Complementary helicity interchange of optically switchable supramolecular-enantiomeric helicenes with (-)-gel-sol-(+)-gel transition ternary logic.

A gallamide-containing pseudoenantiomeric helicene pair bearing a (10R,11R)-dimethoxymethyldibenzosuberane core can self-assemble by intermolecular amide H-bonding and π-π stacking into bundled helical fibers with helical tunnels of complementary helicity in CH2Cl2. The helicenes undergo excellent complementary photoswitchings of ternary logic at 280, 318, and 343 nm through (-)-gel-sol-(+)-gel interconversion.

Dielectrophoretic placement of quasi-zero-, one-, and two-dimensional nanomaterials into nanogap for electrical characterizations.

DEP is one of promising techniques for positioning nanomaterials into the desirable location for nanoelectronic applications. In contrast, the lithography technique is commonly used to make ultra-thin conducting wires and narrow gaps but, due to the limit of patterning resolution, it is not feasible to make electrical contacts on ultra-small nanomaterials for a bottom-up device fabrication. Thus, integrating the lithography and dielectrophoresis, a real bottom-up fabrication can be achieved. In this work, the device with the nanogap in between two nanofinger-electrodes is made using electron-beam lithography from top down and the ultra-small nanomaterials, such as colloidal PbSe quantum dots, polyaniline nanofibers, and reduced-graphene-oxide flakes, are placed in the nanogap by DEP from bottom up. The threshold electric field for the DEP placement of PbSe nanocrystals was roughly estimated to be about 8.3 × 10(4) V/cm under our experimental configuration. After the DEP process, several procedures for reducing contact resistances are attempted and measurements of intrinsic electron transport in versatile nanomaterials are performed. It is experimentally confirmed that electron transport in both PbSe nanocrystal arrays and polyaniline nanofibers agrees well with Prof. Ping Sheng's model of granular metallic conduction. In addition, electron transport in reduced-graphene-oxide flakes follows Mott's 2D variable-range-hopping model. This study illustrates an integration of the electron-beam lithography and the DEP techniques for a precise manipulation of nanomaterials into electronic circuits for characterization of intrinsic properties.

Nano approach investigation of the conduction mechanism in polyaniline nanofibers.

A nanotechnological approach is applied to measurements of the electric field dependence of resistance under a high electric field while in low voltage. With this technique, the conduction mechanism on a mesoscopic scale is explored in a single, nonagglomerated nanofiber. Polyaniline nanofibers are prepared by vigorous mixing of aniline and oxidation agent ammonium persulfate in acid solution. They exhibit a uniform nanoscale morphology rather than agglomeration as that produced via conventional chemical oxidation. The as-synthesized polyaniline nanofibers are doped (dedoped) with a HCl acid (NH(3) base), and their temperature behaviors of resistances follow an exponential function with an exponent of T(-1/2). To measure the conduction mechanism in a single nanofiber, the dielectrophoresis technique is implemented to position nanofibers on top of two electrodes with a nanogap of 100-600 nm, patterned by electron-beam lithography. After the devices are irradiated by electron beam to reduce contact resistances, their temperature behaviors and electric field dependences are unveiled. The experimental results agree well with the theoretical model of charging energy limited tunneling. Other theoretical models such as Efros-Shklovskii and Mott's one-dimensional hopping conduction are excluded after comparisons and arguments. Through fitting, the size of the conductive grain, separation distance between two grains, and charging energy per grain in a single polyaniline nanofiber are estimated to be about 4.9 nm, 2.8 nm, and 78 meV, respectively. The nanotechnological approach, where the nanogap and the dielectrophoresis technique are used for single nanofiber device fabrication, is applied for determination of mesoscopic charge transport in a polyaniline conducting polymer.

Modulation of photoswitching profiles by 10,11-dialkoxymethyl substituents in c(2)-symmetric dibenzosuberane-based helicenes.

A series of C(2)-symmetric, 10,11-disubstituted dibenzosuberane (DBS)-based helicenes 6 a-c with a common 7-bromo-α-tetralin-based bottom fragment were synthesized. Their absolute stereochemistry was determined to be 10R,11R,P after reductive desulfurization of the corresponding (10R,11R,1'S)-episulfides with complete stereospecificity. Photoisomerization of the diastereomerically pure (P)-6 c in hexane led to virtually exclusive formation of the opposite M-form diastereomer (P/M', <1:>99) at 290 nm. The preferential return of (M')-6 c to (P)-6 c was also achieved with high selectivity (P/M', 90:10) at 330 nm. Molecular simulations of (P)-6 c and (M')-6 c with both DBS conformations suggest that the selectivities of photoswitching are controlled by the conformation of the top DBS template as evidenced by their (1)H NMR spectra. Doping 6 c into a nematic liquid crystal (E7) led to a cholesteric mesophase with modulated pitches, reversible helical senses, and with a switch memory of ternary logic.