Organic semiconductors that function in field-effect transistors are further modified so that they will produce electronic signals in the presence of vapors related to nerve agents (phosphonate esters) and explosives (nitroaromatics).
Two strategies are investigated: First, they are blended with compatible compounds that bind to these vapors, namely heterocyclic oligomers with hydrogen bonding and Lewis acidic groups for the phosphonates and electron pi-donors for the nitroaromatics. Second, they are also deposited as ultrathin films just a few monolayers in height. Semiconductor film thickness plays an important role in the performance of chemical sensors based on OFETs, because the first few molecular layers of organic semiconductor at the organic/dielectric interface define the conduction channels. Ultra-thin ChemFETs with only a few molecular monolayers of semiconductors have conduction channels exposed to the environment. Analyte molecules can directly adsorb near conduction channels of the ultrathin OFETs without having to diffuse through the dense organic film.
We systematically studied the thickness dependence of mobility of OFETs with 5,5 -Bis(4- hexylphenyl)-2,2 -bithiophene (6PTTP6) acting as semiconductor, as shown in Figure 1. Figure 2 shows the AFM imagines of ultra-thin 6PTTP6 films at various thickness. Based on these results, we found that the threshold thickness of a working 6PTTP6 FET is 4 nm.
OFETs were then fabricated according to the two above strategies. Structures of devices are shown in Figure 3. Transistors exposed to these vapors show 70% change in mobility when the vapors are present at 5 part per million levels, as shown in Figure 4. Devices are much more sensitive to target vapors than they are to humidity. Arrays of these sensors will be designed so that their cumulative responses will be characteristic of a particular vapor of interest.
