Jane Frommer

Nanoscale Three-Dimensional Patterning of Molecular Resists by Scanning Probes

Patterning a Molecular Glass Lithographic patterning for device fabrication is usually based on initiating polymerization reactions with photons or electrons in a molecular resist. However, patterning can be achieved by mechanically removing a hard resist with scanning probe microscopy tips, but in many cases the resolution is low and excess material is left on the surface. Pires et al. (p. 732, published online 22 April) found that thin films of organic molecules could form glasses through weak interactions and be patterned to tens of nanometers with a heated scanning probe tip. These patterns could be transferred to other substrates or sculpted into three-dimensional shapes by successive rounds of patterning. A molecular glass can be patterned to dimensions of tens of nanometers with a heated scanning probe tip. For patterning organic resists, optical and electron beam lithography are the most established methods; however, at resolutions below 30 nanometers, inherent problems result from unwanted exposure of the resist in nearby areas. We present a scanning probe lithography method based on the local desorption of a glassy organic resist by a heatable probe. We demonstrate patterning at a half pitch down to 15 nanometers without proximity corrections and with throughputs approaching those of Gaussian electron beam lithography at similar resolution. These patterns can be transferred to other substrates, and material can be removed in successive steps in order to fabricate complex three-dimensional structures.

Probe‐Based 3‐D Nanolithography Using Self‐Amplified Depolymerization Polymers

Scanning probes are capable of addressing and modifying surface structures on the atomic scale, [ 1 ] a capability that has been exploited to create molecular logic devices. [ 2 ] However, in realworld applications, the production of nanoscale patterns and devices requires substantial throughput capabilities in combination with suffi cient tip endurance to address areas on the order of 0.1–1 mm 2 at high resolution. At a typical pixel pitch of 10 nm, this translates to 10 8 –10 10 pixels being written with a single tip. Therefore a highly sensitive patterning approach that is gentle on the tip would be indispensable. Besides the well-established method of local anodic oxidation, [ 3 – 5 ] recent developments in this direction are the fi eld-induced deposition of materials [ 6 , 7 ] and the tip-induced modifi cation or removal of thermomechanically responsive organic materials. [ 8 – 12 ] In addition, it has been shown that on polymeric substrates the wear on a sliding silicon tip can be virtually eliminated, [ 13 ] which is a prerequisite for high-resolution patterning on technologically viable scales. Other stimuli have been used to structure polymers locally using atomic force microscopy (AFM) tips, e.g., mechanical forces in plowing [ 14 ] and ultrasonic patterning [ 15 ] or electron irradiation using fi eld emission from the tip. [ 16 ] In this paper, we describe the fabrication of twoand threedimensional structures based on the local removal of a resist polymer using heated tips. Previous experiments have shown that suffi cient energy is provided by heated tips to break the chemical bonds of a Diels–Alder material, [ 11 ] which subsequently decomposes into volatile monomer units. However, the overall patterning effi ciency is low. The effi ciency can be dramatically enhanced by using self-amplifi ed depolymerization (SAD) polymers. Here, the breaking of a single bond induces the spontaneous depolymerization of the entire polymer chain, [ 17 , 18 ] a concept that was fi rst demonstrated in the early 80’s as a dry lithography approach. Recently it was discovered that using phthalaldehyde SAD polymers two-dimensional

Self-assembled ferrimagnet--polymer composites for magnetic recording media.

A self-assembled magnetic recording medium was created using colloidal ferrimagnetic building blocks. Monodisperse cobalt ferrite nanoparticles (CoFe(2)O(4)) were synthesized using solution-based methods and then stabilized in solution using the amphiphilic diblock copolymer, poly(acrylic acid)-b-poly(styrene) (PAA-PS). The acid groups of the acrylate block bound the polymer to the nanoparticle surface via multivalent interactions, while the styrene block afforded the magnetic nanoparticle--polymer complex solubility in organic solvents. Moreover, the diblock copolymer improved the colloidal stability of the ferrimagnetic CoFe(2)O(4) nanoparticles by reducing the strong interparticle magnetic interactions, which typically caused the ferrimagnetic nanoparticles to irreversibly aggregate. The nanoparticle--polymer complex was spin-coated onto a silicon substrate to afford self-organized thin film arrays, with the interparticle spacing determined by the molecular weight of the diblock copolymer. The thin film composite was also exposed to an external magnetic field while simultaneously heated above the glass transition temperature of poly(styrene) to allow the nanoparticles to physically rotate to align their easy axes with the direction of the magnetic field. In order to demonstrate that this self-assembled ferrimagnet--polymer composite was suitable as a magnetic recording media, read/write cycles were demonstrated using a contact magnetic tester. This work provides a simple route to synthesizing stabilized ferrimagnetic nanocrystals that are suitable for developing magnetic recording media.

Interfacial glass transition profiles in ultrathin, spin cast polymer films

Interfacial glass transition temperature (T(g)) profiles in spin cast, ultrathin films of polystyrene and derivatives were investigated using shear-modulated scanning force microscopy. The transitions were measured as a function of film thickness (delta), molecular weight, and crosslinking density. The T(g)(delta) profiles were nonmonotonic and exhibited two regimes: (a) a sublayer extending about 10 nm from the substrate, with T(g) values lowered up to approximately 10 degrees C below the bulk value, and (b) an intermediate regime extending over 200 nm beyond the sublayer, with T(g) values exceeding the bulk value by up to 10 degrees C. Increasing the molecular weight was found to shift the T(g)(delta) profiles further from the substrate interface, on the order of 10 nm/kDa. Crosslinking the precast films elevated the absolute T(g) values, but had no effect on the spatial length scale of the T(g)(delta) profiles. These results are explained in the context of film preparation history and its influence on molecular mobility. Specifically, the observed rheological anisotropy is interpreted based on the combined effects of shear-induced structuring and thermally activated interdiffusion.

Broad-spectrum antimicrobial supramolecular assemblies with distinctive size and shape

With the increased prevalence of antibiotic-resistant infections, there is an urgent need for innovative antimicrobial treatments. One such area being actively explored is the use of self-assembling cationic polymers. This relatively new class of materials was inspired by biologically pervasive cationic host defense peptides. The antimicrobial action of both the synthetic polymers and naturally occurring peptides is believed to be complemented by their three-dimensional structure. In an effort to evaluate shape effects on antimicrobial materials, triblock polymers were polymerized from an assembly directing terephthalamide-bisurea core. Simple changes to this core, such as the addition of a methylene spacer, served to direct self-assembly into distinct morphologies-spheres and rods. Computational modeling also demonstrated how subtle core changes could directly alter urea stacking motifs manifesting in unique multidirectional hydrogen-bond networks despite the vast majority of material consisting of poly(lactide) (interior block) and cationic polycarbonates (exterior block). Upon testing the spherical and rod-like morphologies for antimicrobial properties, it was found that both possessed broad-spectrum activity (Gram-negative and Gram-positive bacteria as well as fungi) with minimal hemolysis, although only the rod-like assemblies were effective against Candida albicans.

Ultrafast molecule sorting and delivery by atomic force microscopy

An atomic force microscope (AFM) is tailored to perform ultrafast electrophoretic differentiation of molecules on populations of <0.1zeptomoles (10−22moles) on the surface of a probe tip. The driving force for differentiation is a large electric field applied over the length of an AFM tip that results in enhanced differential mobilities stemming from the confinement of the water layer on the tip surface. In a demonstration on DNA oligonucleotides, a 5-mer and a 16-mer exhibit migration times of 15 and 5ms, respectively, approximately five orders of magnitude faster than in conventional capillary electrophoresis.

Three-Dimensional Nanoprinting via Scanning Probe Lithography-Delivered Layer-by-Layer Deposition

Three-dimensional (3D) printing has been a very active area of research and development due to its capability to produce 3D objects by design. Miniaturization and improvement of spatial resolution are major challenges in current 3D printing technology development. This work reports advances in miniaturizing 3D printing to the nanometer scale using scanning probe microscopy in conjunction with local material delivery. Using polyelectrolyte polymers and complexes, we have demonstrated the concept of layer-by-layer nanoprinting by design. Nanometer precision is achieved in all three dimensions, as well as in interlayer registry. The approach enables production of designed functional 3D materials with nanometer resolution and, as such, creates a platform for conducting scientific research in designed 3D nanoenvironments as well. In doing so, it enables production of nanomaterials and scaffolds for photonics devices, biomedicine, and tissue engineering.

High-throughput directed self-assembly of core-shell ferrimagnetic nanoparticle arrays.

Magnetic nanoparticles (MNPs) provide a set of building blocks for constructing stimuli-responsive nanoscale materials with properties that are unique to this scale. The size and the composition of MNPs are tunable to meet the requirements for a range of applications including biosensors and data storage. Although many of these technologies would significantly benefit from the organization of nanoparticles into higher-order architectures, the precise placement and arrangement of nanoparticles over large areas of a surface remain a challenge. Herein, we demonstrate the viability of magnetic nanoparticles for patterned recording media utilizing a template-directed self-assembly process to afford well-defined nanostructures of magnetic nanoparticles and access these assemblies using magnetic force microscopy and a magnetic recording head. Photolithographically defined holes were utilized as templates to form assemblies of ferrimagnetic nanoparticle rings or pillars selectively over a large area (>1 cm(2)) in just 30 s. This approach is applicable to other nanoparticle systems as well and enables their high-throughput self-assembly for future advanced device fabrication.

Scanning probe microscopy of organics, an update

Abstract Scanning probe microscopy as applied to organic materials has evolved from the original demonstrations that the technique could successfully and non-invasively image these soft samples [1]. One evolutionary pathway has been to switch from qualitative to more quantitative measurements. Among those studies are measurements of adhesion [2–5], elasticity [6], friction [7], and quantification of individual bonding interactions [8–11]. The use of this technique to derive absolute values for these properties is still limited by uncertainties such as the degree to which the instrument itself contributes to the measured signals, an exact knowledge of the geometry of the interface between sample and probe, and which intermolecular interactions are contributing to measured signals. In this overview, we will address these issues, with particular attention to the “two-body” (or “many-body”) nature of the tip-sample interaction. These multi-body interactions will be discussed in the contexts of adhesion, delamination, friction, and bio-specificity.

Molecular surface structure of tetracene mapped by the atomic force microscope

The atomic force microscope has been used to record molecular structure on free‐standing organic crystals. A crystal of tetracene has been imaged with molecular resolution which allows the assignment of lattice parameters to the surface layer. The intermolecular spacings on the surface of tetracene correspond remarkably closely with those in the bulk. It is even possible to distinguish between the two translationally inequivalent molecules of the unit cell. The mechanism for using force microscopy to distinguish between different molecular orientations is discussed.