Tuning polymorphism and orientation in organic semiconductor thin films via post-deposition processing.

Though both the crystal structure and molecular orientation of organic semiconductors are known to impact charge transport in thin-film devices, separately accessing different polymorphs and varying the out-of-plane molecular orientation is challenging, typically requiring stringent control over film deposition conditions, film thickness, and substrate chemistry. Here we demonstrate independent tuning of the crystalline polymorph and molecular orientation in thin films of contorted hexabenzocoronene, c-HBC, during post-deposition processing without the need to adjust deposition conditions. Three polymorphs are observed, two of which have not been previously reported. Using our ability to independently tune the crystal structure and out-of-plane molecular orientation in thin films of c-HBC, we have decoupled and evaluated the effects that molecular packing and orientation have on device performance in thin-film transistors (TFTs). In the case of TFTs comprising c-HBC, polymorphism and molecular orientation are equally important; independently changing either one affects the field-effect mobility by an order of magnitude.

Channel length scaling in graphene field-effect transistors studied with pulsed current-voltage measurements.

We investigate current saturation at short channel lengths in graphene field-effect transistors (GFETs). Saturation is necessary to achieve low-output conductance required for device power gain. Dual-channel pulsed current-voltage measurements are performed to eliminate the significant effects of trapped charge in the gate dielectric, a problem common to all oxide-based dielectric films on graphene. With pulsed measurements, graphene transistors with channel lengths as small as 130 nm achieve output conductance as low as 0.3 mS/µm in saturation. The transconductance of the devices is independent of channel length, consistent with a velocity saturation model of high-field transport. Saturation velocities have a density dependence consistent with diffusive transport limited by optical phonon emission.

Synthesis and Characterization of a Pyromellitic Diimide-Based Polymer with C- and N-Main Chain links: Matrix for Solution-Processable n-Channel Field-effect Transistors.

A highly soluble pyromellitic diimide-based polymer was obtained through imidization polymerization. The novel architecture features diimide subunits linked alternately at 3-6 and N,N' positions. The polymer is highly transparent in the near ultraviolet-visible regions. Smooth and uniform thin-films were obtained through spin-coating even after blending the polymer with PCBM in 1:9 polymer:PCBM weight ratio. While the polymer itself has modest electron mobility in typical bottom-gate top-contact OFETs, an electron mobility of 3 × 10(-3) cm(2)V(-1)s(-1) was achieved for the blend, which increased to 10(-2) cm(2)V(-1)s(-1) on exposure to propylamine. Thus, polyimides are demonstrated as promising binder materials for solution-processible n-channel semiconductor blends, of which very few examples are known.

Digital Inverter Amine Sensing via Synergistic Responses by n and p Organic Semiconductors.

Chemiresistors and sensitive OFETs have been substantially developed as cheap, scalable, and versatile sensing platforms. While new materials are expanding OFET sensing capabilities, the device architectures have changed little. Here we report higher order logic circuits utilizing OFETs sensitive to amine vapors. The circuits depend on the synergistic responses of paired p- and n-channel organic semiconductors, including an unprecedented analyte-induced current increase by the n-channel semiconductor. This represents the first step towards 'intelligent sensors' that utilize analog signal changes in sensitive OFETs to produce direct digital readouts suitable for further logic operations.

Expeditious synthesis of contorted hexabenzocoronenes.

Contorted hexabenzocoronenes (HBCs) have been synthesized in an expedited manner utilizing a double Barton-Kellogg olefination reaction and a subsequent Scholl cyclization. The scope of both transformations was investigated using a series of pentacene quinones and double olefin precursors. The utility of these reactions to help create functionalized and oligomeric HBCs in a rapid manner is demonstrated.

Transferring self-assembled, nanoscale cables into electrical devices.

This study details a new derivative of the contorted HBCs that self-organizes into one-dimensional, single-crystalline fibers. X-ray diffraction, transmission electron microscopy, and electron diffraction studies show that they have an orthorhombic unit cell with dimensions of 5.8 nm x 4.5 nm x 0.45 nm. Each fiber is composed of a few thousands columns. A method is put forth that utilizes elastomer stamps to manipulate and position isolated fibers in organic field effect transistors.