Suk Gyu Hahm

Programmable bipolar and unipolar nonvolatile memory devices based on poly(2-(N-carbazolyl)ethyl methacrylate) end-capped with fullerene.

A novel polymer, poly(2-(N -carbazolyl)ethyl methacrylate) end-capped with fullerene (PCzMA-C(60) ), has been synthesized via living anionic polymerization. Electrically programmable flash memory devices were easily fabricated with this polymer by using solution coating and metal deposition. This polymer was found in these devices to exhibit bipolar and unipolar switching behaviors with a high ON/OFF current ratio, a long retention time, high reliability, and low power consumption. The excellent properties and easy processability of this polymer open up the possibility of the mass production of high performance nonvolatile memory devices at low cost.

Electrically bistable nonvolatile switching devices fabricated with a high performance polyimide bearing diphenylcarbamyl moieties

In this study, novel nonvolatile memory devices, based on a high performance polyimide, poly(3,3′-bis(diphenylcarbamyloxy)-4,4′-biphenylene hexafluoroisopropylidenediphthalimide) (6F-HAB-DPC PI), were fabricated with a simple conventional solution coating process. The devices were found to exhibit programmable, rewritable nonvolatile memory characteristics with a high ON/OFF current ratio of up to 109, a long retention time in both ON and OFF states, and low power consumption. Moreover, the active 6F-HAB-DPC PI layer is thermally and dimensionally stable and thus hybridization with a complementary metal-oxide-semiconductor platform is feasible. The advantageous properties and ease of fabrication of the 6F-HAB-DPC PI based devices open up the possibility of the mass production of high performance digital nonvolatile polymer memory devices at low cost.

Programmable digital nonvolatile memory behaviors of donor-acceptor polyimides bearing triphenylamine derivatives: Effects of substituents

Two aromatic polyimides bearing triphenylamine (TPA) derivatives with reasonably high molecular weights were synthesized: poly(N-(2,4,6-trimethylphenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-Me3 PI) and poly(N-(4-dimethylaminophenyl)-N,N-4,4′-diphenylene hexafluoroisopropylidenediphthalimide) (6F-TPA-NMe2 PI). These polymers were thermally and dimensionally very stable, providing high-quality nanoscale thin films using a conventional solution coating process. The film densities, optical properties, and electrochemical properties were determined. The polymers displayed a different nonvolatile memory behavior that depended on the substituents of the TPA unit. The 6F-TPA-Me3 PI film showed a unipolar write-once-read-many-times (WORM) memory behavior, whereas the 6F-TPA-NMe2 PI film revealed unipolar and bipolar switching memory behavior. All PI films displayed excellent retention in both the OFF- and ON-states, even under ambient conditions. The ON/OFF current ratio was high, up to 108–109. All memory behaviors were governed by a mechanism that involved trap-limited space charge limited conduction and local filament formation. The memory characteristics may originate from the electron-donating TPA and substituents and from the electron-accepting hexafluoroisopropylidenyl and imide units in the polymer backbone, which acted as effective charge-trapping sites. The film density was found to significantly influence the memory behavior. This study demonstrated that the thermally and dimensionally stable 6F-TPA-Me3 and 6F-TPA-NMe2 PIs are suitable active materials for the low-cost mass production of high-performance programmable memory devices that can be operated with very low power consumption. Moreover, the memory mode and its polarity may be tuned by changing the substituent on the TPA unit.

Rubbed Thin Films of Well-defined Brush Polyimides for Flat-Panel Liquid Crystal Displays: Surface Morphology, Molecular Orientation, and Liquid Crystal Alignability

A series of well-defined brush polyimide (PI) composed of two 4-n-alkyloxyphenyloxy bristles per repeat unit on a semi-rigid poly(4,4′-methylenyldiphenylene pyromellitimide), Cm-PMDA-MDA PIs, were synthesized and their nanoscale thin films prepared by conventional spin-coating of their soluble poly(amic acid) precursor solutions and subsequent drying and thermal imidization in a nitrogen atmosphere. All the PIs were determined to be a positively birefringent polymer. The surface morphology and molecular orientation of each PI in films before and after rubbing were investigated in detail by atomic force microscopy, optical retardation analysis, and linearly polarized infrared spectroscopy. The sequence of the rubbing-induced polymer segmental orientations was further investigated in detail. In addition, the liquid crystal alignment and pretilt ability of the rubbed PI films were examined, and their thermal stability investigated. The present study provides important information on the sequence of the polymer segmental orientations induced by rubbing and additionally the mechanisms of the alignment and pretilt of liquid crystal molecules in contact with the rubbed PI film surface.

A Lattice-Strained Organic Single-Crystal Nanowire Array Fabricated via Solution-Phase Nanograting-Assisted Pattern Transfer for Use in High-Mobility Organic Field-Effect Transistors.

A 50 nm-wide 6,13-bis(triisopropylsilylethynyl) pentacene nanowire (NW) array is fabricated on a centimeter-sized substrate via a facile nanograting-assisted pattern-transfer method. NW growth under a nanoconfined space adopts a lattice-strained packing motif of the NWs for strong intermolecular electronic coupling, and thus a NW-based organic field-effect transistor shows high field-effect mobility up to 9.71 cm(2) V(-1) s(-1) .

The Alignment of Liquid Crystals on the Film Surfaces of Soluble Aromatic Polyimides Bearing t-Butylphenyl and Trimethylsilylphenyl Side Groups

With the study goal of firstly elucidating the anisotropic interactions between oriented polymer chain segments and liquid crystal (LC) molecules, and secondly of determining the contributions of the chemical components of the polymer segments to the film surface topography, LC alignment, pretilt, and anchoring energy, we synthesized three dianhydrides, 1,4-bis(4′-t-butylphenyl)pyromellitic dianhydride (BBPD), 1,4-bis(4′-trimethylsilylphenyl)pyromellitic dianhydride (BTPD), and 2,2′-bis(4″-tert-butylphenyl)-4,4′,5,5′-biphenyltetracarboxylic dianhydride (BBBPAn), and a series of their organosoluble polyimides, BBPD-ODA, BBPD-MDA, BBPD-FDA, BTPD-FDA, and BBBPAn-FDA, which contain the diamines 4,4′-oxydianiline (ODA), 4,4′-methylenediamine (MDA), and 4,4′-(hexafluoroisopropylidene) dianiline (FDA). All the polyimides were determined to be positive birefringent polymers, regardless of the chemical components. Although all the rubbed polyimide films exhibited microgrooves which were created by rubbing process, the film surface topography varied depending on the polyimides. In all the rubbed films, the polymer chains were unidirectionally oriented along the rubbing direction. However, the degree of in-plane birefringence in the rubbed film varied depending on the polyimides. The rubbing-aligned polymer chains in the polyimide films effectively induced the alignment of nematic LCs along their orientation directors by anisotropic interactions between the preferentially oriented polymer chain segments and the LCs. The azimuthal and polar anchoring energies of the LCs ranged from 0.45×10−4 – 1.37×10−4 J/m2 and from 0.86×10−5 – 4.26×10−5 J/m2, respectively, depending on the polyimides. The pretilt angles of the LCs were in the range 0.10–0.62°. In summary, the soluble aromatic polyimides reported here are promising LC alignment layer candidates for the production of advanced LC display devices.

Nanowires: A Lattice-Strained Organic Single-Crystal Nanowire Array Fabricated via Solution-Phase Nanograting-Assisted Pattern Transfer for Use in High-Mobility Organic Field-Effect Transistors (Adv. Mater. 16/2016)

S. H. Kim, S. G. Hahm, C. E. Park, and co-workers fabricate a 50 nm-wide organic single-crystalline nanowire array on a centimeter-sized substrate via a facile roll-to-plate process, as described on page 3209. Nanowire growth in a nano-confined space adopts a lattice-strained and single-crystalline packing motif, which can be harnessed for strong intermolecular electronic coupling. Thus, nanowire-based field-effect transistors show extremely high field-effect mobilities up to 9.71 cm(2) V(-1) s(-1) .