Effect of Functional Groups on the Sensing Properties of Silicon Nanowires toward Volatile Compounds

Molecular layers attached to a silicon nanowire field effect transistor (SiNW FET) can serve as antennas for signal transduction of volatile organic compounds (VOCs). Nevertheless, the mutual relationship between the molecular layers and VOCs is still a puzzle. In the present paper, we explore the effect of the molecular layer's end (functional) groups on the sensing properties of VOCs. Toward this end, SiNW FETs were modified with tailor-made molecular layers that have the same backbone but differ in their end groups. Changes in the threshold voltage (ΔVth) and changes in the mobility (Δμh) were then recorded upon exposure to various VOCs. Model-based analysis indicates that the interaction between molecular layers and VOCs can be classified to three main scenarios: (a) dipole-dipole interaction between the molecular layer and the polar VOCs; (b) induced dipole-dipole interaction between the molecular layers and the nonpolar VOCs; and (c) molecular layer tilt as a result of VOCs diffusion. Based on these scenarios, it is likely that the electron-donating/withdrawing properties of the functional groups control the dipole moment orientation of the adsorbed VOCs and, as a result, determine the direction (or sign) of the ΔVth. Additionally, it is likely the diffusion of VOCs into the molecular layer, determined by the type of functional groups, is the main reason for the Δμh responses. The reported findings are expected to provide an efficient way to design chemical sensors that are based on SiNW FETs to nonpolar VOCs, which do not exchange carriers with the molecular layers.