We have introduced a simple method for the construction of complex 2D molecular structures using UV-photopatterned self-assembled monolayers (SAMs) and selective surface reactions. This approach integrates recent, previously incompatible, developments in nanoscience, organic/molecular electronics and biotechnology, and serves as the foundation for our synthetic efforts. It has significant advantages over other methods: it is extensible to many different types of materials, easily scaled up, and affords precise nanoscale placement.
The basic process has three steps. First, an image in one SAM is created using UV light shone through a mask (UV-photopatterning). Second, a SAM of different composition is adsorbed in the areas where the first SAM has been removed. Finally, physical vapor deposition (PVD) or chemical vapor deposition (CVD) is employed to deposit metal atoms on the patterned SAM surface. By using SAMs with different terminal groups, we can control where and how metals deposit on the construct. Figure 1 displays two examples of selective depositions on patterned SAM surfaces. In Figure 1a, PVD was employed to deposit magnesium on a patterned -COOH/ -CH3 terminated SAM surface, while in Figure 1b, CVD was used to deposit alumina on a patterned -COOH/-CH3 terminated SAM surface.
UV photolithography is already widely used in the production of microelectronics, and many proposed molecular electronic devices consist of functionalized SAMs between metal contacts. Our method is therefore compatible with state-of-the-art fabrication technology and provides a way to incorporate molecular electronic devices into conventional microelectronics forthe production of “hybrid” circuits.
Figure 1: (a) Optical image and negative ion mass spectrum image (area: 100x100 µm2) of a -COOH/-CH3 patterned SAM after deposition of 80 Å Mg. Deposited Mg reacts with -COOH groups to form a metal-organic complex, indicated by the appearance of MgOH- ions. Mg penetrates through the -CH3 terminated SAM to the Au/S interface, as indicated by the formation of AuMgS- ions. Mass spectrum image: primary ion Au+; kinetic energy 25 keV. (b) Optical image and positive ion mass spectrum image (area 500x500 µm2) of a -COOH/-CH3 patterned SAM after exposure to trimethylaluminum (TMA) vapor for 3 minutes. TMA reacts with the -COOH terminated SAM and deposits an alumina overlayer as indicated by the formation of Al+ ions. No reaction is observed with the -CH3 terminated SAM. Mass spectrum image: primary ion Bi+; kinetic energy 25 keV.
The effectiveness of this patterning strategy requires that we have a detailed understanding of the formation of metallic contacts to organic thin films. In our group we investigate the formation of metallic contacts using physical vapor deposition (PVD), chemical vapor deposition (CVD) and electroless deposition.