Growth kinetics and morphology of self-assembled monolayers formed by contact printing 7-octenyltrichlorosilane and octadecyltrichlorosilane on Si(100) wafers

The growth kinetics and morphologies of self-assembled monolayers deposited by contact printing 7-octenyltrichlorosilane (OCT) and octadecyltrichlorosilane (OTS) on Si(100) were studied by ellipsometry and atomic force microscopy. We found that, for both OCT and OTS, full monolayers could be obtained at room temperature after printing times of 120-180 s; the printing-based monolayer assembly processes follow apparent Langmuir adsorption kinetics, with the measured film growth rates increasing both with the ambient humidity and with concentration of the ink used to load the stamp. At a dew point of 10 degrees C and an ink concentration (in toluene) of 50 mM, the observed film growth rate constant is 0.05 s(-)(1). When the printing was carried out at a lower ambient humidity (dew points of 1-3 degrees C), the measured rates of assembly were approximately a factor of 2 slower. Increasing the deposition temperature from 25 to 45 degrees C under these conditions increased the film growth rate only slightly. The morphology of the films depends on the identity of the ink. Uniform, high-coverage films could be obtained readily from the eight-carbon chain length adsorbate OCT, provided that the stamp was not overloaded with the ink; for high concentrations outside of the optimal range, the surface presented significant numbers of adsorbed particles ascribed, in part, to siloxane polymers formed by hydrolysis of the ink on the stamp before printing. In marked contrast, for the 18-carbon adsorbate OTS, the printed films always consisted of a mixture of a uniform monolayer plus adsorbed polysiloxane particles. The different film morphologies seen for OCT and OTS are proposed to result from the different transfer efficiencies of the organotrichlorosilane relative to polysiloxane hydrolysis products formed during the printing process. These transfer efficiencies exhibit sensitivities related to the permeation of the poly(dimethylsiloxane) (PDMS) stamp by the silane reagents. Short-chain inks such as OCT evidently permeate the PDMS stamp more deeply than longer-chain inks such as OTS. This difference, and the different diffusion rates of ink vs oligomeric silane hydrolysis products, determines the film morphology obtained by contact printing. The mass transfer dynamics of the process thus yield surface layers derived from varying quantities of siloxane oligomers, which subsequently transfer to the substrate along with unhydrolyzed silane adsorbate during the printing step. The structural evolution of the contact-printed films so obtained is strikingly different from that of SAMs prepared by immersion.     

Numerical and experimental study of critical roof collapse conditions in soft lithography

One of the major issues during soft lithographic processes is that, if the pressing force on the stamp becomes too high, the stamp may erroneously come into contact with the substrate in zones where contact is not intended. This decreases the patterning accuracy and may lead to badly or nonperforming electronic devices and is therefore undesired. Design rules, available at an early stage in the design phase, are desired to speed-up the development of this technique. Ultimately, these rules should give an indication of the critical pressure that can safely be applied on the stamp thereby avoiding unwanted contact between the stamp and the substrate. To obtain these critical pressures, numerical analyses of the deformation behavior of two characteristic configurations in the microstructured surface pattern of the rubber stamp are performed. The deformation behavior of the rubber is modeled according to a Gaussian and a non-Gaussian approach, leading to a neo-Hookean and Arruda-Boyce constitutive model, respectively. Besides these material nonlinearities, geometrical nonlinearities are taken into account as well. The calculated pressure at which undesired contact takes place (the roof collapse pressure) is compared to experimentally obtained values for two particular types of structures, and the results are in agreement within the error margins of the experiments and those ensuing from the assumptions of the numerical simulations.     

Double-ink dip-pen nanolithography studies elucidate molecular transport

We have investigated the transport mechanism of the inks most typically used in dip-pen nanolithography by patterning both 16-mercaptohexadecanoic acid (MHDA) and 1-octadecanethiol (ODT) on the same Au{111} substrate. Several pattern geometries were used to probe ink transport from the tip to the sample during patterning of both dots (stationary tip) and lines (moving tip). When ODT was written on top of a pre-existing MHDA structure, the ODT was observed at the outsides of the MHDA structure, and the transport rate increased. In the reverse case, the MHDA was also observed on the outsides of the previously patterned ODT features; however, the transport rate was reduced. Furthermore, the shapes of pre-existing patterns of one ink were not changed by deposition of the other ink. These results highlight the important role hydrophobicity plays, both of the substrate as well as of the inks, in determining transport properties and thereby patterns produced in dip-pen nanolithography.     

High-speed microcontact printing

We have demonstrated microcontact printing (muCP) of self-assembled monolayers in the millisecond regime. The contact formation and separation of the stamp and substrate was studied with high-speed video recordings. Using high ink concentrations and contact times as short as 1 ms, we printed monolayers of hexadecanethiol on Au, which served as a selective etch resist. High-speed muCP yields defect-free monolayers that are independent of the dimensions of the printed patterns, have high contrast between printed and unprinted areas, and enable perfect reproducibility of prints.     

Poly(dimethylsiloxane) contamination in microcontact printing and its influence on patterning oligonucleotides

It is well-established that, during microcontact printing (muCP) using poly(dimethylsiloxane) (PDMS)-based stamps, some unexpected siloxane fragments can be transferred from the stamp to the surface of the sample. This so-called contamination effect coexists with the delivery of the molecules constituting the ink and by this way influences the printing process. The real impact of this contamination for the muCP technique is still partially unknown. In this work, we investigate the kinetics of this contamination process through the surface characterization of both the sample and the stamp after imprinting. The way both the curing conditions of the PDMS material and the contact time influence the degree of contamination of the surface is investigated on silicon and glass substrates. We propose a cleaning process of the stamp during several hours which eliminates any trace of contamination during printing. We show that hydrophobicity recovery of PDMS surfaces after hydrophilic treatment using oxygen plasma is considerably slowed down when the PDMS material is cleaned using our procedure. Finally, by comparing cleaned and uncleaned PDMS stamps, we show the influence of contamination on the quality of muCP using fluorescent DNA molecules as an ink. Surprisingly, we observe that the amount of DNA molecules transferred during muCP is higher for the uncleaned stamp, highlighting the positive impact of the presence of low molecular weight siloxane fragments on the muCP process. This result is attributed to the better adsorption of oligonucleotides on the stamp surface in presence of these contaminating molecules.     

Adsorption kinetics of phosphate and arsenate on goethite. A comparative study

The adsorption kinetics of phosphate and arsenate on goethite is studied and compared. Batch adsorption experiments were performed at different adsorbate concentrations, pH, temperatures and stirring rates. For both oxoanions the adsorption rate increases by increasing adsorbate concentration, decreasing pH and increasing temperature. It does not change by changing stirring rate. The adsorption takes place in two processes: a fast one that takes place in less than 5 min and a slow one that takes place in several hours or more. The rate of the slow process does not depend directly on the concentration of phosphate or arsenate in solution, but depends linearly on the amount of phosphate or arsenate that was adsorbed during the fast process. Apparent activation energies and absence of stirring rate effects suggest that the slow process is controlled by diffusion into pores, although the evidence is not conclusive. The similarities in the adsorption kinetics of phosphate and arsenate are quantitatively shown by using a three-parameters equation that takes into account both the fast and the slow processes. These similarities are in line with the similar reactivity that phosphate and arsenate have in general and may be important for theoretical and experimental studies of the fate of these oxoanions in the environment.     

Microscale plasma-initiated patterning (muPIP)

A novel technique to create biomolecular micropatterns of varying complexity on several types of polymer substrates is presented. This method uses a patterned PDMS stamp to preferentially expose or protect areas of an underlying polymer substrate from oxygen plasma. Following plasma treatment, the substrate is immersed in a biomolecular ink, whereby molecules preferentially adsorb to either the plasma-exposed or plasma-protected substrate regions, depending on the particular substrate/ink combination. Using this method, polyethylene (PE), polystyrene (PS), poly(methyl methacrylate) (PMMA), poly(dimethylsiloxane) (PDMS), and poly(hydroxybutyrate/hydroxyvalerate) (PHBV) were micropatterned with different aqueous-based biomolecular inks (i.e., goat anti-rabbit immunoglobulin G, poly-l-lysine, and bovine serum albumin (BSA)). Water contact angle measurements performed on substrates after oxygen plasma exposure showed that the hydrophilicity of substrate areas exposed to plasma was significantly greater than that of areas protected from plasma by the PDMS stamp. In addition, scanning electron microscopy results demonstrated that substrate areas exposed to plasma were physically modified (e.g., roughened) compared to adjacent, protected areas. Areas in contact with a patterned PDMS stamp during plasma exposure were found to be physically unaffected by plasma treatment, and exhibited spatial features/dimensions consistent with the corresponding features of the patterned stamp. Last, protein patterns of BSA on the polymer substrates were stable and distinct after 4 weeks of incubation at 37 degrees C.     

High-resolution direct patterning of gold nanoparticles by the microfluidic molding process

A novel microfluidic molding process was used to form microscale features of gold nanoparticles on polyimide, glass, and silicon substrates. This technique uses permeation pumping to pattern and concentrate a nanoparticle ink inside microfluidic channels created in a porous polymer template in contact with a substrate. The nanoparticle ink is self-concentrated in the microchannels, resulting in dense, close-packed nanoparticle features. The method allows for better control over the structure of printed features at a resolution that is comparable to inkjet printing, and is purely additive with no residual layers or etching required. The process uses low temperatures and pressures and takes place in an ambient environment. After patterning, the gold nanoparticles were sintered into continuous and conductive gold traces.     

Effect of stamp deformation on the quality of microcontact printing: theory and experiment

Microcontact printing (microCP) is an effective way to generate micrometer- or submicrometer-sized patterns on a variety of substrates. However, the fidelity of the final pattern depends critically on the coupled phenomena of stamp deformation, fluid transfer between surfaces, and the ability of the ink to self-assemble on the substrate. In particular, stamp deformation can produce undesirable effects that limit the practice and precision of microCP. Experimental observations and comparison with theoretical predictions are presented here for three of the most undesirable consequences of stamp deformation: (1) roof collapse of low aspect ratio recesses, (2) buckling of high aspect ratio plates, and (3) lateral sticking of high aspect ratio plates. Stamp behavior was observed visually with an inverted optical microscope while load-displacement data were collected during compression and retraction of stamps. Additionally, a "robotic stamper" was used to deliver ink patterns in precise locations on substrates. These monomolecular ink patterns were then observed in high contrast using the surface potential scanning mode of an atomic force microscope. Theoretical models based on continuum mechanics were used to accurately predict both physical deformation of the stamp and the resultant inking patterns. The close agreement between these models and the experimental data presented clearly demonstrates the essential considerations one must weigh when designing stamp geometry, material, and loading conditions for optimal pattern fidelity.     

Direct printing of self-assembled lipid tubules on substrates

Lipid tubules formed by rolled-up bilayer sheets have shown promise in drug delivery systems, nanofluidics, and microelectronics. Here we report a method for directly printing lipid tubules on substrates. Preformed lipid tubules of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine are aligned in the recessed channels of a thin poly(dimethylsiloxane) (PDMS) stamp. The aligned lipid tubules then serve as an "ink" for microcontact printing. We demonstrate that two-dimensional (2-D) arrays of aligned lipid tubules can be transferred onto planar, patterned, and curved substrates from the recessed channels of the PDMS stamp by bringing the tubule-inked PDMS stamp into contact with these substrates. We show that the 2-D array of aligned lipid tubules can be transcribed into a 2-D array of aligned silica cylinders through templated sol-gel condensation of tetraethoxysilane.     

Surface energy modified chips for detection of conformational states and enzymatic activity in biomolecules

A novel patterning method for anchoring biomolecules and noncovalent assembled conjugated polyelectrolyte (CPE)/biomolecule complexes to a chip surface is presented. The surface energy of a hydrophilic substrate is modified using an elastomeric poly(dimethylsiloxane) (PDMS) stamp, containing a relief pattern. Modification takes place on the parts where the PDMS stamp is in conformal contact with the substrate and leaves low molecular weight PDMS residues on the surface resulting in a hydrophobic modification, and then biomolecules and CPE/biomolecule complexes are then adsorbed in a specific pattern. The method constitutes a discrimination system for different conformations in biomolecules using CPEs as reporters and the PDMS modified substrates as the discriminator. Detection of different conformations in two biomacromolecules, a synthetic peptide (JR2E) and a protein (calmodulin), reported by the CPE and resolved by fluorescence was demonstrated. Also, excellent enzyme activity in patterned CPE/horseradish peroxidase (HRP) enzyme was shown, demonstrating that this method can be used to pattern biomolecules with their activity retained. The method presented could be useful in various biochip applications, such as analyzing proteins and peptides in large-scale production, in making metabolic chips, and for making multi-microarrays.     

Template-directed self-assembly of alkanethiol monolayers: selective growth on preexisting monolayer edges

Self-assembled monolayers were investigated for their suitability as two-dimensional scaffolds for the selective growth of alkanethiol edge structures. Heterostructures with chemical contrast could be grown, whose dimensions were governed by both the initial pattern sizes and the process time. n-Octadecanethiol (ODT) was made to expand from the edges of 16-mercaptohexadecanoic acid (MHDA) monolayer patterns. Likewise, 11-mercaptoundecanol (MUD) was grown on MHDA and on ODT monolayer edges. The results of these experiments are in accordance with the moving boundary model for monolayer spreading. In addition to such surface-bound spreading, a vapor-phase contribution to lateral monolayer growth was identified. MUD was observed to be an excellent ink for creating chemical contrast by means of regioselective deposition from a vapor phase. As a proof of principle, ribbons of 11-mercaptoundecanol with submicrometer widths were grown on pentaerythritol-tetrakis(3-mercaptopropionate) edges, and submicrometer wide gold lines were produced by subsequent wet-chemical etching.     

Temporal evolution of the composition of mixed monolayers on TiO2 surfaces: evidence for a dimerization-induced chelate effect

Mixed monolayers of octanoic acid (OA) and 16-mercaptohexadecanoic acid (MHDA) were adsorbed to nanocrystalline TiO(2) films from mixed solutions in tetrahydrofuran. For a range of solution compositions, the mole fraction of MHDA within the mixed monolayers (chi (MHDA,surf)) exceeded that of the coadsorption solution. In addition, chi (MHDA,surf) increased with time, while the sum of the surface coverages of MHDA and OA remained constant. To account for these effects, we propose a mechanism involving disulfide formation between the terminal thiol groups of surface-adsorbed MHDA molecules. Disulfide formation leads to an increase in the surface adduct formation constant ( K(ad)) of dimeric MHDA, causing the gradual displacement of OA from the surface. The mechanism is supported by spectroscopic evidence and desorption kinetics. These are the first examples of mixed monolayers that undergo time-dependent compositional changes as a result of covalent bond formation between surfactants. Our findings illustrate that dimerization and other intermolecular interactions between surfactants may dramatically influence the composition and terminal functionalization of a wide range of mixed monolayer systems.     

Surface patterning by microcontact chemistry

In this Feature Article we describe recent progress in covalent surface patterning by microcontact chemistry. Microcontact chemistry is a variation of microcontact printing based on the transfer of reactive "ink" molecules from a microstructured, elastomeric stamp onto surfaces modified with complementary reactive groups, leading to a chemical reaction in the area of contact. In comparison with other lithographic methods, microcontact chemistry has a number of advantageous properties including very short patterning times, low consumption of ink molecules, high resolution and large area patterning. During the past 5 years we and many others have investigated a set of different reactions that allow the modification of flat and also spherical surfaces in an effective way. Especially click-type reactions were found to be versatile for substrate patterning by microcontact chemistry and were applied for chemical modification of reactive self-assembled monolayers and polymer surfaces. Microcontact chemistry has already found broad application for the production of functional surfaces and was also used for the preparation of DNA, RNA, and carbohydrate microarrays, for the immobilization of proteins and cells and for the development of sensors.     

Adsorption of CdSe nanoparticles to thiolated TiO2 surfaces: influence of intralayer disulfide formation on CdSe surface coverage

Mixed monolayers of hexadecanoic acid (HDA) and 16-mercaptohexadecanoic acid (MHDA) were adsorbed to nanocrystalline TiO2 films, and CdSe nanoparticles were attached to the mixed monolayer functionalized surfaces. IR absorption spectroscopy was used to characterize the equilibrium binding of HDA and MHDA to TiO2. Surface adduct formation constants (Kad) of (4+/-2)x10(3) M(-1) and (6+/-4)x10(3) M(-1) were measured for HDA and MHDA, respectively. CdSe nanoparticles were adsorbed to the terminal thiol groups of MHDA. The surface coverage of CdSe was greater on mixed monolayers, consisting of approximately 12% MHDA and 88% HDA, than on pure MHDA monolayers. A mechanism is proposed wherein intralayer disulfide formation between MHDA thiol groups causes decreased reactivity toward CdSe nanoparticles. Disulfide formation is less significant at low fractional surface coverages of MHDA. The mechanism is supported by an increase of CdSe adsorption upon chemical reduction of surface disulfides to thiols. Our findings highlight the effect of intermolecular interactions on the affinity of nanoparticles for monolayer-functionalized surfaces.     

Rheological behavior of silica suspensions in aqueous solutions of associating polymer

Associating polymers are hydrophilic long-chain molecules containing a small amount of hydrophobic groups. The aqueous solutions show viscoelastic responses above some critical concentrations because a three-dimensional structure is formed by association of hydrophobic groups. When the associating polymers are added to silica suspensions at low concentrations, the flocculation is induced by bridging mechanisms, and the flow of suspensions become shear-thinning. For suspensions prepared with polymer solutions in which the associating network is developed, the viscosity decreases, shows a minimum, and then increases with increasing particle concentration. The viscosity decrease may arise from the breakdown of associating network due to adsorption of polymer chains onto the silica surfaces. As the particle concentration is increased, the polymer concentration in solution is decreased, and finally, all polymer chains are adsorbed on the surfaces. Beyond this point, the partial coverage of particle surfaces takes place and strong interactions are generated between particles by polymer bridging. Since the stable suspensions are converted to highly flocculated systems, the viscosity is increased and the flow becomes shear-thinning. The concentration effect of silica particles on the viscosity behavior of suspensions can be explained by a combination of viscosity decrease in solution due to polymer adsorption and viscosity increase due to flocculation.     

Molecular scale modeling of polymer imprint nanolithography

We present the results of large-scale molecular dynamics simulations of two different nanolithographic processes, step-flash imprint lithography (SFIL), and hot embossing. We insert rigid stamps into an entangled bead-spring polymer melt above the glass transition temperature. After equilibration, the polymer is then hardened in one of two ways, depending on the specific process to be modeled. For SFIL, we cross-link the polymer chains by introducing bonds between neighboring beads. To model hot embossing, we instead cool the melt to below the glass transition temperature. We then study the ability of these methods to retain features by removing the stamps, both with a zero-stress removal process in which stamp atoms are instantaneously deleted from the system as well as a more physical process in which the stamp is pulled from the hardened polymer at fixed velocity. We find that it is necessary to coat the stamp with an antifriction coating to achieve clean removal of the stamp. We further find that a high density of cross-links is necessary for good feature retention in the SFIL process. The hot embossing process results in good feature retention at all length scales studied as long as coated, low surface energy stamps are used.     

Removal of solvent-based ink from printed surface of HDPE bottles by alkyltrimethylammonium bromides: effects of surfactant concentration and alkyl chain length

Three alkyltrimethylammonium bromides (i.e., dodecyl-, tetradecyl-, and hexadecyltrimethylammonium bromide or DTAB, TTAB, and CTAB, respectively) were used to remove a blue solvent-based ink from a printed surface of high-density polyethylene bottles. Either an increase in the alkyl chain length or the surfactant concentration was found to increase the deinking efficiency. Complete deinking was achieved at concentrations about 3, 8, and 24 times of the critical micelle concentration (CMC) of CTAB, TTAB, and DTAB, respectively. For CTAB, ink removal started at a concentration close to or less than its CMC and increased appreciably at concentrations greater than its CMC, while for TTAB and DTAB, significant deinking was only achieved at concentrations much greater than their CMCs. Corresponding to the deinking efficiency of CTAB in the CMC region, the zeta potential of ink particles was found to increase with increasing alkyl chain length and concentration of the surfactants, which later leveled off at some higher concentrations. Wettability of the surfactants on an ink surface increased with increasing alkyl chain length and concentration of the surfactants. Lastly, solubilization of ink binder in the surfactant micelles was found to increase with increasing alkyl chain length and surfactant concentration. We conclude that adsorption of surfactant on the ink pigment is crucial to deinking due to modification of wettability, zeta potential, pigment/water interfacial tension, and dispersion stability. Solubilization of binder (epoxy) into micelles is necessary for good deinking because the dissolution of the binder is required before the pigment particles can be released from the polymer surface.     

Diffusion of alkanethiols in PDMS and its implications on microcontact printing (muCP)

n-Alkanethiols HS-(CH2)n-CH3 such as hexadecanethiol (HDT, n = 15), octadecanethiol (ODT, n = 17), and eicosanethiol (ECT, n = 19) have been shown to provide highly protective etch resists on microcontact-printed noble metals. As the quality of the printed pattern strongly depends on the mobility of the ink compound, we focused on understanding the diffusion behavior of HDT, ODT, and ECT in poly(dimethylsiloxane) (PDMS) stamps. We used a commercial PDMS material (Sylgard184), which is commonly used for microcontact printing (muCP), and a custom-synthesized one with a higher modulus. On the basis of linear-diffusion experiments, which maintained realistic printing conditions, we showed that the ink transport in the stamp follows Fick's law of diffusion. We then determined the diffusion coefficient by analytical and numerical modeling of the diffusion experiments. Numerical calculations were carried out with the finite-difference method applying more realistic boundary conditions (ink adsorption). Values for the diffusion coefficients of the three ink compounds in the two different PDMS materials all are on the order of (4-7) x 10(-7) cm2 s(-1). The scope and limits of the mathematical models are discussed. To demonstrate the potential of such models for microcontact printing, we simulate multiple printing cycles of an inked stamp and compare the results with experimental data.     

微接触印刷法制造聚合物多层次准三维立体微结构

Microstructures of various polymers, such as polystyrene and polymethyl methacrylate, were fabricated with microcontact printing, directly using the corresponding dilute polymeric solutions as “inks”, whose concentrations were about 10 g/L. By repeatedly cross-stamping with the inks, multilayer quasi-three-dimensional polymeric microstructures could be obtained. Both optical photographs and SEM photos showed clear microstructures, which were nearly accurate replication of the original patterns in the PDMS stamps. Microlines of poly-bis-(p-toluene sulfonate)-2,4-hexadiyne-1,6-diol) (PTS) were also fabricated by first processed microcontact printing with solution of the corresponding monomer TS/acetone as ink, then followed with UV polymerization of the monomer micropatterns at solid state. Unlike small molecule processes, the molecules of polymeric inks did not self assembly on the surface of substrates. The formation of polymeric microstructures could be ascribed to the fact that, after volatilization of solvents, polymers tend to stick to the surface of glass substrate which has higher surface free energy (about 72 mN/m), but not to the surface of PDMS stamp which has lower surface free energy (about 20 mN/m). Also the microcontact printing process was studied with optical microscopy, and the main factor--volatilization time of solvent was discussed. The results showed that the volatilization time of solvent is very crucial to the process of polymeric microcontact printing, and with too longer or too shorter volatilization time, the obtained microstructures would become discontinuous or distorted, respectively. For example, with a polystyrene/chloroform solution as ink, the optimal volatilization time was about 15~20 s.