Ajay Virkar

Crystalline Ultrasmooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors

Crystalline self-assembled monolayers (SAMs) of organosilane compounds such as octadecyltrimethoxysilane (OTMS) and octadecyltrichlorosilane (OTCS) were deposited by a simple, spin-casting technique onto Si/SiO(2) substrates. Fabrication of the OTMS SAMs and characterization using ellipsometry, contact angle, atomic force microscopy (AFM), grazing angle attenuated total reflectance Fourier transform infrared (GATR-FTIR) spectroscopy and grazing incidence X-ray diffraction (GIXD) are described. The characterization confirms that these monolayers exhibit a well-packed crystalline phase and a remarkably high degree of smoothness. Semiconductors deposited by vapor deposition onto the crystalline OTS SAM grow in a favorable two-dimensional layered growth manner which is generally preferred morphologically for high charge carrier transport. On the OTMS SAM treated dielectric, pentacene OFETs showed hole mobilities as high as 3.0 cm(2)/V x s, while electron mobilities as high as 5.3 cm(2)/V x s were demonstrated for C(60).

Organic Semiconductor Growth and Morphology Considerations for Organic Thin‐Film Transistors

Analogous to conventional inorganic semiconductors, the performance of organic semiconductors is directly related to their molecular packing, crystallinity, growth mode, and purity. In order to achieve the best possible performance, it is critical to understand how organic semiconductors nucleate and grow. Clever use of surface and dielectric modification chemistry can allow one to control the growth and morphology, which greatly influence the electrical properties of the organic transistor. In this Review, the nucleation and growth of organic semiconductors on dielectric surfaces is addressed. The first part of the Review concentrates on small-molecule organic semiconductors. The role of deposition conditions on film formation is described. The modification of the dielectric interface using polymers or self-assembled mono-layers and their effect on organic-semiconductor growth and performance is also discussed. The goal of this Review is primarily to discuss the thin-film formation of organic semiconducting species. The patterning of single crystals is discussed, while their nucleation and growth has been described elsewhere (see the Review by Liu et. al).([¹]) The second part of the Review focuses on polymeric semiconductors. The dependence of physico-chemical properties, such as chain length (i.e., molecular weight) of the constituting macromolecule, and the influence of small molecular species on, e.g., melting temperature, as well as routes to induce order in such macromolecules, are described.

Energetics and stability of pentacene thin films on amorphous and crystalline octadecylsilane modified surfaces

Charge transport in organic thin film transistors (OTFTs) is directly related to the morphology and growth of the organic semiconductor at the dielectric interface. The most commonly used dielectric interface in OTFT research is alkylsilane-modified silicon oxide (SiO2). In this report, the nucleation, energetics, and stability of pentacene thin films on methyl-terminated surfaces are discussed. The density of the terminal methyl group was found to be an important parameter for controlling the growth of organic semiconductors. Pentacene growth is two-dimensional (2D) on SiO2 dielectrics modified with a crystalline, densely packed octadecylsilane (OTS) monolayer. However, it is primarily three-dimensional (3D) on SiO2 dielectrics modified with an amorphous OTS layer. Beyond a critical OTS density, the interaction between the OTS and pentacene exceeds the pentacene interlayer interaction energy engendering 2D growth which is preferential for high charge carrier mobility. The nucleation density is also much higher on the crystalline OTS compared to the amorphous OTS. The sub-monolayer thin films of pentacene were found to be much more stable on the ordered OTS compared to disordered OTS. Atomic force microscopy (AFM) and Monte Carlo simulations were used to develop a thorough analysis of pentacene film growth and energetics on OTS surfaces.

Preparation of crystalline dielectric modification silane layer by spin-coating and its improvements on organic transistor performance

Self-assembled monolayers (SAMs) of alkyl silane compounds have been used for modifying gate dielectrics surface of organic field-effect transistors (OFETs) and they have frequently shown improvement of FET performances. In this paper we deposited alkyl silane SAMs by simple spin-coating technique onto Si/SiO2 substrates. Spin-cast octadecyltrimethoxysilane (OTMS) SAMs had ultra smooth crystalline surface and provided an excellent dielectric surface for OFETs. In fact on the OTMS SAM treated dielectric, pentacene OFETs showed hole mobilities over 2.0 cm2/Vs and electron mobilties over 1.0 and 5.0 cm2/Vs were demonstrated for 3,4:9,10-perylene diimide derivative and C60, respectively. Fabrication technique and characterizations of the OTMS SAMs is described.

A Crystalline alkylsilane dielectric surface-modification layer: a general strategy for high performance organic thin-film transistors

It has been well established that in organic thin film transistors (OTFTs), charge transport occurs within the first few monolayers of the semiconductor at the semiconductor/dielectric interface. Understanding and engineering the semiconductor-dielectric is therefore critical. Large discrepancies in performance, even with seemingly identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. Here, we used the Langmuir-Blodgett technique to study the effect of an octadecylsilane dielectric modification layer on OTFT performance. We found a crystalline, dense OTS monolayer promotes two-dimensional growth in a variety of organic semiconductors. Mobilities as high as 5.3 cm2/Vs and 2.2 cm2/Vs were demonstrated on crystalline OTS for C60 and pentacene, respectively. Finally, we also developed a simple, scalable spin-coating method to produce crystalline OTS. This work represents a significant step towards a general approach for morphological control of organic semiconductors which is directly linked to their thin film transistor performance.