Development of new technology sometimes requires the revision of previous attempts.
When molecular rectifiers were first proposed by Aviram and Ratner
in 1974, and when the possibilities of molecular electronic devices were discussed
in 1980s, it was very difficult to
characterize the structures and functions on a molecular level.
But, time has changed !Scanning probe techniques such as scanning tunneling microscopy (STM) and atomic force microscopy (AFM) have enabled us to observe and manipulate single molecules, and the concept of molecular devices now attracts many researchers.
For fabricating the molecular devices, we must control the molecular position precisely and also the orientation. Several groups have succeeded in the direct positioning of individual molecules using scanning probe techniques, but this takes time and it is difficult to control the molecular orientation. By patterning self-assembled monolayers (SAMs), we can manipulate the position of molecules whose orientation is already ordered. SAMs are organic molecular films covering solid surfaces spontaneously with well-ordered and oriented molecules, exposing specific functional groups outside which modify bulk surface properties. There are many possible applications for SAMs. For example, in biochemical fields, SAMs can be designed to bind specific proteins so that they immobilize molecules on the substrates.
Patterning SAM films is of great interest from the viewpoint of a high-resolution monolayer resist. For example, a very promising microcontact printing method was developed by Whitesides and coworkers, with which the linewidth can be reduced to as small as 100 nm. Piner et al. demonstrated that the AFM probe can be used as a `dip-pen' to draw 30-nm-wide lines with SAM molecules. We are searching for a way to control the phase separation of binary SAM for producing a lateral pattern of less than 100 nm. To create smaller structures down to 10 nm, these patterned SAMs must be trimmed further by other techniques such as scanning probe lithography. For patterning SAMs, we studied the phase separation of binary component monolayers, molecular extraction, and growth control on patterned substrates.
SAMs can be used not only as the resist, but also as components of electronic devices. Garnier et al. developed field-effect transistor (FET) composed of organic semiconductors (thiophen oligomers) patterned by a printing technique. Photoemitting devices have also been fabricated with organic materials. Since they used multilayer organic films, the dimensions of the devices are large and the voltage required for the function is high (30-40 V). Monolayer film of organic semiconductors could reduce the operating voltages and the size of the devices. For this type of application, conductive SAM films are a good candidate. Therefore, we prepared conjugated molecules and measured their in-plane conductivities.
Another application of SAMs is an interface material tailoring the work function of metal electrodes for efficient carrier injection to electronic devices. Campbell and coworkers and Zehner et al. demonstrated that the Schottky energy barrier of Ag and Cu on organic materials can be tuned by covering the metals with SAM films. We applied the STM technique to characterize the local variation of barrier height of SAM films with a molecular resolution.
