Principal goals in organic thin‐film transistor (OTFT) gate dielectric research include achieving: (i) low gate leakage currents and good chemical/thermal stability, (ii) minimized interface trap state densities to maximize charge transport efficiency, (iii) compatibility with both p‐ and n‐ channel organic semiconductors, (iv) enhanced capacitance to lower OTFT operating voltages, and (v) efficient fabrication via solution‐phase processing methods. In this Review, we focus on a prominent class of alternative gate dielectric materials: self‐assembled monolayers (SAMs) and multilayers (SAMTs) of organic molecules having good insulating properties and large capacitance values, requisite properties for addressing these challenges. We first describe the formation and properties of SAMs on various surfaces (metals and oxides), followed by a discussion of fundamental factors governing charge transport through SAMs. The last section focuses on the roles that SAMs and SAMTs play in OTFTs, such as surface treatments, gate dielectrics, and finally as the semiconductor layer in ultra‐thin OTFTs.

A series of 0−3 metal oxide−polyolefin nanocomposites are synthesized via in situ olefin polymerization, using the following single-site metallocene catalysts: C 2-symmetric dichloro[rac-ethylenebisindenyl]zirconium(IV), Me2Si( t BuN)(η5-C5Me4)TiCl2, and (η5-C5Me5)TiCl3 immobilized on methylaluminoxane (MAO)-treated BaTiO3, ZrO2, 3-mol %-yttria-stabilized zirconia, 8-mol %-yttria-stabilized zirconia, sphere-shaped TiO2 nanoparticles, and rod-shaped TiO2 nanoparticles. The resulting composite materials are structurally characterized via X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), 13C nuclear magnetic resonance (NMR) spectroscopy, and differential scanning calorimetry (DSC). TEM analysis shows that the nanoparticles are well-dispersed in the polymer matrix, with each individual nanoparticle surrounded by polymer. Electrical measurements reveal that most of these nanocomposites have leakage current densities of ∼10−6−10−8 A/cm2; relative permittivities increase as the nanoparticle volume fraction increases, with measured values as high as 6.1. At the same volume fraction, rod-shaped TiO2 nanoparticle−isotactic polypropylene nanocomposites exhibit significantly greater permittivities than the corresponding sphere-shaped TiO2 nanoparticle−isotactic polypropylene nanocomposites. Effective medium theories fail to give a quantitative description of the capacitance behavior, but do aid substantially in interpreting the trends qualitatively. The energy storage densities of these nanocomposites are estimated to be as high as 9.4 J/cm3.

New carbonyl-functionalized quaterthiophenes, 5, 5' ''-diheptanoyl-2,2':5',2' ':5' ',2' ''-quaterthiophene (DHCO-4T), 5, 5' ''-diperfluorohexylcarbonyl-2,2':5',2' ':5' ',2' ''-quaterthiophene (DFHCO-4T), and 2,7-[bis-(5-perfluorohexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b']-dithiophen-4-one (DFHCO-4TCO) have been synthesized and characterized. Field-effect transistors fabricated with these materials exhibit high electron mobilities both in a vacuum (up to 0.6 cm2 V-1 s-1) and in air (up to 0.02 cm2 V-1 s-1) and very high Ion:Ioff currents ratios (>107). DHCO-4T is the first organic material exhibiting excellent ambipolar transport (mue/muh up to 0.1/0.01 cm2 V-1 s-1, (Ion:Ioff)e/(Ion:Ioff)h up to 107/108 for the same device) over a broad range of deposition temperatures. These materials are therefore promising for organic complementary circuits.

Aluminum oxide encapsulated high-permittivity (ε) BaTiO 3 and ZrO 2 core-shell nanoparticles having variable Al 2 O 3 shell thicknesses were prepared via a layer-by-layer methylaluminoxane coating process. Subsequent chemisorptive activation of the single-site metallocene catalyst [rac-ethylenebisindenyl]zirconium dichloride (EBIZrCl 2 ) on these Al 2 O 3 -encapsulated nanoparticles, followed by propylene addition, affords 0-3 metal oxide-isotactic polypropylene nanocomposites. Nanocomposite microstructure is analyzed by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, differential scanning calorimetry, atomic force microscopy, and Raman spectroscopy. The in situ polymerization process yields homogeneously dispersed nanoparticles in a polyolefin matrix. Electrical measurements indicate that as the concentration of the filler nanoparticles increases, the effective permittivity of the nanocomposites increases, affording ε values as high as 6.2. The effective permittivites of such composites can be predicted by the Maxwell-Garnett formalism using the effective medium theory for volume fractions (ν f ) of nanoparticles below 0.06. The nanocomposites have leakage current densities of ∼10 -7 -10 -9 A/cm 2 at an electric field of 10 5 V/cm, and very low dielectric loss in the frequency range 100 Hz-1 MHz. Increasing the Al 2 O 3 shell thickness dramatically suppresses the leakage current and high field dielectric loss in these nanocomposites.

The 0−3 metal oxide−isotactic polypropylene nanocomposites are synthesized via in situ propylene polymerization using the C 2-symmetric metallocene catalyst dichloro[rac-ethylenebisindenyl]zirconium(IV) (EBIZrCl2) immobilized on methylaluminoxane (MAO)-treated barium titanate (BaTiO3) or titanium dioxide (TiO2) nanoparticles. The composite materials are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and 13C nuclear magnetic resonance (NMR) spectroscopy. It is shown that the nanoparticles are homogeneously dispersed in the polyolefin matrices. Electrical measurements reveal nanocomposite leakage current densities of ∼10-6 to 10-9 A/cm2, permittivities as high as 6.1, and breakdown strengths of ∼4 MV/cm. Energy densities are estimated to be as high as 9.4 J/cm3.

We report here on the systematic investigation of photoinduced intramolecular electron transfer (IET) in a series of donor-bridge-acceptor molecules as a means of understanding electron transport through the bridge. Perylenebisimide chromophores connected by various oligophenylene bridges are studied because their electron-transfer behavior can readily be monitored by following changes in the fluorescence intensity. We find dramatic switching of the IET behavior as the solvent polarity (dielectric constant) is increased. By combining steady-state and time-resolved fluorescence spectroscopy in a variety of solvents at multiple temperatures with standard theories of electron transfer, we determine parameters governing the IET behavior of these dimers, such as the electronic coupling through the bridges. We also deploy available ab initio quantum chemical methods to calculate the through-space component of the electronic coupling matrix element. Single-molecule investigations of the electron-transfer behavior also show that IET can be switched reversibly by a similar mechanism in an isolated individual molecule.

The generalizable synthesis, comparative molecular physicochemical properties, film microstructures/morphologies, and field-effect transistor (FET) response characteristics of a series of six carbonyl-derivatized quaterthiophenes is described. These compounds are as follows:  5, 5‘ ‘‘-diheptanoyl-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (1), spiro[4H-cyclopenta[2,1-b:3,4-b‘]dithiophene-4,2‘-[1,3] dioxolane], 2,6-bis-(5-hexyl carbonylthien-2-yl) (2), 2,7-[bis-(5-hexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b‘]-dithiophen-4-one (3), 5, 5‘ ‘‘-diperfluorohexylcarbonyl-2,2‘:5‘,2‘ ‘:5‘ ‘,2‘ ‘‘-quaterthiophene (4), spiro[4H-cyclopenta[2,1-b:3,4-b‘]dithiophene-4,2‘- [1,3]dioxolane], 2,6-bis-(5-perfluorohexylcarbonylthien-2-yl) (5), and 2,7-[bis-(5-perfluorohexylcarbonylthien-2-yl)]-4H-cyclopenta[2,1-b:3,4-b‘]-dithiophen-4-one (6). Optical and electrochemical data demonstrate that terminal/central carbonyl-functionalization of the quaterthiophene core strongly lowers both HOMO and LUMO energies. However, the extent of LUMO lowering is far greater than HOMO lowering with the outcome that the carbonyl-containing quaterthiophenes exhibit lower energy gaps than the corresponding parent systems. This greater LUMO stabilization is confirmed by electrochemical data and fully explained by DFT computations. OTFT measurements show that all of the six semiconductors are FET-active, and very large n-type (up to 0.32 cm2/Vs), p-type (up to 0.04 cm2/Vs), and ambipolar (up to 0.12 cm2/Vs for electrons, 0.008 cm2/Vs for holes) mobilites are observed depending on the exact quaterthiophene backbone architecture. A simple Schottky injection barrier model in combination with molecular packing and thin-film molecular orientation/morphology characteristics of 1−6 explain the observed OFET performance trends. Finally, FET majority charge carrier inversion (p-type → n-type) via in situ chemical deprotection of the central carbonyl functionality (5 and 6) is demonstrated for the first time and is attractive for sensor functions as well as for patterning complementary circuits. The latter is demonstrated in a simple contact patterning process.

Developing alternative high dielectric constant (k) materials for use as gate dielectrics is essential for continued advances in conventional inorganic CMOS and organic thin film transistors (OTFTs). Thicker films of high-k materials suppress tunneling leakage currents while providing effective capacitances comparable to those of thin films of lower-k materials. Self-assembled monolayers (SAMs) and multilayers offer attractive options for alternative OTFT gate dielectrics. One class of materials, organosilane-based self-assembled nanodielectrics (SANDs), has been shown to form robust films with excellent insulating and surface passivation properties, enhancing both organic and inorganic TFT performance and lowering device operating voltages. Since gate leakage current through the dielectric is one factor limiting continued TFT performance improvements, we investigate here the current (voltage, temperature) (I (V,T)) transport characteristics of SAND types II (pi-conjugated layer) and III (sigma-saturated + pi-conjugated layers) in Si/native SiO(2)/SAND/Au metal-insulator-metal (MIS) devices over the temperature range -60 to +100 degrees C. It is found that the location of the pi-conjugated layer with respect to the Si/SiO(2) substrate surface in combination with a saturated alkylsilane tunneling barrier is crucial in controlling the overall leakage current through the various SAND structures. For small applied voltages, hopping transport dominates at all temperatures for the pi-conjugated system (type II). However, for type III SANDs, the sigma- and pi-monolayers dominate the transport in two different transport regimes: hopping between +25 degrees C and +100 degrees C, and an apparent switch to tunneling for temperatures below 25 degrees C. The sigma-saturated alkylsilane tunneling barrier functions to reduce type III current leakage by blocking injected electrons, and by enabling bulk-dominated (Poole-Frenkel) transport vs electrode-dominated (Schottky) transport in type II SANDs. These observations provide insights for designing next-generation self-assembled gate dielectrics, since the bulk-dominated transport resulting from combining sigma- and pi-layers should enable realization of gate dielectrics with further enhanced performance.