled release of bioactive molecules. (3-Aminopropyl)triethoxysilane (APTES) is able to self-assemble to form monolayers on TiO₂ surfaces, but its adsorption geometry and solar-induced photodegradation pathways are not well understood. We here employ advanced experimental (XPS, NEXAFS, AFM, HR-TEM, and FT-IR) and theoretical (plane-wave DFT) tools to investigate the preferential interaction mode of APTES on anatase TiO₂. We demonstrate that monomeric APTES chemisorption should proceed through covalent Si-O-Ti bonds. Although dimerization of the silane through Si-O-Si bonds is possible, further polymerization on the surface is scarcely probable. Terminal amino groups are expected to be partially involved in strong charge-assisted hydrogen bonds with surface hydroxyl groups of TiO₂, resulting in a reduced propensity to react with other species. Solar-induced mineralization proceeds through preferential cleavage of the alkyl groups, leading to the rapid loss of the terminal NH₂ moieties, whereas the Si-bearing head of APTES undergoes slower oxidation and remains bound to the surface. The suitability of employing the silane as a linker with other chemical species is discussed in the context of controlled degradation of APTES monolayers for drug release and surface patterning.

Polymer Surface Engineering for Efficient Printing of Highly Conductive Metal Nanoparticle Inks

An approach to polymer surface modification using self-assembled layers (SALs) of functional alkoxysilanes has been developed in order to improve the printability of silver nanoparticle inks and enhance adhesion between the metal conducting layer and the flexible polymer substrate. The SALs have been fully characterized by AFM, XPS, and WCA, and the resulting printability, adhesion, and electrical conductivity of the screen-printed metal contacts have been estimated by cross-cut tape test and 4-point probe measurements. It was shown that (3-mercaptopropyl)trimethoxysilane SALs enable significant adhesion improvements for both aqueous- and organic-based silver inks, approaching nearly 100% for PEN and PDMS substrates while exhibiting relatively low sheet resistance up to 0.1 Ω/sq. It was demonstrated that SALs containing functional -SH or -NH2 end groups offer the opportunity to increase the affinity of the polymer substrates to silver inks and thus to achieve efficient patterning of highly conductive structures on flexible and stretchable substrates.

Organic chemistry on surfaces: Direct cyclopropanation by dihalocarbene addition to vinyl terminated self-assembled monolayers (SAMs)

C11-Vinyl-terminated self-assembled monolayers (SAMs) on silica surfaces are successfully modified in C–C bond forming reactions with dihalocarbenes to generate SAMs, terminated with dihalo- (fluoro, chloro, bromo) cyclopropane motifs with about 30% surface coverage.

Soft liquid phase adsorption for fabrication of organic semiconductor films on wettability patterned surfaces

We report a soft liquid-phase adsorption (SLPA) technique for the fabrication of organic semiconductor films on wettability-patterned substrates using toluene/water emulsions. Wettability-patterned substrates were obtained by the UV-ozone treatment of self-assembled monolayers of silane coupling agents on glass plates using a metal mask. Organic semiconductor polymer films were formed selectively on the hydrophobic part of the wettability-patterned substrates. The thickness of the films fabricated by the SLPA technique is significantly larger than that of the films fabricated by dip-coating and spin-coating techniques. The film thickness can be controlled by adjusting the volume ratio of toluene to water, immersion angle, immersion temperature, and immersion time. The SLPA technique allows for the direct production of organic semiconductor films on wettability-patterned substrates with minimized material consumption and reduced number of fabrication steps.

Near-field photochemical and radiation-induced chemical fabrication of nanopatterns of a self-assembled silane monolayer

A general concept for parallel near-field photochemical and radiation-induced chemical processes for the fabrication of nanopatterns of a self-assembled monolayer (SAM) of (3-aminopropyl)triethoxysilane (APTES) is explored with three different processes: 1) a near-field photochemical process by photochemical bleaching of a monomolecular layer of dye molecules chemically bound to an APTES SAM, 2) a chemical process induced by oxygen plasma etching as well as 3) a combined near-field UV-photochemical and ozone-induced chemical process, which is applied directly to an APTES SAM. All approaches employ a sandwich configuration of the surface-supported SAM, and a lithographic mask in form of gold nanostructures fabricated through colloidal sphere lithography (CL), which is either exposed to visible light, oxygen plasma or an UV-ozone atmosphere. The gold mask has the function to inhibit the photochemical reactions by highly localized near-field interactions between metal mask and SAM and to inhibit the radiation-induced chemical reactions by casting a highly localized shadow. The removal of the gold mask reveals the SAM nanopattern.

Orthogonality in organic, polymer, and supramolecular chemistry: from Merrifield to click chemistry

The concept of orthogonality has been applied to many areas of chemistry, ranging from wave functions to chromatography. But it was Barany and Merrifield's orthogonal protecting group strategy that paved the way for solid phase peptide syntheses, other important classes of biomaterials such as oligosaccharides and oligonucleotides, and ultimately to a term in widespread usage that is focused on chemical reactivity and binding selectivity. The orthogonal protection strategy has been extended to the development of orthogonal activation, and recently the click reaction, for streamlining organic synthesis. The click reaction and its variants are considered orthogonal as the components react together in high yield and in the presence of many other functional groups. Likewise, supramolecular building blocks can also be orthogonal, thereby enabling programmed self-assembly, a superb strategy to create complex architectures. Overall, orthogonal reactions and supramolecular interactions have dramatically improved the syntheses, the preparation of functional materials, and the self-assembly of nanoscale structures.

"Soft" liquid-phase adsorption for the fabrication of solution processable organic material films on wettability-patterned surfaces

We discuss a "soft" liquid-phase adsorption (SLPA) technique for the fabrication of organic films on wettability-patterned templates by using methanol/hexane and toluene/water emulsions. The emulsions are stable for several hours at room temperature, and the diameters of the dispersed phase are estimated to be several micrometers using dynamic light scattering. The templates are fabricated by exposing self-assembled monolayers to an ultraviolet/ozone atmosphere through a shadow mask. Fluorescent dye and semiconductor polymer films are formed selectively on the hydrophilic region of the substrates and the hydrophobic region of the self-assembled monolayer, respectively. The thickness is significantly larger than those of the films fabricated by conventional film-forming techniques such as spin-coating and dip-coating, respectively. These patterned films serve as photoluminescent films. These results demonstrate that the SLPA technique allows for the fabrication of organic films on wettability-patterned templates using solution-processable materials. This technique will find application to the fabrication of electronic and photonic devices with small material consumption and few film-forming processes.

Vapor-based tri-functional coatings

The tri-functional coating synthesized via CVD copolymerization is comprised of distinguished anchoring sites of acetylene, maleimide, and ketone that can synergically undergo specific conjugation reactions to render surfaces with distinct biological functions, simultaneously. In addition, these tri-functional coatings can be fabricated in a micro-structured fashion on non-conventional surfaces.

Wettability of bare and fluorinated silanes: a combined approach based on surface free energy evaluations and dipole moment calculations

The assessment of the surface free energy (SFE) of a material permits to control and predict a large number of physicochemical properties of a solid surface and its reactivity. Here, the surface energies of a series of bare and fluorinated silanes are determined by means of different semi-empirical models on the grounds of contact angle determinations for different solvents. Literature data are also considered in order to produce a series of films with increasing SFE. Wetting envelopes (WE) are obtained for the various surfaces in order to predict the wettability of the films by numerous commonly employed solvents. The polar and disperse components of the surface energies are obtained by the Owens-Wendt-Rabel-Kaelbe (OWRK) method; the values of the polar components are compared with gas-phase dipole moments obtained by theoretical calculations employing semi-empirical Hamiltonians. The sequences of the polar components of the SFE and of the calculated dipole moments for the different molecules are strictly the same. The interplay between theoretical and experimental approaches proves efficient in predicting the behavior of different systems and it could be employed in tuning the SFE of a solid surface with respect to its final applications.

Terpyridine-functionalized surfaces: redox-active, switchable, and electroactive nanoarchitecturesgland

Terpyridines represent versatile functional supramolecular building blocks that are easily integrated in numerous devices and can readily modify surfaces. In particular, redox-active complexes with terpyridine ligands have been attached to surfaces, either by covalent or non-covalent interactions, and form highly ordered mono- or multilayer systems, since electronic and charge transport properties are major topics of interest. Their applications in nanoelectronics are a driving force for understanding and enabling the utilization of the supramolecular properties of terpyridines for surface modification. This area of research has received increasing attention during the last decade leading into the supramacromolecular regime. This Progress Report presents an overview of the state-of-the-art of surface modifications utilizing terpyridine systems and highlights main results, as well as modern trends, in this research area.