Organosilane Monolayer Research Articles

Nanoscopic Surface Architecture Based on Scanning Probe Electrochemistry and Molecular Self-Assembly

Coplanar nanostructures consisting of two different types of organosilane monolayers have been fabricated using scanning probe microscopes (SPMs) and served as templates for the area-selective immobilization of various materials. The first organosilane monolayer, which had been uniformly prepared on a substrate, was locally degraded through electrochemistry of adsorbed water at the junction of the monolayer and the SPM probe. This probe-scanned region chemisorbed molecules of the second organosilane, resulting in the creation of a self-assembled monolayer (SAM) confined to the SPM-defined pattern. Latex nanoparticles or proteins selectively assembled onto this patterned SAM.

Imaging Bacteriophage T4 on Patterned Organosilane Monolayers by Scanning Force Microscopy

A patterned organosilane monolayer consisting of CH3-terminated islands surrounded by a CF3-terminated continuous phase is used as a template for controlling the adsorption of biological molecules. In coadsorption of bovine serine albumin (BSA) and bacteriophage T4 and its tail, the T4 adheres to the CF3-terminated surfaces while the BSA is restricted to the CH3-terminated islands. The strong affinity of the T4 for the continuous phase allows detailed imaging by scanning force microscopy in both topographic and frictional force modes. Structural features of the tail tubes are clearly recognized. The baseplates at end of the tubes exhibit different configurations, and in images of T4 obtained by frictional force one can clearly distinguish head, neck, tail, baseplate and fibers. For giant T4 with elongated heads, the surface lattice of the head and the sheath annuli of the tail are resolved in some detail.

AFM lithography in constant current mode

An organosilane monolayer consisting of trimethylsilyl ([ - ) groups prepared on the native oxide of a silicon substrate effectively served as a resist material for AFM-based nanolithography. The patterning of this resist was performed through its electrochemical degradation locally induced around the contact point of a conductive AFM probe while biasing the sample substrate positively. In the region where the probe passed, the monolayer resist was degraded and the underlying silicon oxide surface was selectively uncovered. The number of electrons injected into the probe-scanned region was controlled by conducting the AFM lithography in constant current mode. By means of this constant current AFM lithography a sufficient amount of electrons could be injected even at high probe-scan rates faster than . It was demonstrated that the TMS monolayer resist was sensitive enough to allow line drawing at a probe scan rate of .

Scanning probe nanofabrication of chemically active areas on substrate covered with organosilane monolayers

An organosilane monolayer self-assembled onto Si substrates was patterned using a scanning tunneling microscope or an atomic force microscope with a conductive probe. The tip-scanned area was etched by wet etching, while the unscanned area remained resistant to the etching. Consequently, the tip-scanned patterns were transferred to the substrate Si using the scanned area as an etching window. Moreover, the tip-scanned area preferentially reacted with other organosilane molecules, e.g., 3-aminopropyltrimethoxysilane, to form a monolayer confined to the scanned pattern. Next, onto this amino-terminated monolayer, aldehyde-modified latex nanoparticles were immobilized area selectively through chemical reaction between the amino and aldehyde groups. It was demonstrated that in the areas where the probe tip of the microscopes had been scanned, the surface was terminated with hydroxyl groups and became active to various chemical treatments.

Measurements of Single-Molecule Bond-Rupture Forces between Self-Assembled Monolayers of Organosilanes with the Atomic Force Microscope

Atomic force microscopy (AFM) tips and glass surfaces were modified with various organosilanes to determine magnitude and dispersion information about single-molecule bond-rupture forces. X-ray photoelectron spectroscopy (XPS) and contact-angle measurements were used to study and quantify organosilane adsorption on the glass surface and on SiO2-coated AFM tips. Hydrogen bond interactions between hydroxyl- and thiol-terminated groups on the tip and surface were detected and measured. Differentiation between the functionalities of the acetate- and thioacetate-terminated silanes and their reduced forms produced by on-surface reduction (the alcohol and thiol, respectively) was also accomplished. The experiments demonstrate the complementary information that can be obtained from AFM and XPS and illustrate how they can be used to determine the nature of the surface after an organic transformation has occurred to the functional groups present. They also represent a first step in detecting chemical reactions on a localized scale and in measuring the dispersion in the single-molecule bond-rupture force when it exists.

Charge Dissipation on Chemically Treated Thin Silicon Oxide in Air

We studied the microscopic charge dissipation of densely contact-electrified charges on silicon oxides with and without a trimethylsilyl (TMS) organosilane monolayer using a modified atomic force microscope in air. Here, the TMS film was used to change a hydrophilic surface to a hydrophobic one. As a result, for both of the deposited negative and positive charges, it was clarified that the TMS film can suppress the rapid dissipation induced by surface diffusion. For the positive charge, however, the TMS film enhanced the charge dissipation induced by recombination through the TMS film and silicon oxide.

Electroless Deposition of Metals onto Organosilane Monolayers

Deposition of gold and copper onto monolayers of 3‐aminopropylsilane and 3‐mercaptopropylsilane is described. The depositions described here are performed without the aid of a palladium or tin/palladium catalyst. Strongly adherent metal particles attached to the modified surfaces can be produced by placing the silane surfaces either in aqueous solutions of the metal ion of interest ( or ), followed by exposure of the metal/silane surface to a solution containing only reducing agent (denoted as sequentially prepared films), or aqueous solutions of a reducing agent mixed with the metal ions (denoted as autocatalytic films). Scanning force microscopy, in contact mode, was used to examine the morphology of the resulting nanometer‐sized metal deposits. The metal deposits have been used as nucleation layers for the electroless deposition of conductive, reflective copper films.

Organosilane Monolayer Resists for Scanning Probe Lithography.

We report the preparation of organosilane monolayers by means of chemical vapor deposition and scanning probe lithography (SPL) using these monolayers as resists. Two types of monolayers having different alkyl groups, that is, trimethylsilyl [(CH3)3Si-, TMS] and octadecylsilyl [CH3(CH2)17Si-, ODS] monolayers, were deposited on Si substrate from precursor molecules of hexamethyldisilazane [(CH3)3Si-NH-Si(CH3)3, ] and octadecyltrimethoxysilane [CH3(CH2)17Si(OCH3)3], respectively. The thicknesses of these monolayers as estimated by ellipsometry were 0.4 and 2.0nm, respectively, and, therefore, were concluded to be defined by their alkyl chain lengths. Using an atomic force microscope (AFM) with an electrically conductive probe, a Si sample deposited with a TMS or ODS monolayer was patterned by flowing current through the AFM-probe/sample junction. The pattern thus fabricated on the monolayer was transferred to the substrate Si by chemical etching in an aqueous solution of NH4F and H2O2. The etching proceeded area-selectively in the regions where the probe had passed, since, in these regions, the monolayer had been electrochemically degraded. The patterning was entirely accomplished through probe scanning without any need for further developing steps. The resist can thus be regarded as being self-developing. Both TMS and ODS successfully served as such SPL resists.

Nanoscale patterning of an organosilane monolayer on the basis of tip-induced electrochemistry in atomic force microscopy

An organosilane trimethylsilyl (TMS) monolayer prepared on silicon (Si) substrate by chemical vapor deposition was successfully applied as a self-developing resist for atomic force microscope (AFM) lithography. The thickness of the monolayer was less than 1 nm. This resist was locally degraded due to electrochemical reactions induced in the junction between a conductive AFM probe and a Si–TMS sample. The generated pattern on the sample was then transferred to the Si substrate by chemical etching using the degraded region as an etching window. Degradation of the monolayer proceeded with both positive and negative sample biases. However, the absolute values of the voltage at which the probe-scanned region began to show etching were +3.0 for Vs>0 and −5.0 V for Vs<0, in a 60% relative humidity air atmosphere. Faster patterning was achieved through increased current flow by applying a higher bias voltage. A 500 μm/s line drawing at Vs=+20.0 V with 2–3 nA was obtained. The number of injected electrons was estimated to be hundreds of times larger than the number of TMS groups in the scanned area. We therefore concluded that only a small part of the current flowing through the probe-sample junction is actually responsible for the degradation.

Electrochemical nanolithography using scanning probe microscopy: fabrication of patterned metal structures on silicon substrates

The application of scanning probe anodization to nanolithography is described. The electrochemical processing of material surfaces at nanometer scale both laterally and vertically was conducted by scanning probe anodization, which used the tip-sample junction of a scanning probe microscope connected with an adsorbed water column as a minute electrochemical cell. Silicon (Si) samples whose surfaces were terminated with hydrogen (Si−H samples) or covered with an organosilane monolayer were used. Silicon oxide (SiO x ) patterns were prepared on the Si−H sample surfaces by the use of anodization of Si, while in the second case the organosilane layer was selectively degraded by anodic corrosion. Furthermore, pattern transfer processes that fabricated metal nanostructures using these patterned SiO x or organosilane layers as templates were developed. These processes are based on area-selective electroless plating where selectivity depends on the difference in the chemical reactivity between the surface modified by scanning probe anodization and the unmodified surface.

Fabrication of patterned DNA surfaces

Two photolithographic methods are described for the formation of patterned single or multiple DNA species on SiO2 substrates. In the first approach, substrates are treated with a photochemically labile organosilane monolayer film. Irradiation of these surfaces with patterned deep UV (193 nm) light results in patterned chemically reactive groups which are then reacted with heterobifunctional crosslinking molecules. Covalent attachment of modified synthetic DNA oligomers to the crosslinker results in stable DNA patterns. Alternatively, a photoresist is spin-coated over a silane film which had been previously modified with the heterobifunctional crosslinker. Upon patterned irradiation and subsequent development, the underlying crosslinker-modified layer is revealed, and is then reacted with a chemically modified DNA. Feature dimensions to 1 micron are observed when a single fluorescent DNA is attached to the surface. By performing sequential exposures, we have successfully immobilized two distinguishable DNA oligomers on a single surface. Synthetic DNA immobilized in this manner retains the ability to hybridize to its complementary strand, suggesting that these approaches may find utility in the development of miniaturized DNA-based biosensors.

Scanning Probe Lithography Using a Trimethylsilyl Organosilane Monolayer Resist

A trimethylsilyl organosilane monolayer was applied as a self-developing resist for scanning probe microscope (SPM) lithography under ambient conditions. Both a scanning tunneling microscope and an atomic force microscope with a conductive probe were applied and shown to be effective for this lithography. The resist was prepared on a silicon (Si) substrate by chemical vapor deposition, and was patterned through the localized degradation of the organosilane monolayer induced by scanning an SPM tip while positively (+5∼+10 V) biasing the substrate. The degradation rate was enhanced by increasing the concentration of atmospheric water vapor or oxygen. The patterning mechanism was identified as tip-induced anodic oxidation, i.e., scanning probe anodization. The pattern was transferred to the Si substrate through selective chemical etching in which the degraded monolayer regions served as etching windows. Consequently, fine grooves as narrow as 30 nm could be fabricated.

Scanning probe anodization: Nanolithography using thin films of anodically oxidizable materials as resists

We report scanning probe microscope (SPM) nanolithography that uses thin films patternable by scanning probe anodization as resists. An organosilane monolayer composed of trimethylsilyl [−Si(CH3)3] groups was prepared by chemical vapor deposition on a Si substrate. It was then patterned through the localized degradation of the monolayer as a result of anodic reactions induced beneath a SPM tip. The fabricated patterns on the resist were transferred into the substrate Si by area-selective chemical etching in an aqueous NH4F/H2O2 solution. Consequently, grooves narrower than 100 nm were successfully fabricated. We have also developed another resist system for SPM lithography that is compatible with a wide range of surfaces, including insulators. The resist consisted of a thin Ti film covered with a fluoroalkylsilane monolayer. The monolayer was degraded along a tip-scanning trace. This patterned resist was etched in a dilute HF solution. The chemical etching of the underlying Ti film proceeded selectively in the region where the monolayer had been degraded, and the pattern was transferred into the Ti resist film.

Scanning force microscopic studies of surface structure and protein adsorption behavior of organosilane monolayers

Alkyltrichlorosilanes, [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) and their mixed monolayers were polymerized on the water subphase and were subsequently immobilized onto the silicon wafer surface by covalent bonding. Atomic force microscopic (AFM) observation of the [n-octadecyltrichlorosilane(OTS)/FOETS] mixed monolayer revealed that the crystalline OTS formed circular domains of ∼1–2 μm in diameter that were surrounded by an amorphous FOETS matrix. Lateral force microscopic (LFM) and scanning viscoelasticity microscopic (SVM) observations of the (OTS/FOETS) mixed monolayer could also distinguish the difference in the physical and mechanical properties between domain and matrix. The (alkylsilane/FOETS) mixed monolayers with a shorter alkyl chain such as n-dodecyltrichlorosilane (DDTS) did not show the phase-separated structure maybe due to the lack in crystallinity of DDTS component. AFM observation of the [crystalline fatty acid (nonreactive)/FOETS (reactive)] mixed monolayer also revealed the phase-separated structure. The crystalline fatty acid domains in the mixed monolayer were extracted by solvent and then, the monolayer of FOETS with the holes of ∼1–2 μm in diameter, that is, the (Si/FOETS) monolayer was finally obtained. The Si phase of (Si/FOETS) was backfilled with the another organosilane by chemisorption. Then, the [(3-mercaptopropyl) trimethoxysilane (MTS)/FOETS] mixed monolayer was newly obtained by the chemisorption of MTS. The absorption of bovine serum albumin (BSA) onto mixed monolayers was studied by exposure of the mixed monolayers to a BSA solution. BSA was preferentially adsorbed onto the MTS part due to a specific reaction between disulfide bond (–S–S–) in BSA and SH groups in MTS. On the other hand, in the case of (OTS/FOETS) mixed monolayer, BSA adsorbed preferentially onto the FOETS matrix in order to minimize the interfacial free energy in the aqueous environment.

Phase-Separated Two-Component Self-Assembled Organosilane Monolayers and Their Use in Selective Adsorption of a Protein

Stable mixed monolayers are prepared by combining Langmuir−Blodgett (LB) deposition with self-assembly. Islands of stable organosilane monolayers are formed on mica by the LB technique and the bare...

Spectroscopic characterization of self-assembled organosilane monolayer films

We have used the scanning tunneling microscope (STM) to investigate self-assembled monolayer films formed from three organosilane precursors. We have studied these films in order to understand better the interaction between them and the microscope and their utility as resist materials for STM-based lithography. These films were chemisorbed onto hydrogen-passivated silicon substrates. One was studied immediately after film growth; another sample of that film, as well as the other two, were studied four weeks later. We compared topography, morphology, and site-specific current–voltage spectroscopy of the films with each other before and after patterning the films. Patterning consisted of operating the STM at tip biases expected to induce a modification of the film that inhibits an electroless metallization process we have described previously. Two films did not exhibit observable changes after operation at conditions expected to induce exposure. We attribute this to high resist sensitivity and a low voltage threshold for exposure, and thus inadvertant exposure. The film that was studied when fresh and after aging in air exhibited changes attributable to oxidation of the substrate. We conclude that the STM is interacting directly with the films and is sensitive to their chemical composition. The effects of exposure on the individual molecular precursors are slight. The molecules remain present on the surface, even though the chemical functionality is greatly modified.

Degradation of a Trimethylsilyl Monolayer on Silicon Substrates Induced by Scanning Probe Anodization

We report on nanolithography using a scanning probe microscope (SPM) as an exposure tool of an organosilane resist consisting of a trimethylsilyl monolayer. This monolayer was prepared on silicon substrates by chemical vapor surface modification with hexamethyldisilazane. The SPM exposure of the resist was conducted under air, nitrogen (N 2 ), and oxygen (O 2 ) atmospheres. The trimethylsilyl monolayer was locally degraded along a scanning trace at a sample bias of +5.0 V. The degradation of the organosilane monolayer was induced irrespective of the atmosphere. This degraded region served as an etching window in an aqueous solution of ammonium fluoride and hydrogen peroxide. Consequently, etched grooves of 80 nm in width were fabricated. The degradation mechanism is discussed in terms of SPM tip induced anodic oxidation.

Area-selective formation of an organosilane monolayer on silicon oxide nanopatterns fabricated by scanning probe anodization

An organosilane monolayer was formed from a precursor vapor (trimethylchlorosilane, TMCS) onto the surface of silicon oxide (SiOx) nanopatterns surrounded by hydrogen-terminated silicon (Si–H). The nanopattern was fabricated with a resolution of 20 nm by scanning probe anodization, that is, localized anodization induced beneath the tip of a scanning probe microscope. The area-selectivity arises from the difference in the chemical reactivity of the vapor between the SiOx and Si–H surfaces. The TMCS-coated SiOx patterns showed a resistivity to chemical etching with an aqueous solution of ammonium fluoride and hydrogen peroxide. Lateral force microscopy with an organosilane-coated probe also indicated the presence of the monolayer on SiOx through a friction force contrast between the monolayer-coated and uncoated regions.

Control of Density in Self-Assembled Organosilane Monolayers by Langmuir-Blodgett Deposition

Control of Density in Self-Assembled Organosilane Monolayers by Langmuir-Blodgett Deposition

Phase Separated Morphology of an Immobilized Organosilane Monolayer Studied by a Scanning Probe Microscope

Alkyltrichlorosilanes and [2-(perfluorooctyl)ethyl]trichlorosilane (FOETS) mixed monolayers were prepared on the water subphase and were covalently immobilized onto the silicon wafer surface. Atomic force microscopic observation of the octadecyltrichlorosilane (OTS)/FOETS mixed monolayer revealed that the crystalline OTS formed circular domains of ca. 1-2 μm diameter, which were surrounded by a sealike amorphous FOETS even though molar fraction of OTS was above 75%. The alkylsilane/FOETS mixed monolayers with shorter alkyl chain length such as n-dodecyltrichlorosilane (DDTS) did not show the phase-separated structure. Also, the OTS/FOETS mixed monolayer prepared by a chemisorption method did not show distinct phase separation. Friction force microscopic observation of the OTS/FOETS mixed monolayer revealed that the OTS domain had higher frictional coefficient compared with that of the FOETS matrix. On the basis of scanning viscoelasticity microscopic observation, it was revealed that the crystalline OTS domain had higher modulus than that of the amorphous FOETS matrix. Morphological observation of the stearic acid (nonreactive)/FOETS (reactive) mixed monolayer also exhibited the phase-separated structure. The stearic acid domain in the mixed monolayer was easily extracted by solvent, and then the ghost monolayer of FOETS with holes of ca. 1-3 μm diameter was finally obtained