Self-assembled Monolayer Templates Research Articles

Disordered Phase-Assisted Growth of Organic Semiconductor Crystals on Self-Assembled Monolayer Templates.

Achieving high mobility and bias stability is a challenging obstacle in the advancement of organic thin-film transistors (OTFTs). To this end, the fabrication of high-quality organic semiconductor (OSC) thin films is critical for OTFTs. Self-assembled monolayers (SAMs) have been used as growth templates for high-crystalline OSC thin films. Despite significant research progress in the growth of OSC on SAMs, a detailed understanding of the growth mechanism of the OSC thin films on a SAM template is lacking, which has limited its use. In this study, the effects of the structure (thickness and molecular packing) of SAM on the nucleation and growth behavior of the OSC thin films were investigated. We found that disordered SAM molecules assisted in the surface diffusion of the OSC molecules and resulted in a small nucleation density and large grain size of the OSC thin films. Moreover, a thick SAM with disordered SAM molecules on the top was found to be beneficial for the high mobility and bias stability of the OTFTs.

Impact of self-assembled monolayer templates on electrodeposition of Pt particles

Active templating is commonly used to fabricate nanoparticles. We have demonstrated that self-assembled monolayers (SAMs) on gold can be employed as active templates for nanoparticle synthesis. This work further expands the use of active template by evaluating the effect of thiol terminal groups (CH3, COOH and OH) and their chain lengths (C6, C10/C11 and C16) on electrochemical Pt particle growth. The result suggests that the size, distribution and the potential difference (ΔEdep = ELOP - EFOP) between the first observable peak (EFOP) and the last observable peak (ELOP) in the cathodic region, are strongly affected by the terminal groups and their hydrophilicity/hydrophobicity. Methyl terminal groups lead to the expected ΔEdep > 0, but surprisingly resulted in an increase in the particle count with a decrease in the average particle size with increasing thiol length. Both hydrophilic terminal groups demonstrated ΔEdep < 0 with the expected decrease in particle count with an increase in their size by increasing the thiol chain length. The results suggest thiol motion and hydrophobicity significantly influence particle growth behaviour.

A mechanistic study of Pt particle electrodeposition and growth on a self-assembled monolayer as an active template

• Application of monolayers on gold as templates for deposition of Pt particles. • Evaluation of particle growth using cyclic voltammetry and chronoamperometry. • Evaluation of growth mechanism with SH model using chronoamperometric deposition. • Evaluation of growth mechanism using time dependant image J analysis of the SEM. • Comparing mechanism of growth evaluated using SH model with SEM analysis. Platinum is an essential component in many commercial catalytic processes. However, due to its cost and availability considerable effort is made to control the amount, location, and accessibility of the platinum species on an electrode surface. The current study investigates a low-cost, and versatile method for the deposition of Pt particles using a self-assembled monolayer (SAM) as an active assisted template on a gold electrode. Alkane (HSCH 2 (CH 2 ) 8 CH 3 ), alcohol (HSCH 2 (CH 2 ) 9 CH 2 OH) and carboxylate (HSCH 2 (CH 2 ) 8 CH 2 COOH) terminated monolayers were used to probe the impact of hydrophobicity on particle size, population density and growth mechanism. Cyclic voltammetry (CV) and chronoamperometry (CA) were employed to induce particle growth. From the CA data a dimensionless theoretical plot was calculated based on the Scharifker and Hills model to establish the mechanism as either progressive or instantaneous. The time-dependent study revealed a progressive nucleation mechanism for Pt particles synthesized on all three electrode surfaces, whereas microscopy and image analysis revealed an instantaneous nucleation mechanism for the alkane template only and progressive nucleation pathways for the alcohol and carboxylate SAM surfaces. The results indicated that SAM templating could be used to effectively control particle growth in a predictable fashion, however there is a limitation in application of the existing model due to a combination of factors including the migration of the SAM monomers on the electrode surface, the hydrophobicity of the terminal moieties and the occurrence of secondary and subsequent nucleation and growth phases. In addition, post-particle deposition CV was used to determine the impact of the SAM template on the active area of the Pt particles available for catalysis and showed that the hydrophobic SAM coatings gave significantly higher activity.

Synthesis of Noble Metal Nanoparticles By Atomic Layer Deposition Utilizing Self-Assembled Monolayers Templates

Self-assembled monolayers (SAMs) are widely used as surface modification structures for thin films synthesis and designed nanostructures fabrication1. Alkyltrichlorosilanes is a group of SAMs widely applied in the field of area-selective atomic layer deposition (AS-ALD). The closely packed octadecyltrichlorosilane (ODTS) SAMs could act as monolayer resist to prohibit ALD2, while the pinholes on unsaturated ODTS SAMs could act as nucleation sites to initiate ALD growth. Thus the distribution and structure of pinholes, including its size and density, are important factors for utilizing SAMs template to fabricate nanoparticles via ALD. Many studies characterize SAMs and pinholes by atomic force microscopy (AFM), scanning electron microscopy (SEM), scanning tunneling microscope (STM), etc3. Most of these methods could provide the microscopic information on the pinhole structures, yet the information from these methods is quite localized and hard to obtain statistical average and full-scale information about the pinhole size and distribution density. Here we report utilizing electrochemical impedance spectroscopy (EIS) to study the size distribution and loading of initial nanoparticles grown in those pinhole sites. EIS is a powerful and surface sensitive characterization technique to obtain the average and global characters of the detected surfaces4,5. In this study, the properties of unsaturated ODTS SAMs are characterized with EIS methods. The relationship between SAMs growth time and the pinholes properties has been established. With the SAMs immersing time increasing, the sizes and density distribution are both reduced. For SAMs sample immersed over long time, at least 24 hours, negligible pinholes are observed. Furthermore, Pd ALD process has been carried on those SAMs modified substrates. The distribution of Pd nanoparticles corresponds well with the pinholes. The results show that the nanoparticles grown on SAMs modified substrates depend on the pinholes properties. With SAMs modified substrate, ALD grown noble metal nanoparticles with uniform size and controlled loading density can be achieved.

Plasmon-enhanced ultraviolet photoluminescence from highly ordered ZnO nanorods/graphene hybrid structure decorated with Au nanospheres

Highly ordered and vertically aligned ZnO nanorods using an inverted self-assembled monolayer (SAM) template was fabricated via the hydrothermal process, and few-layer graphene was transferred on the surface of the ZnO nanorod arrays with 20 nm Au nanospheres coverage on the graphene surface. The band edge emission of the ZnO nanorods/graphene hybrid structures with Au nanospheres was increased by approximately three times and the defect-related emission was significantly reduced compared with the pristine ZnO nanorods arrays. The improved PL intensity can be attributed to the resonant coupling between the exciton emission of ZnO nanorods and plasmonic effects of graphene-Au structures. Our results will be promising for designing and fabricating ordered ZnO nanorod-based optical and optoelectronic devices.

Principal component analysis (PCA)-assisted time-of-flight secondary-ion mass spectrometry (ToF-SIMS): a versatile method for the investigation of self-assembled monolayers and multilayers as precursors for the bottom-up approach of nanoscaled devices.

The production of high-quality self-assembled monolayers (SAMs) followed by layer-by-layer (LbL) self-assembly of macrocycles is essential for nanotechnology applications based on functional surface films. To help interpret the large amount of data generated by a standard ToF-SIMS measurement, principal component analysis (PCA) was used. For two examples, the advantages of a combination of ToF-SIMS and PCA for quality control and for the optimization of layer-by-layer self-assembly are shown. The first example investigates how different cleaning methods influence the quality of SAM template formation. The second example focuses on the LbL self-assembly of macrocycles and the corresponding stepwise surface modification.

Controlled Wetting on Electrodeposited Oxide Thin Films: From Hydrophilic to Superhydrophobic

We have explored how surface morphology and structure affect wetting properties of electrodeposited oxide thin films notwithstanding chemical modifications. Microstripes of self-assembled monolayers (SAMs) on gold were prepared using a microcontact printing (μCP) technique, which served as molecular templates to guide the electrochemical deposition of zirconia in aqueous solution. The wetting properties of the thus-prepared zirconia oxide thin films are shown to be tunable; i.e., a wide range of wettabilities from hydrophilic to superhydrophobic can be obtained by simply varying the SAM template and the electrodeposition conditions (potential scan rate and number of cycles). In particular, a “two-tier” micro/nanoscale roughness was achieved on the gold substrate patterned with alternating stripes of 1-octadecanethiol and 6-mercapto-1-hexanol SAMs, which leads to a superhydrophobic surface (water contact angle ∼150°). Of great significance is the demonstrated ability herein to convert an intrinsically hydrophilic into a hydrophobic surface by changing the conditions for materials fabrication, which does not involve any chemical modifications.

Deposition of Ordered Layers of Tetralactam Macrocycles and Ether Rotaxanes on Pyridine-Terminated Self-Assembled Monolayers on Gold

The deposition of tetralactam macrocycles and the corresponding benzyl ether rotaxanes on gold substrates is investigated for the first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated self-assembled monolayers (SAMs) are developed that are used as well-ordered template layers. The two SAMs differ with respect to the rigidity of the terminal pyridines as shown by angle-resolved near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. The template layers are then used for the metal-mediated self-assembly of macrocylces and rotaxanes on solid supports. The SAM with the more rigid terminal pyridine shows a higher coverage with the macrocycles and is therefore preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited supramolecules to be oriented preferentially upright. This order is only achieved for the macrocycles through the deposition on the more rigid SAM template, whereas rotaxanes form oriented layers on both SAMs. Time-of-flight secondary-ion mass spectrometry analysis was used to determine the deposition time required for the self-assembly process.

Ultraviolet electroluminescence from ordered ZnO nanorod array/p-GaN light emitting diodes

The highly ordered and aligned ZnO nanorod arrays were grown on p-GaN substrates via a facile hydrothermal process assisted by the inverted self-assembled monolayer template, from which the ZnO nanorod/p-GaN heterojunction light emitting diodes (LEDs) were fabricated. The ZnO nanorod-based LEDs exhibit a stronger ultraviolet emission of 390 nm than the ZnO film-based counterpart, which is attributed to the low density of interfacial defects, the improved light extraction efficiency, and carrier injection efficiency through the nano-sized junctions. Furthermore, the LED with the 300 nm ZnO nanorods has a better electroluminescence performance compared with the device with the 500 nm nanorods.

Combined effect of ion concentration and functional groups on surface chemistry modulated CaCO3 crystallization

The effects of self-assembled monolayer templates terminated with carboxyl, hydroxyl, amino and methyl groups (–COOH, –OH, –NH2 and –CH3) on CaCO3 crystallization are compared in aqueous solutions of low, medium and high Ca2+ concentrations. On the –COOH surface, only calcite rhombohedra are crystallized in the solutions at three different concentrations, while size, shape and orientation of the formed calcites are distinct in various Ca2+ concentration solutions. In low Ca2+ concentration solution, vaterites (111) and (200) are formed on –OH and –NH2 surfaces respectively while no crystals are observed on –CH3 surface. In high Ca2+ concentration solution, calcite (104) is formed on the –OH surface. Calcites and small amounts of vaterites and aragonites are formed on –NH2 and –CH3 surfaces. In medium concentration solutions, the results are similar to the case of high concentration with a lower density of crystals. From the view of the two well-known mechanisms for biomineralization, ion absorption and particle-based crystallization, the results in our study are discussed. Our findings can advance the understanding of CaCO3 biomineralization in nature and also supply guidance for the design of advanced biomaterials.

In-situ guidance of individual neuronal processes by wet femtosecond-laser processing of self-assembled monolayers.

In-situ guidance of neuronal processes (neurites) is demonstrated by applying wet femtosecond-laser processing to an organosilane self-assembled monolayer (SAM) template. By scanning focused laser beam between cell adhesion sites, on which primary neurons adhered and extended their neurites, we succeeded in guiding the neurites along the laser-scanning line. This guidance was accomplished by multiphoton laser ablation of cytophobic SAM layer and subsequent adsorption of cell adhesion molecule, laminin, onto the ablated region. This technique allows us to arbitrarily design neuronal networks in vitro.

Controllable Growth of Highly Ordered ZnO Nanorod Arrays via Inverted Self-Assembled Monolayer Template

This article presents a facile and effective approach to the controllable growth of highly ordered and vertically aligned ZnO nanorod arrays on the GaN substrate via a hydrothermal route by using the TiO(2) ring template deriving from the polystyrene microsphere self-assembled monolayer. The size of TiO(2) ring template can be flexibly tuned from 50 to 400 nm for the 500 nm polystyrene microspheres by varying the time of reactive ion etching and the concentration of TiO(2) sol. As a result, the diameter of the individual ZnO nanorods can be potentially tuned over a wide range. The combination of several characterization techniques has demonstrated that the ordered ZnO nanorods are highly uniform in diameter and height with perfect alignment and are epitaxially grown along [0001] direction. This work provides a novel and accessible route to prepare oriented and aligned ZnO nanorod arrays with high crystalline quality.

Assembly of single-walled carbon nanotubes on patterns of Au nanoparticles

We report on the assembly of single-walled carbon nanotubes (SWNTs) and gold nanoparticles (NPs) hybrid structure without any surface modification of SWNTs on patterns of Au nanoparticles (NPs). Microscale Au NP patterns were created on composite self-assembled monolayer (SAM) templates of octadecanethiol (ODT) and octanedithiol (OD) through self-assembly of Au NPs via the thiol-Au chemical bond onto the OD region. On such templates, we observed extensive adhesion and strong affinity of SWNTs on the Au NPs and no SWNT on ODT. We also examined systematically the adhesion of SWNTs on ODT with varying coverage of vapour-deposited Au. We observed little SWNT attachment even when there are high-density of Au clusters on the ODT SAM. Extensive adhesion of SWNTs is observed only when the coverage of ODT by Au is almost complete. Dynamic contact angle measurements of dichlorobenzene on the ODT/Au substrates revealed a direct correlation between the surface wettability and the SWNT assembly on a molecular template.

Single Chain Fragment Variable Recombinant Antibody as a Template for Fc Sensors

A label free immunosensor for detection of Fc receptors expressed on cell surface was developed and characterized using a Quartz Crystal Microbalance (QCM) transducer. Taking advantage of the characteristics of single chain fragment variable (scFv) recombinant antibody and the multivalency of an antibody, the engineered recombinant scFv was immobilized onto preformed functionalized self-assembled monolayers (SAMs) template surface. The monomeric scFv can bind with the CH1 region of any rabbit IgG to form a highly oriented IgG layer with its Fc portion pointing toward a solution phase. This results in a highly oriented Fc sensor that can be used to study the thermodynamics and kinetics of binding between the Fc portion of immunoglobulin and the cell surface Fc receptor (FcR), an important area of the immune system. The Fc sensor was used to study the binding between Staphylococcus aureus and the Fc receptor on macrophage. Parallel characterization of cell surface Fc receptors in the same samples by ELISA was also performed.

Zirconia electrodeposited on a self-assembled monolayer on a gold electrode for sensitive determination of parathion

A self-assembled monolayer (SAM) template was employed for the electrodeposition of zirconia nanoparticles (ZNPs) film on a gold electrode. The ZNPs film was highly stable and could selectively adsorb molecules containing phosphoric groups. Therefore, a sensor was developed for the determination of parathion based on the ZNPs electrodeposited on the SAM modified gold electrode. The electrochemistry of parathion on the present electrode was studied using cyclic voltammetry and square-wave voltammetry. A linear adsorptive stripping voltammetric response over the concentration range from 0.005 to 1.0 μg ml−1 (after a 2-min adsorption) was obtained under the optimized operational parameters of square wave voltammetry. The limit of detection was 0.8 ng ml−1 (S/N = 3). The ZNPs modified on SAM surface of gold electrode exhibits acceptable reproducibility (RSD 4.16%, n = 10) and a good stability. The electrode was applied to the determination of parathion in real samples, and satisfactory results were obtained.

Site-Selective Biofunctionalization of Aluminum Nitride Surfaces Using Patterned Organosilane Self-Assembled Monolayers

Surface biochemical functionalization of group-III nitride semiconductors has recently attracted much interest because of their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave (SAW) sensor structures. To demonstrate the possibility of site-selective biofunctionalization on AlN surfaces, we have fabricated two-dimensional antibody micropatterns on AlN surfaces by using patterned self-assembled monolayer (SAM) templates. Patterned SAM templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Because the patterned SAM templates have different surface properties on the same surface, clear imaging contrast of SAM micropatterns can be observed by field-emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5-1.5 kV. Furthermore, the contrast in surface potential of the binary SAM microstructures was confirmed by selective adsorption of negatively charged colloidal gold nanoparticles (AuNPs). The immobilization of AuNPs was limited on the positively charged amino-terminated regions, while they were scarcely found on the surface regions terminated by methyl groups. In this work, selective immobilization of green fluorescent protein (GFP) antibodies was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody-EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Formation of Antibody Microarrays on Aluminum Nitride Surface Using Patterned Organosilane Self-Assembled Monolayers

AbstractSurface biofunctionalization of group-III nitride semiconductors has recently attracted much interest due to their biocompatibility, nontoxicity, and long-term chemical stability under demanding physiochemical conditions for chemical and biological sensing. Among III-nitrides, aluminum nitride (AlN) and aluminum gallium nitride (AlGaN) are particularly important because they are often used as the sensing surfaces for sensors based on field-effect transistor or surface acoustic wave sensor structures. Patterned self-assembled monolayer (SAM) templates are composed of two types of organosilane molecules terminated with different functional groups (amino and methyl), which were fabricated on AlN/sapphire substrates by combining photolithography, lift-off process, and self-assembly technique. Clear imaging contrast of SAM micropatterns can be observed by field emission scanning electron microscopy (FE-SEM) operating at a low accelerating voltage in the range of 0.5–1.5 kV. In this work, the formation of green fluorescent protein (GFP) antibody microarrays was demonstrated by the specific protein binding of enhanced GFP (EGFP) labeling. The observed strong fluorescent signal from antibody functionalized regions on the SAM-patterned AlN surface indicates the retained biological activity of specific molecular recognition resulting from the antibody–EGFP interaction. The results reported here show that micropatterning of organosilane SAMs by the combination of photolithographic process and lift-off technique is a practical approach for the fabrication of reaction regions on AlN-based bioanalytical microdevices.

Nano/Micro Patterning of Inorganic Thin Films

Abstract The present review is focused on the site-selective deposition of inorganic thin films on self-assembled monolayer (SAM) templates. SAMs were used to modify the substrate surface with chemical functional groups. The patterned SAM substrate was prepared by exposing the SAM substrate to UV light through photomasks for photocleavage. The patterned SAM was used as the templates for deposition of ceramic thin films in aqueous solutions. The preparation and characterization methods of SAMs, and the preparation of the patterned SAM templates are explained. Solution chemistry for the deposition of ceramic oxides and the physics and chemistry of interfaces are described. Experimental examples are given to explain how these aspects influence the formation of films and their properties. Various examples of site-selective deposition of ceramic oxides on SAM templates are explained with classification into three categories: patterning by site-selective attachment of homogeneously nucleated particles, patterning by surface-selective heterogeneous nucleation and growth, and patterning by catalyst-induced heterogeneous nucleation. We also describe pattern-deposition on flexible substrates and future views to improve the pattern resolution.

Engineering Peptide Linkers for scFv Immunosensors

Using A10B single-chain fragment variable (scFv) as a model system, we demonstrated that the flexibility of scFv linker engineering can be combined with the inherent quick and adaptable characters of surface coupling chemistry (e.g., electrostatic, hydrogen bonding, or covalent attachment) to attach scFv to preformed functionalized self-assembled monolayers (SAMs). Six arginines, which were separated by glycine or serine as spacer, were incorporated in the peptide linker to form a 15-mer peptide linker (RGRGRGRGRSRGGGS). The polycationic arginine peptide was engineered into the A10B scFv-RG3 to favor its adsorption at anionic charged template surface (11-mercaptoundecanoic acid (MUA) and poly(sodium 4-styrenesulfonate (PSS))). This new approach was compared with the other engineered scFv constructs. Our results demonstrated that the anionic charged SAM template facilitated the oriented immobilization of scFvs on the SAM template surface as well as reduced the possibility of protein denaturation when directly immobilized on the solid surface. A 42-fold improvement of detection limits using MUA/A10B scFv-RG3 (less than 0.2 nM experimentally determined) was achieved compared to A10B Fab antibody and a 5-fold improvement was observed compared to A10B scFv that was engineered with a cysteine in the linker sequence. Using protein A-coated gold nanoparticles, a picomolar experimental detection limit was achieved. With 20 amino acids to choose from, engineered recombinant scFv in combination with SAM technology and nanoparticle mass amplification provide an emerging strategy for the development of highly sensitive and specific scFv immunosensors.

Assembly of Nanomaterials Through Highly Ordered Self-Assembled Monolayers and Peptide-Organic Hybrid Conjugates as Templates

Synthesis and processing techniques have now been established for obtaining high quality monodisperse nanocrystals of various metallic and semiconducting materials, fullerenes of distinct properties, single- and multi-wall carbon nanotubes, polymeric dendrimers with tailored functionalities, as well as other nanophase constructs. The next key step towards novel applications of nanostructured materials concerns their positioning, arrangement, and connection into functional networks without mutual aggregation. In this review, we highlight the recent progress of using anthracene- and pyrene-based self-assembling molecules with tunable energetic (pi-pi interactions, hydrogen bonding, dipole-dipole interactions) and variable geometries to create stable, highly ordered, and rigid self-assembled monolayer (SAM) templates with adjustable superlattices on crystalline substrates. Based on aromatic SAM templates, stable and highly ordered self-assembled structures of optoelectronically active C60 have been obtained and shown to exhibit desirable electrical and optoelectronic properties, such as nonlinear transporting effect for molecular electronics and efficient photocurrent generation for mimicking photosynthesis in nature. By using genetically engineered polypeptides with surface recognition for specific inorganics, selective integration of nanoparticles onto aromatic SAM templates have also been realized. Through a combination of spatially confined surface chemical reaction and microcontact printing, sub-micron arrays of peptide-organic hybrid conjugates were successfully generated to serve as templates to achieve the patterned assembly of nanoparticles.