The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals1,2 whose dimensions are much smaller than those that can be realized using lithographic techniques3,4,5,6. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties7,8,9. Preliminary studies of both the electrical10,11,12,13 and optical properties14,15,16 of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots3,4,5,6 and small metallic grains17.

A single-electron transistor made from a cadmium selenide nanocrystal

In this contribution we review the motivations for, and recent advances in, new gate dielectric materials for incorporation into organic thin-film transistors (OTFTs) for organic electronics. After a general introduction to OTFT materials, operating principles, and processing requirements for optimizing low-cost organic electronics, this review focuses on three classes of OTFT-compatible dielectrics: i) inorganic (high-k) materials; ii) polymeric materials; and iii) self-assembled mono- and/multilayer materials. The principal goals in this active research area are tunable and reduced OTFT operating voltages, leading to decreased device power consumption while providing excellent dielectric/insulator properties and efficient low-cost solution-phase processing characteristics.

Gate Dielectrics for Organic Field‐Effect Transistors: New Opportunities for Organic Electronics

The techniques of colloidal chemistry permit the routine creation of semiconductor nanocrystals, whose dimensions are much smaller than those that can be realized using lithographic techniques. The sizes of such nanocrystals can be varied systematically to study quantum size effects or to make novel electronic or optical materials with tailored properties. Preliminary studies of both the electrical and optical properties of individual nanocrystals have been performed recently. These studies show clearly that a single excess charge on a nanocrystal can markedly influence its properties. Here we present measurements of electrical transport in a single-electron transistor made from a colloidal nanocrystal of cadmium selenide. This device structure enables the number of charge carriers on the nanocrystal to be tuned directly, and so permits the measurement of the energy required for adding successive charge carriers. Such measurements are invaluable in understanding the energy-level spectra of small electronic systems, as has been shown by similar studies of lithographically patterned quantum dots and small metallic grains.

The field of organic electronics has been developed vastly in the past two decades due to its promise for low cost, lightweight, mechanical flexibility, versatility of chemical design and synthesis, and ease of processing. The performance and lifetime of these devices, such as organic light-emitting diodes (OLEDs), photovoltaics (OPVs), and field-effect transistors (OFETs), are critically dependent on the properties of both active materials and their interfaces. Interfacial properties can be controlled ranging from simple wettability or adhesion between different materials to direct modifications of the electronic structure of the materials. In this Feature Article, the strategies of utilizing surfactant-modified cathodes, hole-transporting buffer layers, and self-assembled monolayer (SAM)-modified anodes are highlighted. In addition to enabling the production of high-efficiency OLEDs, control of interfaces in both conventional and inverted polymer solar cells is shown to enhance their efficiency and stability; and the tailoring of source–drain electrode–semiconductor interfaces, dielectric–semiconductor interfaces, and ultrathin dielectrics is shown to allow for high-performance OFETs.

Interface Engineering for Organic Electronics

Organic thin film transistors (TFTs) are of interest for a variety of large-area electronic applications, such as displays, sensors and electronic barcodes. One of the key problems with existing organic TFTs is their large operating voltage, which often exceeds 20 V. This is due to poor capacitive coupling through relatively thick gate dielectric layers: these dielectrics are usually either inorganic oxides or nitrides, or insulating polymers, and are often thicker than 100 nm to minimize gate leakage currents. Here we demonstrate a manufacturing process for TFTs with a 2.5-nm-thick molecular self-assembled monolayer (SAM) gate dielectric and a high-mobility organic semiconductor (pentacene). These TFTs operate with supply voltages of less than 2 V, yet have gate currents that are lower than those of advanced silicon field-effect transistors with SiO2 dielectrics. These results should therefore increase the prospects of using organic TFTs in low-power applications (such as portable devices). Moreover, molecular SAMs may even be of interest for advanced silicon transistors where the continued reduction in dielectric thickness leads to ever greater gate leakage and power dissipation.

Low-voltage organic transistors with an amorphous molecular gate dielectric

Self-assembled monolayers of long chain alkanes deposited on silicon wafers using an optimally designed procedure exhibit very large energy barriers ( $\ensuremath{\sim}45$ eV) to carrier tunneling The dc conductivity is found to be $\ensuremath{\sim}46\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ S/cm, close to that of bulk polyethylene, and independent of monolayer thickness This demonstrates that the tunneling contribution to the overall conductivity can be made negligible when the organic structure is densely packed and highly ordered Distinctly higher conductivity values are observed with disordered monolayers

Suppression of charge carrier tunneling through organic self-assembled monolayers.

It is demonstrated that monolayers of organic molecules (long chain hydrocarbons) as thin as 1.9 nm, deposited by the self‐assembly technique on silicon, form high performance electrically insulating barriers. Their properties are compared with those of silicon dioxide. Leakage current densities through the organic monolayers of the order of 10−8–10−7A/cm2 have been obtained. These values are 4–5 decades lower than those for silicon dioxide of equivalent thickness. Larger tunneling barriers for organic monolayers than for silicon dioxide explain these results.

J. V. Davidovits

Papers

Suppression of charge carrier tunneling through organic self-assembled monolayers.

Organic insulating films of nanometer thicknesses

Ultra-low conductivity through insulating self-assembled organic monolayers