Jonathan W. Choi

A dual functional layer for block copolymer self-assembly and the growth of nanopatterned polymer brushes.

We present a versatile method for fabricating nanopatterned polymer brushes using a cross-linked thin film made from a random copolymer consisting of an inimer (p-(2-bromoisobutyloylmethyl)styrene), styrene, and glycidyl methacrylate (GMA). The amount of inimer was held constant at 20 or 30% while the relative amount of styrene to GMA was varied to induce perpendicular domain orientation in an overlying P(S-b-MMA) block copolymer (BCP) film for lamellar and cylindrical morphologies. A cylinder forming BCP blend with PMMA homopolymer was assembled to create a perpendicular hexagonal array of cylinders, which allowed access to a nanoporous template without the loss of initiator functionality. Surface-initiated ATRP of 2-hydroxyethyl methacrylate was conducted through the pores to generate a dense array of nanopatterned brushes. Alternatively, gold was deposited into the nanopores, and brushes were grown around the dots after removal of the template. This is the first example of combining the chemistry of nonpreferential surfaces with surface-initiated growth of polymer chains.

Transfer of pre-assembled block copolymer thin film to nanopattern unconventional substrates.

In this work, we demonstrate that a preassembled block copolymer (BCP) thin film can be floated, transferred, and utilized to effectively nanopattern unconventional substrates. As target substrates, we chose Cu foil and graphene/Cu foil since they cannot be nanopatterned via conventional processes due to the high surface roughness and susceptibility to harsh processing chemicals and etchants. Perpendicular hexagonal PMMA cylinder arrays in diblock copolymer poly(styrene-block-methyl methacrylate) [P(S-b-MMA)] thin films were preassembled on sacrificial SiO2/Si substrates. The BCP thin film was floated at the air/water interface off of a SiO2/Si substrate and then collected with the target substrate, leading to well-defined nanoporous PS templates on these uneven surfaces. We further show that the nanoporous template can be used for a subtractive process to fabricate nanoperforated graphene on Cu foil in sub-20 nm dimension, and for an additive process to create aluminum oxide nanodot arrays without any polymeric residues or use of harsh chemicals and etchants.

Molecular orientation and photoswitching kinetics on single-walled carbon nanotubes by optical second harmonic generation

Electrical conduction through chromophore-functionalized nanotubes can be modulated by light with wavelengths expected to isomerize the chromophores. Here, we use second harmonic generation to directly measure the orientation and photoisomerization kinetics of azo-benzene chromophores on single-walled carbon nanotubes. We find a net chromophore orientation with an average chromophore tilt angle of 40° ± 3°. We show that this angle can be reduced effectively to zero with an applied corona field. Periodic illumination with unpolarized 495 nm light induces reversible trans-cis switching, enabling the extraction of switching time scales both with and without an applied electric field.

A facile route for fabricating graphene nanoribbon array transistors using graphoepitaxy of a symmetric block copolymer

We report a facile route to form densely packed graphene nanoribbon (GNR) arrays via graphoepitaxial assembly of symmetric P(S-b-MMA). Since guiding channels for graphoepitaxy are the source and drain electrodes in field effect transistor (FET) geometry, we avoid laborious nanopatterning and FET device fabrication processes. By grafting a random copolymer brush on the graphene FET device, perpendicular lamellar domains are aligned normal to the electrode direction, resulting in line arrays connecting the two electrodes. Through optimization of the reactive ion etching conditions, the vertically oriented lamellar domains were transferred to the underlying graphene, leading to GNR arrays that act as conducting channels. This is a simple and efficient fabrication process using the fundamental concepts developed for the graphoepitaxial assembly of symmetric BCPs to create densely packed sub- 20 nm GNR arrays, compared to conventional fabrication process.

Photo-induced refractive index and topographical surface gratings in functionalized nanocarbon solid film

We demonstrate that a single-walled carbon nanotube network noncovalently coupled with a pyrene-modified azo-benzene chromophore functions as a host matrix for a broad range of photo-orientation and photomechanical effects. The chromophore could be efficiently reoriented through repeated trans-cis-trans isomerization under linearly polarized 480 nm light, with Δn of 0.012 at 650 nm and fast characteristic rise-times of 0.12 s. Erasable phase diffraction gratings could also be written, with permanent surface relief gratings forming at sufficiently long irradiation times. In addition to demonstrating a mechanism for photo-manipulation of single-walled carbon nanotubes, these results show photo-orientation of chromophores in azo-functionalized single-walled carbon nanotube networks as a path towards the photosensitive tuning of the electrostatic environment of the nanotube.

Growth and characterization of horizontal GaN wires on silicon

We report the growth of in-plane GaN wires on silicon by metalorganic chemical vapor deposition. Triangular-shaped GaN microwires with semi-polar sidewalls are observed to grow on top of a GaN/Si template patterned with nano-porous SiO2. With a length-to-thickness ratio ∼200, the GaN wires are well aligned along the three equivalent 〈112¯0〉 directions. Micro-Raman measurements indicate negligible stress and a low defect density inside the wires. Stacking faults were found to be the only defect type in the GaN wire by cross-sectional transmission electron microscopy. The GaN wires exhibited high conductivity, and the resistivity was 20–30 mΩ cm, regardless of the wire thickness. With proper heterostructure and doping design, these highly aligned GaN wires are promising for photonic and electronic applications monolithically integrated on silicon.