Sokol Ndoni

Surface Modification of Nanoporous 1,2-Polybutadiene by Atom Transfer Radical Polymerization or Click Chemistry

Surface-initiated atom transfer radical polymerization (ATRP) and click chemistry were used to obtain functional nanoporous polymers based on nanoporous 1,2-polybutadiene (PB) with gyroid morphology. The ATRP monolith initiator was prepared by immobilizing bromoester initiators onto the pore walls through two different methodologies: (1) three-step chemical conversion of double bonds of PB into bromoisobutyrate, and (2) photochemical functionalization of PB with bromoisobutyrate groups. Azide functional groups were attached onto the pore walls before click reaction with alkynated MPEG. Following ATRP-grafting of hydrophilic polyacrylates and click of MPEG, the originally hydrophobic samples transformed into hydrophilic nanoporous materials. The successful modification was confirmed by infrared spectroscopy, contact angle measurements and measurements of spontaneous water uptake, while the morphology was investigated by small-angle X-ray scattering and transmission electron microscopy.

UV patterned nanoporous solid-liquid core waveguides

Nanoporous Solid-Liquid core waveguides were prepared by UV induced surface modification of hydrophobic nanoporous polymers. With this method, the index contrast (deltan = 0.20) is a result of selective water infiltration. The waveguide core is defined by UV light, rendering the exposed part of a nanoporous polymer block hydrophilic. A propagation loss of 0.62 dB/mm and a bend loss of 0.81 dB/90 degrees for bend radius as low as 1.75 mm was obtained in these multimode waveguides.

Wafer-Scale Nanopillars Derived from Block Copolymer Lithography for Surface-Enhanced Raman Spectroscopy.

We report a novel nanofabrication process via block copolymer lithography using solvent vapor annealing. The nanolithography process is facile and scalable, enabling fabrication of highly ordered periodic patterns over entire wafers as substrates for surface-enhanced Raman spectroscopy (SERS). Direct silicon etching with high aspect ratio templated by the block copolymer mask is realized without any intermediate layer or external precursors. Uniquely, an atomic layer deposition (ALD)-assisted method is introduced to allow reversing of the morphology relative to the initial pattern. As a result, highly ordered silicon nanopillar arrays are fabricated with controlled aspect ratios. After metallization, the resulting nanopillar arrays are suitable for SERS applications. These structures readily exhibit an average SERS enhancement factor of above 10(8), SERS uniformities of 8.5% relative standard deviation across 4 cm, and 6.5% relative standard deviation over 5 × 5 mm(2) surface area, as well as a very low SERS background. The as-prepared SERS substrate, with a good enhancement and large-area uniformity, is promising for practical SERS sensing applications.

Substrate tolerant direct block copolymer nanolithography

Block copolymer (BC) self-assembly constitutes a powerful platform for nanolithography. However, there is a need for a general approach to BC lithography that critically considers all the steps from substrate preparation to the final pattern transfer. We present a procedure that significantly simplifies the main stream BC lithography process, showing a broad substrate tolerance and allowing for efficient pattern transfer over wafer scale. PDMS-rich poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) copolymers are directly applied on substrates including polymers, silicon and graphene. A single oxygen plasma treatment enables formation of the oxidized PDMS hard mask, PS block removal and polymer or graphene substrate patterning.

Fast & scalable pattern transfer via block copolymer nanolithography

A fully scalable and efficient pattern transfer process based on block copolymer (BCP) self-assembling directly on various substrates is demonstrated. PS-rich and PDMS-rich poly(styrene-b-dimethylsiloxane) (PS-b-PDMS) copolymers are used to give monolayer sphere morphology after spin-casting of solutions with selective solvents relative to the majority block. The pattern is directly formed during spin-casting at room temperature, which takes less than 20 seconds, without any preliminary surface treatment of the substrate and without any subsequent annealing. The self-assembled BCPs are transformed into hard lithography masks by oxidation of PDMS in oxygen plasma. The hard masks are then used to fabricate full wafer scale arrays of nano-pillars and nano-wells on various substrates, including polymers and silicon. The demonstrated BCP nanolithography process opens up numerous applications not relying on long range lateral order, including fabrication of substrates for catalysis, solar cells, sensors, ultrafiltration membranes and templating of semiconductors or metals.

Physisorption of SDS in a Hydrocarbon Nanoporous Polymer

Surface modification of nanoporous 1,2-polybutadiene of pore diameter approximately 15 nm was accomplished by physisorption of sodium dodecyl sulfate (SDS) in water. Loading of the aqueous solution and the accompanying physisorption of SDS into the hydrophobic nanoporous films were investigated in a wide range of concentrations. The loading showed varying dependence on the SDS concentration. No loading was observed for SDS concentrations below 4.0 mM. At concentrations above 5.0 mM, the initial part of loading showed a linear dependence on the square root of time, which can be interpreted as diffusion-controlled dynamics. Both the specific equilibrium loading and the final SDS adsorption reached plateau values at concentrations above 6.8 mM. The infiltration of SDS into the nanoporous film was mainly followed by gravimetry and for a few samples confirmed by X-ray photoelectron spectroscopy (XPS). The SDS adsorption isotherm can be well described by the Langmuir model, consistent with a monolayer adsorption onto the pore walls. The surface modified by the surfactant clearly showed water wettability, in contrast to the original hydrophobic nanoporous 1,2-PB. We also looked at the release process of SDS out of the nanopores in the presence of excess of water or methanol. A discussion of the very different time scales of release in the two media is presented at the end of the paper.

Photocatalytic Nanostructuring of Graphene Guided by Block Copolymer Self-Assembly

Nanostructured graphene exhibits many intriguing properties. For example, precisely controlled graphene nanomeshes can be applied in electronic, photonic, or sensing devices. However, fabrication of nanopatterned graphene with periodic supperlattice remains a challenge. In this work, periodic graphene nanomesh was fabricated by photocatalysis of single-layer graphene suspended on top of TiO2-covered nanopillars, which were produced by combining block copolymer nanolithography with atomic layer deposition. Graphene nanoribbons were also prepared by the same method applied to a line-forming block copolymer template. This mask-free and nonchemical/nonplasma route offers an exciting platform for nanopatterning of graphene and other UV-transparent materials for device engineering.

High quality sub-10 nm graphene nanoribbons by on-chip PS-b-PDMS block copolymer lithography

A block copolymer self-assembly holds great promise as a rapid, cheap and scalable approach to nanolithography. We present a straightforward method for fabrication of sub-10 nm line patterns from a lamellar polystyrene-b-polydimethylsiloxane (SD) block copolymer with a total average molecular weight of 10.5 kg mol−1. Thin SD films directly spin cast onto silicon substrates and on graphene, form regular line patterns of sub-10 nm pitch on the substrates after a few minutes of annealing at 45 °C in the presence of toluene vapour. Perfect pattern alignment was achieved by confining the films inside the trenches of graphoepitaxial substrates. The SD template was furthermore used as a lithographic mask to fabricate high-quality sub-10 nm graphene nanoribbons. This was realized by one step oxygen plasma treatment, which accomplishes three tasks: hardening the PDMS block by oxidation, and etching both the PS block and the graphene under PS. Raman analysis supports the formation of graphene nanoribbons with an average distance between defects corresponding to the oxidized PDMS pitch, with no sign of defects generated in the ribbon channel. This suggests a high degree of protection of the nanoribbons by the hard oxidized PDMS mask formed in situ during oxygen plasma etching.

Nanoporous materials modified with biodegradable polymers as models for drug delivery applications

Polymers play a central role in the development of carriers for diagnostic and therapeutic agents. Especially the use of either degradable polymers or porous materials to encapsulate drug compounds in order to obtain steady drug release profiles has received much attention. We present here a proof of principle for a system combining these two encapsulation methods and consisting of a nanoporous polymer (NP) with the pores filled with a degradable polymer mixed with a drug model. Rhodamine 6G (R6G) mixed with Poly(L-Lactic Acid) (PLLA) were confined within the 14 nm pores of a NP with gyroid morphology derived from a diblock copolymer precursor. Glass transition, crystallization and melting of free and confined PLLA were monitored by differential scanning calorimetry. Release profiles for R6G were measured in methanol-water solvents at pH 13, which works as an accelerated release test by speeding up the hydrolysis of PLLA. The obtained release profiles demonstrate that the degradation of PLLA in nanoporous confinement is significantly slower than the degradation of unconfined PLLA. The release of R6G encapsulated in PLLA becomes correspondingly slower, while the initial burst release virtually disappears. These findings suggest that the presented proof of principle constitutes a promising basis for the development of novel implantable drug delivery systems.

Sub-15nm silicon lines fabrication via PS-b-PDMS block copolymer lithography

This paper describes the fabrication of nanodimensioned silicon structures on silicon wafers from thin films of a poly(styrene)- block-poly(dimethylsiloxane) (PS-b-PDMS) block copolymer (BCP) precursor self-assembling into cylindrical morphology in the bulk. The structure alignment of the PS-b-PDMS (33 k-17 k) was conditioned by applying solvent and solvothermal annealing techniques. BCP nanopatterns formed after the annealing process have been confirmed by scanning electron microscope (SEM) after removal of upper PDMS wetting layer by plasma etching. Silicon nanostructures were obtained by subsequent plasma etching to the underlying substrate by an anisotropic dry etching process. SEM images reveal the formation of silicon nanostructures, notably of sub-15nm dimensions.