Polymer Blend Lithography Research Articles

Tip-Induced Nanopatterning of Ultrathin Polymer Brushes.

Patterned, ultra-thin surface layers can serve as templates for positioning nanoparticlesor targeted self-assembly of molecular structures, for example, block-copolymers. This work investigates the high-resolution, atomic force microscopebased patterning of 2nm thick vinyl-terminated polystyrene brush layers and evaluates the line broadening due to tip degradation. This work compares the patterning properties with those of a silane-based fluorinated self-assembled monolayer (SAM), using molecular heteropatterns generated by modified polymer blend lithography (brush/SAM-PBL). Stable line widths of 20nm (FWHM) over lengths of over 20000µm indicate greatly reduced tip wear, compared to expectations on uncoated SiOx surfaces. The polymerbrush acts as a molecularly thin lubricating layer, thus enabling a 5000 fold increase in tip lifetime, and the brush is bonded weakly enough that it can be removed with surgical accuracy. On traditionally used SAMs, either the tip wear is very high or the molecules are not completely removed. Polymer Phase Amplified Brush Editing is presented, which uses directed self-assembly to amplify the aspect ratio of the molecular structures by a factor of 4. The structures thus amplified allow transfer into silicon/metal heterostructures, fabricating 30nm deep, all-silicon diffraction gratings that could withstand focused high-power 405nm laser irradiation.

Facile fabrication of random plasmonic metasurface via azeotropic solvent-induced polymer template and its application in organic light-emitting diodes

Random plasmonic metasurfaces (RPMs), in which an incident light is diffusely scattered with random phases, have attracted a lot interest due to its broadband optical response. Polymer blend lithography is a widely used nanofabrication method for RPMs. Typically, a high humidity environment is needed to control the phase separation process of polymer blend, which limited the controllability and repeatability of the nanofabrication process. Here, we provide a facile strategy based on azeotropic solvent for phase-separation control of polymer blend. A random nanohole mask is achieved by using H 2 O/MEK azeotropic solvent at normal humidity (∼20%). By depositing silver film on the nanohole mask, we realize a RPM consisting of nanohole-desk units. The scattering spectra centered at 540 nm, 580 nm and 640 nm were realized by changing the size distribution of nanohole-desk units. Furthermore, we used the RPM as an electrode to improve the efficiency of a blue-emission organic light-emitting diode. The facility and scalability of the azeotropic solvent-based nanofabrication method makes this technique a suitable candidate for lots of photonic applications. • A facile strategy based on azeotropic solvent for phase-separation control of polymer blend was reported. • A Ag-based random plasmonic metasurface (RPM) was demonstrated via polymer nanohole mask with controllable optical response. • RPM-based blue-emission OLED was fabricated with improved efficiency and reorganized far-field distribution.

Efficient light-trapping in ultrathin GaAs solar cells using quasi-random photonic crystals

Ultrathin solar cells reduce material usage and allow the use of lower-quality materials thanks to their one order of magnitude smaller thickness than their conventional counterparts. However, efficient photonic light-trapping is required to harvest the incident light efficiently for an otherwise insufficient absorber thickness. Quasi-random photonic crystals are predicted to have high efficient light-trapping while being more robust under angle and thickness variations than simple photonic crystals. Here we experimentally demonstrate a light-trapping solution based on quasi-random photonic crystals fabricated by polymer blend lithography. We control the average lattice parameter by modifying the spin-coating speed. We demonstrate an ultrathin GaAs cell of 260 nm with a rear quasi-random pattern with submicron features, and a Jsc = 26.4 mA/cm2 and an efficiency of 22.35% under the global solar spectrum.

High Figure-of-Merit Transparent Copper–Zinc Oxide Window Electrodes for Organic Photovoltaics

We report a copper - zinc oxide bilayer electrode supported on flexible polyethylene terephthalate (PET) with a sheet resistance of 11.3 Ω sq-1 and average transparency of 84.6 % in the wavelength range of 400 - 800 nm. The copper film is perforated with a dense array of sub-micron diameter apertures fabricated using polymer blend lithography, which imparts broad band anti-reflectivity. We demonstrate proof-of-principle that it is possible to fabricate the polymer mask by dip coating which is a scalable deposition method compatible with roll-to-roll processing. During storage of the electrode at ambient temperature the ZnO layer is spontaneously doped with copper from the underlying copper film and so the thin ZnO layer serves both as an anti-reflecting layer and an excellent electron transport layer. When compared with commercially available indium tin oxide coated (ITO) plastic substrates this electrode exhibits superior stability towards bending deformation, with no change in sheet resistance after bending through a 4 mm radius of curvature one hundred times. Model inverted organic photovoltaic (OPV) devices using this electrode exhibit a champion power conversion efficiency of 8.7%, which is the highest reported efficiency to date for an OPV using a copper based transparent electrode, outperforming identical devices using ITO coated plastic as the transparent electrode.

Local Conductivity Measurements of Metal-Organic Layers By Conductive AFM on Micropatterned Electrodes Produced By Metal Polymer Blend Lithography

The electronic characterization of thin metal-organic layers like SURMOFs is often difficult, since transport contributions in global measurements may be dominated by pin holes with direct electrical contact of top and bottom electrodes. In addition, these measurements average over a wide surface area and no local information can be derived. Using an AFM with a metalized tip, local measurements of the conductivity and I-V characteristics can be extracted. But these measurements have three important draw backs: (I.) the small contact area in the 20-nm-range restricts the range of measureable conductivity due to a limited sensitivity of the electronic amplifiers, (II.) the electric field underneath the AFM Tip is rather inhomogeneous and current contributions in-plane and off-plane are mixed and (III.) the contact area varies individually from tip to tip and with the applied force. With our approach of polymer blend lithography (PBL) we produce billions of small metal islands with diameters in the range of a few hundred nanometers up to a few microns with a simple process (Metal-PBL) [1]. These Islands serve as planar top electrodes, which we can address individually with the AFM-tip in a highly reproducible way. Using a set of AFM scans in the same region, a manifold of data sets concerning conductivity and I-V characteristic can be extracted, since many individual islands are measured simultaneously. 1.: Polymer blend lithography for metal films: large-area patterning with over 1 billion holes/inch2: C. Huang, A Förste, S. Walheim, and Th. Schimmel: Beilstein Journal of Nanotechnology 2015, 6, 1205–1211

A robust AFM-based method for locally measuring the elasticity of samples.

Investigation of the local sample elasticity is of high importance in many scientific domains. In 2014, Herruzo et al. published a new method based on frequency-modulation atomic force microscopy to locally determine the elasticity of samples (Nat. Commun. 2014, 5, 3126). This method gives evidence for the linearity of the relation between the frequency shift of the cantilever first flexural mode Δf1 and the square of the frequency shift of the second flexural mode Δf22. In the present work, we showed that a similar linear relation exists when measuring in contact mode with a certain load FN and propose a new method for determining the elastic modulus of samples from this relation. The measurements were performed in non-dry air at ambient temperature on three different polymers (polystyrene, polypropylene and linear low-density polyethylene) and a self-assembled monolayer of 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) on a silicon oxide substrate perforated with circular holes prepared by polymer blend lithography. For all samples the relation was evidenced by recording Δf1, Δf2 and FN as a function of the Z-displacement curves of the piezoelectric scanner. The occurence of a plastic deformation followed by an elastic deformation is shown and explained. The necessary load FN for measuring in the elastic domain was assessed for each sample, used for mapping the frequency shifts Δf1 and Δf2 and for determining the elastic modulus from Δf22/Δf1. The method was used to give an estimate of the Young’s modulus of the FDTS thin film.

Light trapping in thin film silicon solar cells via phase separated disordered nanopillars.

In this work, we have improved the absorption properties of thin film solar cells by introducing light trapping reflectors deposited onto self-assembled nanostructures. The latter consist of a disordered array of nanopillars and are fabricated by polymer blend lithography. Their broadband light scattering properties are exploited to enhance the photocurrent density of thin film devices, here based on hydrogenated amorphous silicon active layers. We demonstrate that these light scattering nanopillars yield a short-circuit current density increase of +33%rel with respect to equivalent solar cells processed on a planar reflector. Moreover, we experimentally show that they outperform randomly textured substrates that are commonly used for achieving efficient light trapping. Complementary optical simulations are conducted on an accurate 3D model to analyze the superior light harvesting properties of the nanopillar array and to derive general design rules. Our approach allows one to easily tune the morphology of the self-assembled nanostructures, is up-scalable and operated at room temperature, and is applicable to other photovoltaic technologies.

Polymer blend lithography for metal films: large-area patterning with over 1 billion holes/inch(2).

Polymer blend lithography (PBL) is a spin-coating-based technique that makes use of the purely lateral phase separation between two immiscible polymers to fabricate large area nanoscale patterns. In our earlier work (Huang et al. 2012), PBL was demonstrated for the fabrication of patterned self-assembled monolayers. Here, we report a new method based on the technique of polymer blend lithography that allows for the fabrication of metal island arrays or perforated metal films on the nanometer scale, the metal PBL. As the polymer blend system in this work, a mixture of polystyrene (PS) and poly(methyl methacrylate) (PMMA), dissolved in methyl ethyl ketone (MEK) is used. This system forms a purely lateral structure on the substrate at controlled humidity, which means that PS droplets are formed in a PMMA matrix, whereby both phases have direct contact both to the substrate and to the air interface. Therefore, a subsequent selective dissolution of either the PS or PMMA component leaves behind a nanostructured film which can be used as a lithographic mask. We use this lithographic mask for the fabrication of metal patterns by thermal evaporation of the metal, followed by a lift-off process. As a consequence, the resulting metal nanostructure is an exact replica of the pattern of the selectively removed polymer (either a perforated metal film or metal islands). The minimum diameter of these holes or metal islands demonstrated here is about 50 nm. Au, Pd, Cu, Cr and Al templates were fabricated in this work by metal PBL. The wavelength-selective optical transmission spectra due to the localized surface plasmonic effect of the holes in perforated Al films were investigated and compared to the respective hole diameter histograms.

Polymer blend lithography: A versatile method to fabricate nanopatterned self-assembled monolayers

A rapid and cost-effective lithographic method, polymer blend lithography (PBL), is reported to produce patterned self-assembled monolayers (SAM) on solid substrates featuring two or three different chemical functionalities. For the pattern generation we use the phase separation of two immiscible polymers in a blend solution during a spin-coating process. By controlling the spin-coating parameters and conditions, including the ambient atmosphere (humidity), the molar mass of the polystyrene (PS) and poly(methyl methacrylate) (PMMA), and the mass ratio between the two polymers in the blend solution, the formation of a purely lateral morphology (PS islands standing on the substrate while isolated in the PMMA matrix) can be reproducibly induced. Either of the formed phases (PS or PMMA) can be selectively dissolved afterwards, and the remaining phase can be used as a lift-off mask for the formation of a nanopatterned functional silane monolayer. This “monolayer copy” of the polymer phase morphology has a topographic contrast of about 1.3 nm. A demonstration of tuning of the PS island diameter is given by changing the molar mass of PS. Moreover, polymer blend lithography can provide the possibility of fabricating a surface with three different chemical components: This is demonstrated by inducing breath figures (evaporated condensed entity) at higher humidity during the spin-coating process. Here we demonstrate the formation of a lateral pattern consisting of regions covered with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and (3-aminopropyl)triethoxysilane (APTES), and at the same time featuring regions of bare SiOx. The patterning process could be applied even on meter-sized substrates with various functional SAM molecules, making this process suitable for the rapid preparation of quasi two-dimensional nanopatterned functional substrates, e.g., for the template-controlled growth of ZnO nanostructures [1].

Gold mesoflower arrays with sub-10 nm intraparticle gaps for highly sensitive and repeatable surface enhanced Raman spectroscopy

Self-assembling Au mesoflower arrays are prepared using a polymethylmethacrylate (PMMA) template on an iron substrate via a combined top-down/bottom-up nanofabrication strategy. The PMMA template with the holes around 300–500 nm in diameter is first fabricated by using polymer blend lithography on iron substrates, and the highly homogeneous Au mesoflower arrays with less than 10 nm intraparticle gaps are subsequently obtained by an in situ galvanic reaction between HAuCl4 solution and the iron substrate under optimal stirring of the solution as well as reaction time. Owing to the unique mesostructures and uniformity, Raman measurements show that the gold mesoflower arrays obtained demonstrated a strong and reproducible surface enhanced Raman scattering (SERS) enhancement on the order of ∼107–108. The development of a SERS substrate based on the Au mesoflowers with high spatial density of hot spots, relatively low cost and facial synthesis provides a novel strategy for applications in chemical and biomolecular sensing.

Inducible Site-Selective Bottom-Up Assembly of Virus-Derived Nanotube Arrays on RNA-Equipped Wafers

Tobacco mosaic virus (TMV) is a tube-shaped, exceptionally stable plant virus, which is among the biomolecule complexes offering most promising perspectives for nanotechnology applications. Every viral nanotube self-assembles from a single RNA strand and numerous identical coat protein (CP) subunits. Here we demonstrate that biotechnologically engineered RNA species containing the TMV origin of assembly can be selectively attached to solid surfaces via one end and govern the bottom-up growth of surface-linked TMV-like nanotubes in situ on demand. SiO(2) wafers patterned by polymer blend lithography were modified in a chemically selective manner, which allowed positioning of in vitro produced RNA scaffolds into predefined patches on the 100-500 nm scale. The RNA operated as guiding strands for the self-assembly of spatially ordered nanotube 3D arrays on the micrometer scale. This novel approach may promote technically applicable production routes toward a controlled integration of multivalent biotemplates into miniaturized devices to functionalize poorly accessible components prior to use. Furthermore, the results mark a milestone in the experimental verification of viral nucleoprotein complex self-assembly mechanisms.

Templated Self-Assembly of ZnO Films on Monolayer Patterns with Nanoscale Resolution

Lithographically defined self-growing ZnO films were prepared by a bioinspired chemical bath deposition technique (CBD). We observed a high selectivity of ZnO deposition: Teflon-like per-fluoro-decyl-trichlorosilane (FDTS) monolayers repelled ZnO primary particles, whereas amino-functionalized areas of the substrate were selectively covered by a highly anisotropic, oriented, and compact ZnO film with a thickness of 50 nm. The size of the primary particles in our methanol-based solution was approximately 2.5 nm. On the amino substrate they formed agglomerates not larger than 30 nm. Monolayer patterns made by polymer blend lithography were templated with an edge resolution of 30 nm. By using a specialized derivative of microcontact printing, we prepared layout-defined silane templates, which reliably determined the growth of a layout-defined, patterned oxide film with submicrometer lateral resolution.