Two-dimensional Self-assembled Monolayers Research Articles

Time of Flight – Secondary Ion Mass Spectroscopy Profiling of Self-Assembled Monolayer Patterns Based on Vapor Deposition Technique

It is crucial to develop novel metrology techniques in a semiconductor fabrication process for characterizing a thin film with a thickness of a few nanometers, as well as the material profile of the film accurately. Uniform trichlorosilane (1H,1H,2H,2H-perfluorodecyltrichlorosilane) derived self-assembled monolayer film patterns were fabricated by several conventional semiconductor fabrication techniques incorporated, including photolithography, vapor deposition, and lift-off technique. The patterned trichlorosilane self-assembled monolayer patterns were thoroughly characterized by Time-of-Flight Secondary Ion Mass Spectrometry, and precise two-dimensional self-assembled monolayer patterns were reconstructed through corresponding ions and ion clusters feature peaks. Additionally, three-dimensional self-assembled monolayer patterns were reconstructed layer by layer through gas cluster ion beam etching and two-dimensional mapping analysis in an alternate way, which verifies that vapor-based trichlorosilane self-assembled monolayer patterns were precisely fabricated and Time-of-Flight Secondary Ion Mass Spectrometry is highly surface-sensitive to obtain accurate information about the self-assembled monolayer pattern in a point-by-point manner. Time-of-Flight Secondary Ion Mass Spectrometry technique could be used as a metrology technique in the semiconductor process with high-quality self-assembled monolayer micro and nanostructures.

Curvature instability of chiral colloidal membranes on crystallization

Buckling and wrinkling instabilities are failure modes of elastic sheets that are avoided in the traditional material design. Recently, a new paradigm has appeared where these instabilities are instead being utilized for high-performance applications. Multiple approaches such as heterogeneous gelation, capillary stresses, and confinement have been used to shape thin macroscopic elastic sheets. However, it remains a challenge to shape two-dimensional self-assembled monolayers at colloidal or molecular length scales. Here, we show the existence of a curvature instability that arises during the crystallization of finite-sized monolayer membranes of chiral colloidal rods. While the bulk of the membrane crystallizes, its edge remains fluid like and exhibits chiral ordering. The resulting internal stresses cause the flat membrane to buckle macroscopically and wrinkle locally. Our results demonstrate an alternate pathway based on intrinsic stresses instead of the usual external ones to assemble non-Euclidean sheets at the colloidal length scale.

Scanning tunneling microscopy investigation of nano-structured α-K5PW11(M x OH2)O39(M = Mn(II), Co(II), Ni(II), and Zn(II)) Keggin heteropolyacid catalyst monolayers.

Nano-structured α-K5PW11(M x OH2)O39 (M = Mn(II), Co(II), Ni(II), and Zn(II)) Keggin heteropolyacids (HPAs) were investigated by scanning tunneling microscopy (STM) and tunneling spectroscopy (TS) measurements in order to elucidate their redox property and oxidation catalysis. HPA molecules formed two-dimensional self-assembled monolayer arrays on highly oriented pyrolytic graphite (HOPG) surface. Furthermore, HPAs exhibited a distinctive current-voltage behavior referred to as negative differential resistance (NDR) phenomenon. The measured NDR peak voltage of HPAs was correlated with the reduction potential and the absorption edge energy determined by electrochemical method and UV-visible spectroscopy, respectively. NDR peak voltage of HPAs appeared at less negative voltage with increasing reduction potential and with decreasing UV-visible absorption edge energy. The correlations strongly suggested that NDR phenomenon was closely related to the redox property of HPAs. Vapor-phase oxidation of benzyl alcohol to benzaldehyde was carried out as a model reaction to track the oxidation catalysis of HPAs. NDR peak voltage appeared at less negative voltage with increasing yield for benzaldehyde.

Nanotribological characteristics of TiO2 films on sulphonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Silane coupling regent [3-mercaptopropyl trimethoxysilane (MPTS)] was prepared on silicon substrate to form two-dimensional self-assembled monolayer (SAM) and the terminal −SH group in the film was in situ oxidised to −SO3H group to endow the film with good chemisorption ability. Thus, TiO2 thin films were deposited on the oxidised MPTS-SAM to form composite thin films, making use of the chemisorption ability of the −SO3H group. Atomic force microscope (AFM) and contact angle measurements were used to characterise TiO2 films. Adhesive force and friction force of TiO2 thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidity. Results showed that the adhesive force of silicon substrate increases with relative humidity and the adhesive force of TiO2 thin films only increases slightly with relative humidity. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.

Nano-tribological characteristics of TiO2 films on 3-mercaptopropyl trimethoxysilane sulfonated self-assembled monolayer

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was used to prepare twodimensional self-assembled monolayer (SAM) on silicon substrate. The terminal -SH group was in situ oxidized to −SO3H group to endow the film with good chemisorption ability. Then TiO2 thin films were deposited on the oxidized MPTS-SAM to form composite thin films, making use of the chemisorption ability of the −SO3H group. Atomic force microscope (AFM) and contact angle measurements were used to characterize TiO2 films. Adhesive force and friction force of TiO2 thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidities. Results showed that the adhesive force of silicon substrate increases with relative humidities and the adhesive force of TiO2 thin films only increases slightly with relative humidity. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.

Novel self-assembled films of La-based phosphonate 3-aminopropyltriethoxysilane

Silane coupling reagent (3-aminopropyltriethoxysilane (APTES)) was prepared on single-crystal silicon substrates to form two-dimensional self-assembled monolayer (SAM). The terminal –NH 2 groups in the film were in situ phosphorylated to –PO(OH) 2 group to endow the film with good chemisorption ability. Then La-based thin films were deposited on phosphorylated APTES-SAM in order to make good use of the chemisorption ability of –PO(OH) 2 groups. The thickness of the film was determined with ellipsometer, while phase transformation and surface morphology, surface energy, phase composition were analyzed by means of atomic force microscope (AFM), contact angle measurements and X-ray photoelectron spectroscopy (XPS). The results indicated that the terminal –NH 2 groups could be completely transformed into desirable –PO(OH) 2 groups after phosphorylation of APTES-SAM. Detailed XPS analysis of the La 3+ peaks revealed that lanthanum element existed in the films in different states. As a result, conclusion could be made that lanthanum reacted with –PO(OH) 2 groups on the surface of the substrate by chemical bond which would improve the bonding strength between the film and silicon substrate. Since the La-based thin films were well adhered to the silicon substrate, it might find promising application in the surface-modification of single-crystal Si and SiC in microelectromechanical systems (MEMS).

Self-Assembly of Benzodithiophene Derivatives on Graphite: Effects of Substitution Alkoxy Chain Number and Location

Self-assembling of polycyclic benzodithiophene derivatives (PBDs), a family of important optoelectronic organic molecules, at highly oriented pyrolytic graphite was studied with scanning tunneling microscopy. The experimental results showed that variation of the attached alkoxy chain number caused drastic structural changes of the corresponding assemblies. The competitions between the π-conjugated cores and the alkoxy chains led to the structural transformations from relatively compact core-induced line patterns to more open chain-induced networks. Furthermore, the substitution location of the attached alkoxy chain in the PBD molecules also played an important role in tuning the molecular conformations in the two-dimensional self-assembled monolayers. Thermally induced transformation from energetically unfavorable structures to thermodynamically favorable ones was also observed. The changes of the self-assembling structures of the PBDs due to the variation of the attached alkoxy chain number may help in u...

Grafted Functional Polymer Nanostructures Patterned Bottom-Up by Colloidal Lithography and Initiated Chemical Vapor Deposition (iCVD)

Colloidal lithography, a popular inexpensive alternative to conventional lithography, uses two-dimensional self-assembled monolayer arrays of colloidal nanoparticles as a lithographic template. Combined with initiated chemical vapor deposition (iCVD), which offers unprecedented opportunity for producing grafted polymeric layers, this work demonstrates a generic bottom-up process as an inexpensive, simple, and environmentally friendly technique for creating robust well-ordered arrays of functional patterned polymeric nanostructures up to 500 nm in height. These grafted nanobowl patterns are produced for a broad material set of functional organic, fluorinated, and silicon containing polymers. These polymers fully retain the organic functionality of their monomeric precursors, are free of wetting defects, and are robustly tethered to the underlying substrate as shown by their ability to withstand aggressive solvent. Furthermore, using this method we pattern a novel low dielectric constant polymer down to 25 nm without the need for environmentally harmful solvents.

Preparation and tribological characteristics of sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane TiO2 films

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on silicon substrate to form two-dimensional self-assembled monolayer (SAM) and the terminal–SH group in the film was in situ oxidised to–SO3H group to endow the film with good chemisorption ability. Thus, TiO2 thin films were deposited on the oxidised MPTS-SAM to form composite thin films, making use of the chemisorption ability of the–SO3H group. Atomic force microscope (AFM), and contact angle measurements were used to characterise TiO2 films. Adhesive force and friction force of TiO2 thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. Results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. In order to study the effect of capillary force, tests were performed in various relative humidity (RH). Results showed that the adhesive force of silicon substrate increases with RH and the adhesive force of TiO2 thin films only increases slightly with RH. Research showed that surfaces with more hydrophobic property revealed the lower adhesive and friction forces.

Nano-tribological characteristics of lanthanum-based thin films on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on silicon substrate to form two-dimensional Self-Assembled Monolayer (SAM) and the terminal -SH group in the film was in situ oxidized to -SO 3H group to endow the film with good chemisorption ability. Thus, lanthanum-based thin films were deposited on oxidized MPTS-SAM to form rare earth composite thin films (RE thin films), making use of the chemisorption ability of the -SO 3H group. Atomic Force Microscope (AFM). X-ray Photoelectron Spectrometry (XPS), and contact angle measurements were used to characterize the RE thin films. Adhesive force and friction force of the RE thin films and silicon substrate were measured under various applied normal loads and scanning speed of AFM tip. The results showed that the friction force increased with applied normal loads and scanning speed of AFM tip. To study the effect of capillary force, tests were performed in various relative humidities. The results showed that the adhesive force of silicon substrate increased with relative humidity and the adhesive force of RE thin films only increased slightly with relative humidity. Research showed that surfaces with higher hydrophobic property reveal lowered adhesive and friction forces.

Temporal Stability of Thiophene Self-Assembled Monolayers on Au(111)

Temporal stability of close-packed thiophene (TP) self-assembled monolayers (SAMs) on Au(111) was investigated by scanning tunneling microscopy (STM). Molecular-scale STM imaging reveals that the structural transitions of TP SAMs in the structure and size of ordered domains and the distribution of vacancy islands (VIs) occurs after long-term storage for 1 year. During this time period, the ordered domain sizes were remarkably increased and the fraction of the areas of the VIs to the total surface area was steeply decreased from 6.5% to 1.7%. We demonstrate that the temporal factor strongly affects two-dimensional SAM structures and our results will be very useful in designing a molecular template for SAM-based further applications.

Investigation of Tribological Behavior of Lanthanum-Based Thin Films Deposited on Sulfonated Self-Assembled Monolayer

3-mercaptopropyl trimethoxysilane (MPTS) was prepared on glass substrate so as to form a two-dimensional self-assembled monolayer (SAM), and the terminal -SH group in the film was in situ oxidized to -SO 3H group to confer good chemisorption ability to the film. Thus, lanthanum-based thin films were deposited on oxidized MPTS-SAM, making use of the chemisorption ability of -SO 3H group. Atomic force microscopy (AFM) and X-ray photoelectron spectrometry (XPS) and contact angle measurements were used to characterize the thin films. The tribological properties of the as-prepared thin films sliding against a steel ball were evaluated on a friction and wear tester. Tribological experiment shows that the friction coefficient of glass substrate decreases from 0.8 to 0.08 after the rare earth (RE) self-assembled films (SAMs) are formed on its surface. And the RE self-assembled films have longer wear life (500 sliding passes). It is demonstrated that RE self-assembled film exhibits good wear-resistant property. The marked decrease in friction and the longer wear life of RE films are attributed to the excellent adhesion of the film to the substrate and to the special characteristics of the RE elements. The frictional behaviors of RE thin-films-coated silicon surface were sensitive to the applied load and the sliding velocity of the steel ball.

Cholesterol biosensor based on N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane self-assembled monolayer

Cholesterol oxidase (ChOx) has been covalently immobilized onto two-dimensional self-assembled monolayer (SAM) of N-(2-aminoethyl)-3-aminopropyl-trimethoxysilane (AEAPTS) deposited on the indium–tin oxide (ITO) coated glass plates using N-ethyl- N′-(3-dimethylaminopropyl) carbodiimide and N-hydroxysuccinimide (EDC/NHS) chemistry. These ChOx/AEAPTS/ITO bioelectrodes are characterized using contact angle (CA) measurements, UV–visible spectroscopy, atomic force microscopy (AFM), electrochemical impedance technique, and Fourier transform infrared (FT–IR) technique. The covalently immobilized ChOx-modified AEAPTS bioelectrodes are used for the estimation of cholesterol in solution using UV–visible technique. These cholesterol sensing bioelectrodes show linearity as 50 to 500 mg/dl for cholesterol solution, detection limit as 25 mg/dl, sensitivity as 4.499 × 10 −5 Abs (mg/dl) −1, K m value as 58.137 mg/dl (1.5 mM), apparent enzyme activity as 1.81 × 10 −3 U cm −2, shelf life of approximately 10 weeks, and electrode reusability as 10 times.

Self-Assembled Monolayers of Disulfide-Functionalized Diacetylenes on Gold Films and Nanoparticles

This paper reports on the fabrication of self-assembled monolayers containing disulfide-functionalized diacetylenes. To this purpose, two diacetylene derivatives were synthesized: the novel 14-(9H-9-carbazolyl)tetradeca-10,12-diyn-1-yl disulfide (CDS9) and the henicosa-10,12-diyn-1-yl disulfide (DS9). Both monomers were employed to prepare two-dimensional self-assembled monolayers (2D-SAMs) by chemisorption on gold-sputtered platforms and three-dimensional self-assembled monolayers (3D-SAMs) on gold nanoclusters by direct synthesis in organic solvent. The chemisorbed diacetylenes were then converted to the polymeric form by exposure to UV radiation, and their morphological and spectroscopic properties investigated by several techniques. From SERS data it was inferred that both monomers show the same behavior, in that the highly conjugated blue phase was observed in the 3D-SAMs, while only the less extended red conformation was found in the corresponding 2D-SAMs.

Self-assembly of octadecyltrichlorosilane monolayers on silicon-based substrates by chemical vapor deposition

Increasingly, organosilane self-assembled monolayers (SAMs) are used to modify the surfaces of silicon-based sensors and atomic force microscope (AFM) probes. Organosilane SAMs are preferred due to their fast and easy preparation, stability, and applicability to a wide range of substrates. The traditional dip coating method from solution often yields ill-defined particulate aggregates on the two-dimensional SAM. The presence of such three-dimensional aggregates seriously reduces the performance of miniaturized biosensor devices and AFM probes. It is difficult to control the amount of water in solution-based deposition. This paper describes a chemical vapor deposition (CVD) method to deposit octadecyltrichlorosilane (OTS) monolayers on silicon wafers and AFM probes under vacuum condition. OTS coated surfaces with static water contact angle ranging from 20° to 107° can be obtained by controlling the deposition conditions. The silicon substrates and AFM probes after CVD are characterized by AFM, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and contact angle goniometry. The OTS monolayer is in a uniform low-density state below 65%. Above 65%, densely packed crystalline-like domains start to form. It takes 24 h to reach the adsorption saturation. The time span in the CVD deposition is much longer than the solution case and thus allowing precise variation of the substrate hydrophobicity for biosensor applications.

Preparation and characterization of lanthanum-based thin films on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Silane coupling regent (3-mercaptopropyl trimethoxysilane (MPTS)) is prepared on a single-crystal silicon substrate to form two-dimensional self-assembled monolayer (SAM). The terminal thiol groups (–SH) in the film is oxidized to sulfonic acid groups (–SO3H) in situ to enhance the chemisorption property of the film. Lanthanum-based thin films are deposited on the oxidized MPTS-SAM by means of chemisorption with the –SO3H group. The surface energy, chemical composition, phase transformation and surface morphology of the films are analyzed using contact angle measurements, X-ray photoelectron spectroscopy (XPS) and atomic force microscopy. The results show that MPTS is a self-assembled layer on the substrate and that the terminal –SH groups of the self-assembled MPTS films are closer to the air/silane interface than the silicon atoms. The study also shows that the treatment of SAM with 30% nitric acid (HNO3) at 80 °C in sufficient duration can completely transform the terminal groups in the top-most layer into desirable sulfonate groups. Then the substrates coated with the sulfonated MPTS-SAM were immersed in the RE solution to form lanthanum-based thin films. The experimental results indicate that the lanthanum-based thin films are adsorbed on oxidized MPTS-SAM and that the lanthanum element with different states of oxidation exists in the thin films that are deposited on the surface of self-assembly monolayers. Also, shape analyses of the La3+ XPS peaks reveal that the rare-earth film can react with the substrate by chemical bonding and that some lanthanum molecules are adsorbed on the MPTS-SAM. The above results show that it is the –SO3H group that has good chemisorption trend.

Template-directed growth ofp-bromoiodobenzene

Crystals of the title compound, C6H4BrI, were obtained by growth on a two-dimensional self-assembled monolayer template. Single-crystal X-ray diffraction at 110 K revealed a centrosymmetric structure in which the center of the benzene ring lies on an inversion center and the halogen atoms are statistically disordered. The monoclinic structure reported here is isostructural with p-dibromo­benzene but not with p-diiodo­benzene.

Heat capacity measurements of two-dimensional self-assembled hexadecanethiol monolayers on polycrystalline gold

The melting characteristics of hexadecanethiol two-dimensional self-assembled monolayers (SAMs) grown on polycrystalline Au surfaces are obtained via heat capacity measurements using nanocalorimetry at scanning rates of ∼30 000°C∕s. The analyzed amount of alkanethiol in the form of SAMs is typically in the 10−11 mol range. Melting occurs over a broad (full width on the half height ∼100°C) temperature range with a melting temperature of about 70°C, if the alkanethiol is partially desorbed from the Au surface during thermal treatment. SAMs in their as-deposited state without desorption have melting temperatures above 100°C. The heat of fusion is about 20kJ∕mol. Partial desorption of the alkanethiol is directly observed. The melting characteristics of three-dimensional SAMs on Au nanoparticles are comparable to earlier studies using conventional calorimetry.

Preparation and characterization of ZrO 2 thin film on sulfonated self-assembled monolayer of 3-mercaptopropyl trimethoxysilane

Silane coupling reagent (3-mercaptopropyl trimethoxysilane (MPTS)) was prepared on single-crystal Si substrate to form two-dimensional self-assembled monolayer (SAM) and the terminal SH group in the film was in situ oxidized to SO 3H group to endow the film with good chemisorption ability. Thus, ZrO 2 thin film were deposited on the oxidized MPTS-SAM, by enhanced hydrolysis of zirconium sulfate (Zr(SO 4) 2·4H 2O) solution in the presence of HCl at 70 °C, making use of the chemisorption ability of the SO 3H group. The thickness of the films was determined with an ellipsometer, while the chemical feature, phase composition, thermal stability, and morphology of the films were analyzed by means of Fourier transformation infrared spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction analysis, thermogravimetric analysis, and atomic force microscopy. As the results, the as-deposited ZrO 2 film was composed of nanocrystalline tetragonal ZrO 2 (t-ZrO 2) and an amorphous basic zirconium sulfate. The annealing of the films at 500 °C led to crystallization to t-ZrO 2, while further heating to 800 °C eliminated S in the film, and the ZrO 2 film consisted of nanocrystalline tetragonal ZrO 2 and monoclinic ZrO 2 in this case. The as-deposited ZrO 2 thin films were compact and crack-free, showing quadratically-looking features which were not observed previously, while the thickness of the ZrO 2 films decreased with increasing annealing temperature. Since the ZrO 2 film was well adhered to the MPTS-SAM, it might find promising application in the surface-modification of single-crystal Si and SiC in microelectromechanical systems (MEMS).

Structure and thermal behavior of a layered silver hydroxyalkanecarboxylate

The structure and thermal behavior of silver 16-hydroxyhexadecanoate (Ag-HHDA) have been investigated by using various analytical tools. The X-ray diffraction pattern was composed of a series of peaks that could be indexed to (0 k0) reflections of a layered structure. Diffuse reflectance infrared Fourier transform spectroscopy revealed that the alkyl chains in Ag-HHDA as prepared were in an all- trans conformational state. Upon heating the sample, noticeable structural changes took place particularly in two temperature regions. The first structural change occurring at ∼380 K was a partially irreversible one in which the binding state of carboxylate to silver converted from bridging into unidentate. A second dramatic structural change occurring at ∼480 K was a totally irreversible process that could be associated with the decomposition of Ag-HHDA. All of these thermal characteristics of Ag-HHDA are comparable to those of silver stearate (Ag-STA). Separately, we have endeavored without success to intercalate polar molecules into the OH-group terminated layers in Ag-HHDA; the exfoliation of Ag-HHDA in various polar solvents was also unsuccessful. This is indicative of the presence of a rather stronger H-bond in Ag-HHDA, but the comparable thermal characteristics of Ag-HHDA and Ag-STA dictate that the thermal behavior of silver alkanoate is determined exclusively by the silver-to-carboxylate group interaction. This is in sharp contrast to the case of two-dimensional self-assembled monolayers for which the terminal functionalities play a crucial role in determining the structure and thermal stability of entire monolayers.