We report on fabrication of novel field-effect transistors (FETs) based on transition metal dichalcogenides. The unique structure of single crystals of these layered inorganic semiconductors enables fabrication of FETs with intrinsically low field-effect threshold and high charge carrier mobility, comparable to that in the best single-crystal Si FETs (up to 500 cm 2 /Vs for the p-type conductivity in the WSe 2 -based FETs at room temperature). These novel FETs demonstrate ambipolar operation. Owing to mechanical flexibility, they hold potential for applications in "flexible" electronics.In modern electronics, the requirements to field-effect transistors are stringent and often contradictory: e.g., for many applications, a combination of high charge carrier mobility (µ) and mechanical flexibility is desirable. Neither of the developed FETs satisfies these requirements. For example, the process of fabrication of silicon FETs with a relatively high µ ≤ 500 cm 2 /Vs [1,2] is incompatible with flexible substrates. The organic-based FETs, that provide basis for flexible electronics [3,4,5], are notoriously known for their low µ. Although several

The substitution of chloro or bromo groups in tetracene gives rise to the change of crystal structure, having a substantial effect on carrier transport. Halogenated tetracene derivatives were synthesized and grown into single crystals. Monosubstituted 5-bromo- and 5-chlorotetracenes have the herringbone-type structure, while 5,11-dichlorotetracene has the slipped pi stacking structure. Mobility of 5,11-dichlorotetracene was measured to be as high as 1.6 cm2/V.s in single-crystal transistors. The pi stacking structure, which enhances pi orbital overlap and facilitates carrier transport, may thus be responsible for this high mobility.

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Rubrene single crystals have been grown by a vapor-phase process. Two additional compounds that contaminate rubrene have been identified and their structures determined. Single crystals of rubrene show excellent crystallinity and very small rocking curve width. Field effect transistors based on pure rubrene single crystals with colloidal graphite electrodes and Parylene as a dielectric demonstrate a maximal mobility of 13 cm 2 /Vs with strong anisotropy. The mobility increases very slightly with cooling, but decreases significantly at low temperatures.

This study explores the assembly in the crystalline state of a class of pentacenes that are substituted along their long edges with aromatic rings forming rigid, cruciform molecules. The crystals were grown from the gas phase, and their structures were compared with DFT-optimized geometries. Both crystallographic and computed structures show that a planar acene core is the exception rather than the rule. In the assembly of these molecules, the phenyl groups block the herringbone motif and further guide the arrangement of the acene core into higher order structures. The packing for the phenyl-substituted derivatives is dictated by close contacts between the C-H's of the pendant aromatic rings and the carbons at the fusions in the acene backbone. Using thiophene substituents instead of phenyls creates cofacially stacked acenes. In thin films, the thiophene-substituted derivative forms devices with good electrical properties: relatively high mobility, high ON/OFF ratios, and low threshold voltage for device activation. An unusual result is obtained for the decaphenyl pentacene when devices are fabricated on its crystalline surface. Although its acene cores are well isolated from each other, this material still exhibits good electrical properties.

At moderate temperatures in flowing gas, pentacene undergoes a disproportionation reaction to produce 6,13-dihydropentacene (DHP) and a series of polycondensed aromatic hydrocarbons, including the previously unknown peripentacene (PP). The process requires activation by heating to 320 degrees C and is possibly catalyzed by impurities such as DHP, 6,13-pentacenequinone (PQ), Al, or Fe found in the starting materials. These impurities also result in a decrease in the intrinsic field-effect mobility (FEM) of pentacene crystals. Subsequent purifications remove such impurities, thus inhibiting the formation of the disproportionation products and increasing the FEM of pentacene (2.2 cm(2)/Vs). These results clarify the importance of purification of semiconductive materials for measurements of intrinsic mobility and optimal device performance.

Copper phthalocyanine (Cu-Pc) single crystals were grown by physical vapor transport and field effect transistors (FETs) on the surface of these crystals were prepared. These FETs function as p-channel accumulation-mode devices. Charge carrier mobilities of up to 1 cm2/Vs combined with a low field-effect threshold were obtained. These remarkable FET-characteristics, along with the highly stable chemical nature of Cu-Pc make it an attractive candidate for device applications

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