Molecular Ink Research Articles

Phase of molecular ink in nanoscale direct deposition processes

We report the first observation of a phase transition in a nanoscale direct deposition process. This transition involves the melting of molecular ink layers in dip-pen nanolithography, and it is observed by measuring the temperature dependence of the growth rate of the deposited pattern. The results are interpreted using a diffusion equation approach in conjunction with a "double-molecular-layer" model of the adsorbed molecules on the atomic force microscope tip. The theory provides a qualitative explanation for the dependence of the pattern growth rate on solvent and adsorbed water as well as on temperature.

Review of tissue simulating phantoms for optical spectroscopy, imaging and dosimetry

Optical spectroscopy, imaging, and therapy tissue phantoms must have the scattering and absorption properties that are characteristic of human tissues, and over the past few decades, many useful models have been created. In this work, an overview of their composition and properties is outlined, by separating matrix, scattering, and absorbing materials, and discussing the benefits and weaknesses in each category. Matrix materials typically are water, gelatin, agar, polyester or epoxy and polyurethane resin, room-temperature vulcanizing (RTV) silicone, or polyvinyl alcohol gels. The water and hydrogel materials provide a soft medium that is biologically and biochemically compatible with addition of organic molecules, and are optimal for scientific laboratory studies. Polyester, polyurethane, and silicone phantoms are essentially permanent matrix compositions that are suitable for routine calibration and testing of established systems. The most common three choices for scatters have been: (1.) lipid based emulsions, (2.) titanium or aluminum oxide powders, and (3.) polymer microspheres. The choice of absorbers varies widely from hemoglobin and cells for biological simulation, to molecular dyes and ink as less biological but more stable absorbers. This review is an attempt to indicate which sets of phantoms are optimal for specific applications, and provide links to studies that characterize main phantom material properties and recipes.

Patterning Polymerized Lipid Vesicles with Soft Lithography

The applications of soft lithography in patterning polymerized lipid vesicles of 1,2-bis(tricosa-10,12-diynoyl)-sn-glycero-3-phosphocholine on glass substrates are reported. We demonstrate that the polymerized vesicles can be used as a high molecular weight ink to be transferred from a PDMS stamp onto a glass substrate to form two-dimensional stripes with a controlled separation. By combining channel flow with dewetting within microfluidic networks, we assemble the polymerized vesicle into three-dimensional stripes and one-dimension lines on glass substrates. Atomic force microscopy shows that these patterned vesicle structures are stable on glass substrates. The simple, stable, and precise immobilization of lipid vesicles on solid substrates will open up the possibility of integrating them in biosensors and microelectronic devices.

Organic thin-film transistors fabricated by microcontact printing

We have fabricated organic thin-film transistors (OTFTs) using a microcontact printing technique (μCP) that employs thin polydimethylsiloxane stamps on a rigid silicon substrate in order to reduce macroscopic distortions. Systematic variation of the printing pressure, printing time, and concentration of eicosanethiol, the “molecular ink” in the μCP process, permits the fabrication of devices with smaller channel lengths (Leff) than nominally defined by the stamp. Interdigitated Ti/Au electrode structures with Leff down to 100 nm could be fabricated which, after additional surface treatment and vacuum deposition of αα′-dihexylquaterthiophene, yield OTFTs with excellent characteristics.

Anomalous surface diffusion in nanoscale direct deposition processes.

We report the first observation of anomalous diffusion in nanometer scale direct deposition processes utilizing dip-pen nanolithography (DPN). DPN permits quite general nanostructure patterns to be drawn on flat surfaces. Here we demonstrate experimentally, and discuss theoretically, the situation in which the molecular ink in DPN binds weakly to the surface. We observe, for the weak-binding case of 1-dodecylamine on mica, that anomalous diffusion occurs, leading to nearly fractal deposition patterns.

Nanocontact Printing: A Route to Sub-50-nm-Scale Chemical and Biological Patterning

This paper describes the first example of the patterning of surfaces via nanocontact printing with chemically distinct features in the <50 nm scale over 3 × 3 mm2. These ultrasmall features are achieved via a combination of “sharp” and “hard” poly(dimethylsiloxane) stamps, keeping the contact area in the 50 nm domain, and high molecular weight inks to avoid diffusion. The patterns consist of dendrimers or proteins and can be used as a scaffold for further modification with gold nanoparticles.

Molecular Transport from an Atomic Force Microscope Tip: A Comparative Study of Dip-Pen Nanolithography

Dip-pen nanolithography (DPN), the direct transport of a “molecular ink” onto a substrate from an AFM tip, is of interest both as a potential industrial patterning tool and as a means to study molecular behavior. We investigated DPN for the patterning of both octadecanethiol (ODT) and mercaptohexadecanoic acid (MHA). The molecular transport of ODT and MHA does not require a water meniscus but takes place because the molecules are individually mobile. The effects of relative humidity and temperature on the patterning of each molecule were investigated. While the molecular transport rate for MHA increases slightly with increased relative humidity, the rate of molecular transport for ODT exhibits no dependence on relative humidity (although relative humidity affects the resulting molecular distribution). The transport rate of both molecules increases exponentially with temperature, consistent with an activation barrier of about 0.75 eV. Although the molecular fluxes of MHA and ODT vary with tip preparation and atmospheric conditions, both molecules diffuse from an AFM tip at a rate of about 105 molecules/s, corresponding to a patterning time of 100 years/cm2.

Siloxane Polymers for High-Resolution, High-Accuracy Soft Lithography

We report the formulation of siloxane polymers for high-resolution, high-accuracy stamps for soft lithography. With this technique, a molecular, polymeric, or liquid ink is applied to the surface of a stamp and then transferred by conformal contact to a substrate. Stamps for this technique are usually made of a commercial siloxane elastomer with appropriate mechanical properties to achieve conformal contact but are incapable of printing accurate, submicrometer patterns. To formulate better stamp polymers, we used models of rubber-like elasticity as guidelines. Poly(dimethylsiloxane) networks were prepared from vinyl and hydrosilane end-linked polymers and vinyl and hydrosilane copolymers, with varying mass between cross-links and junction functionality. The polymer formulations were characterized by strain at break as well as compression modulus and surface hardness measurements. This resulted in the identification of bimodal polymer networks having mechanical properties that allow the replication of high-density patterns at the 100 nm scale and that withstand the mechanical constraints during use as a stamp material. We also demonstrate advantageous implementations of the formulated polymers in hybrid stamps that achieve submicrometer-dimensional accuracy over large areas.