Deposition of dense siloxane monolayers from water and trimethoxyorganosilane vapor

Review: 3-Aminopropyltriethoxysilane (APTES) Deposition Methods on Oxide Surfaces in Solution and Vapor Phases for Biosensing Applications

Surface functionalization and bioreceptor immobilization are critical processes in developing a highly sensitive and selective biosensor. The silanization process with 3-aminopropyltriethoxysilane (APTES) on oxide surfaces is frequently used for surface functionalization because of beneficial characteristics such as its bifunctional nature and low cost. Optimizing the deposition process of the APTES layer to obtain a monolayer is crucial to having a stable surface and effectively immobilizing the bioreceptors, which leads to the improved repeatability and sensitivity of the biosensor. This review provides an overview of APTES deposition methods, categorized into the solution-phase and vapor-phase, and a comprehensive summary and guide for creating stable APTES monolayers on oxide surfaces for biosensing applications. A brief explanation of APTES is introduced, and the APTES deposition methods with their pre/post-treatments and characterization results are discussed. Lastly, APTES deposition methods on nanoparticles used for biosensors are briefly described.

Silane-Based SAMs Deposited by Spin Coating as a Versatile Alternative Process to Solution Immersion

Functionalization of silica surfaces with silane-based self-assembled monolayers (SAMs) is widely used in material sciences to tune surface properties and introduce terminal functional groups enabling subsequent chemical surface reactions and immobilization of (bio)molecules. Here, we report on the synthesis of four organotrimethoxysilanes with various molecular structures and we compare their grafting by spin coating with the one performed by the conventional solution immersion method. Strikingly, this study clearly demonstrates that the spin coating technique is a versatile, fast, and more convenient alternative process to prepare robust, smooth, and homogeneous SAMs with similar properties and quality as those deposited via immersion. SAMs were characterized by PM-IRRAS, AFM, and wettability measurements. SAMs can undergo several chemical surface modifications, and the reactivity of amine-terminated SAM was confirmed by PM-IRRAS and fluorescence measurements.

Detecting and Removing Defects in Organosilane Self-Assembled Monolayers

Defects occur as self-assembled monolayers form, and the number and type of defects depend on the surface preparation and deposition solvent, among other parameters. Indirect measures to detect defects using a layer property, such as the thickness or bond vibrational frequency, are used routinely for process development but often lack sensitivity. Direct measures using an atomic probe offer a glimpse of defect structures but over a small fraction of the layer. Direct detection after reacting defects by etching or deposition is more common, and this approach has advanced our understanding of how monolayers form and has led to improved monolayers for a variety of applications. Here we show that a series of TiCl₄ gas pulses reacts with defects in organosilane layers on SiO₂ depositing TiO, which was measured by X-ray photoelectron spectroscopy. The defects were silanol groups and siloxane bridge bonds at the interface between the layer and the SiO₂ surface and on agglomerates physisorbed to the layer. As the TiO saturation coverage or the total number of defects decreased, the incubation period in which no TiO was detected became longer. Cleaning the layer by solvent extraction to remove nonpolar agglomerates followed by an aqueous mixture of ammonium hydroxide and hydrogen peroxide, which is Standard Clean 1, a common particle removal step for silicon surfaces, produced an organosilane monolayer without agglomerates based on atomic force microscopy. After a second organosilane immersion, the monolayer density rose to 3.8 molecules/nm². This monolayer inhibited the deposition of TiO on the SiO₂ surface for 250 pulses of TiCl₄ and 200 complete TiO₂ atomic layer deposition cycles using TiCl₄ and water vapor, and it failed at 300 complete cycles. The Standard Clean 1 solution not only removed defects left by solvent extraction but also led to the reorganization of the organosilane layer.

Molecular Layer Deposition of a Highly Stable Silicon Oxycarbide Thin Film Using an Organic Chlorosilane and Water

In this study, molecular layer deposition (MLD) was used to deposit ultrathin films of methylene-bridged silicon oxycarbide (SiOC) using bis(trichlorosilyl)methane and water as precursors at room temperature. By utilizing bifunctional trichlorosilane precursors, films of SiOC can be deposited in a layer-by-layer manner, wherein a water co-reactant circumvents the need for plasma, high temperatures, or highly oxidizing precursors. In this manner, films could be grown without the degradation commonly seen in other SiOC deposition methods. Saturation behavior for both precursors was confirmed for the MLD process, and a constant growth rate of 0.5 ± 0.1 Å/cycle was determined. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy were used to verify the reaction between precursors and to gain insight into the final film composition. Unlike most MLD films, which grow polymers in a linear fashion, XPS analysis indicates that neighboring silanol groups within the films tend to condense, forming a highly cross-linked network structure, whereby, on average, two-thirds of silanol groups undergo a condensation reaction. Further indication of cross-linking is seen by XPS during in situ annealing, which shows exceptional temperature stability of the film up to 600 °C in vacuum, in contrast to linear SiOC films, which are known to degrade below this temperature. The films also exhibit high chemical stability against acids, bases, and solvents. A film density of 1.4 g/cm³ was measured by X-ray reflectivity, while the dielectric constant and refractive index were determined to be 2.6 ± 0.3 and 1.6 ± 0.1, respectively, at a 633 nm wavelength. The low dielectric constant, high ease of deposition, and exceptional thermal and chemical stabilities of this MLD SiOC film suggest that it may have potential applications for electronic devices.

Vapor-Phase Nanopatterning of Aminosilanes with Electron Beam Lithography: Understanding and Minimizing Background Functionalization

Electron beam lithography (EBL) is a highly precise, serial method for patterning surfaces. Positive tone EBL resists enable patterned exposure of the underlying surface, which can be subsequently functionalized for the application of interest. In the case of widely used native oxide-capped silicon surfaces, coupling an activated silane with electron beam lithography would enable nanoscale chemical patterning of the exposed regions. Aminoalkoxysilanes are extremely useful due to their reactive amino functionality but have seen little attention for nanopatterning silicon surfaces with an EBL resist due to background contamination. In this work, we investigated three commercial positive tone EBL resists, PMMA (950k and 495k) and ZEP520A (57k), as templates for vapor-phase patterning of two commonly used aminoalkoxysilanes, 3-aminopropyltrimethoxysilane (APTMS) and 3-aminopropyldiisopropylethoxysilane (APDIPES). The PMMA resists were susceptible to significant background reaction within unpatterned areas, a problem that was particularly acute with APTMS. On the other hand, with both APTMS and APDIPES exposure, unpatterned regions of silicon covered by the ZEP520A resist emerged pristine, as shown both with SEM images of the surfaces of the underlying silicon and through the lack of electrostatically driven binding of negatively charged gold nanoparticles. The ZEP520A resist allowed for the highly selective deposition of these alkoxyaminosilanes in the exposed areas, leaving the unpatterned areas clean, a claim also supported by contact angle measurements with four probe liquids and X-ray photoelectron spectroscopy (XPS). We investigated the mechanistic reasons for the stark contrast between the PMMA resists and ZEP520A, and it was found that the efficacy of resist removal appeared to be the critical factor in reducing the background functionalization. Differences in the molecular weight of the PMMA resists and the resulting influence on APTMS diffusion through the resist films are unlikely to have a significant impact. Area-selective nanopatterning of 15 nm gold nanoparticles using the ZEP520A resist was demonstrated, with no observable background conjugation noted in the unexposed areas on the silicon surface by SEM.

DNA's Chiral Spine of Hydration

The iconic helical structure of DNA is stabilized by the solvation environment, where a change in the hydration state can lead to dramatic changes to the DNA structure. X-ray diffraction experiments at cryogenic temperatures have shown crystallographic water molecules in the minor groove of DNA, which has led to the notion of a spine of hydration of DNA. Here, chiral nonlinear vibrational spectroscopy of two DNA sequences shows that not only do such structural water molecules exist in solution at ambient conditions but that they form a chiral superstructure: a chiral spine of hydration. This is the first observation of a chiral water superstructure templated by a biomolecule. While the biological relevance of a chiral spine of hydration is unknown, the method provides a direct way to interrogate the properties of the hydration environment of DNA and water in biological settings without the use of labels.

Ordered porous structure hybrid films generated by breath figures for directional water penetration

A highly ordered open-pore hybrid film was fabricated by controlling the substrate roughness and wettability. The composite with different wettability on the two side resulted in an attractive unidirectional water-penetration function (see figure).

Conversion of bilayers of PS-b-PDMS block copolymer into closely packed, aligned silica nanopatterns

Block copolymer (BCP) self-assembly is an effective and versatile approach for the production of complex nanopatterned interfaces. Monolayers of BCP films can be harnessed to produce a variety of different patterns, including lines, with specific spacings and order. In this work, bilayers of cylinder-forming polystyrene-block-polydimethylsiloxane block copolymer (PS-b-PDMS) were transformed into arrays of silica lines with half the pitch normally attained for conventional monolayers, with the PDMS acting as the source for the SiOx. The primary hurdle was ensuring the bilayer silica lines were distinctly separate; to attain the control necessary to prevent overlap, a number of variables related to the materials and self-assembly process were investigated in detail. Developing a detailed understanding of BCP film swelling during solvent annealing, blending of the PS-b-PDMS with PS homopolymer, utilization of a surface brush layer, and adjustment of the plasma exposure conditions, distinct and separate silica lines were prepared. On the microscale, the sample coverage of PS-b-PDMS bilayers was investigated and maximized to attain >95% bilayers under defined conditions. The bilayer BCP structures were also amenable to graphoepitaxy, and thus, dense and highly ordered arrays of silica line patterns with tightly controlled width and pitch were fabricated and distributed uniformly across a Si surface.

Squish and CuAAC: additive-free covalent monolayers of discrete molecules in seconds

A terminal alkyne is immobilized rapidly into a full monolayer by squishing a small volume of a solution of the alkyne between an azide-modified surface and a copper plate. The monolayer is covalently attached to the surface through a copper-catalyzed alkyne-azide cycloaddition (CuAAC) reaction, and the coverages of the immobilized electroactive alkyne species are quantified by cyclic voltammetry. A reaction time of less than 20 s is possible with no other reagents required. The procedure is effective under aerobic conditions using either an aqueous or aprotic organic solution of the alkyne (1-100 mM).