Molecule-Terminated, Oxide-Free Silicon Nanowire Field Effect Transistors : Effect of the C-C Bond Nearest to the Surface

We have recently developed silicon nanowire enhanced field effect transistor (FET) devices to improve the sensitivity and diagnostic yield of the current clinical chemiresistors.¹ The silicon nanowire, which has a natural passive oxide layer, can be modified to increase the sensitivity and stability of these devices. Modifications of the device can be made on top of the oxide layer by reacting silanes with oxygen groups;² however, we wish to alkylate the nanowire surface without oxygen with the goal of increased sensitivity and selectivity.

We will report on the alkylation of the silicon nanowires (Si NW) on the field effect transistor, as well as the effect of alkylation on the sensitivity and stability of the sensor. The modification is conducted in a three stage reaction mechanism: (1) removal of the oxide layer via HF etch, (2) Si NW Chlorination with a PCl₅ solution, and (3) alkylation with propylmagnesium chloride, 1-propenylmagnesium bromide, and 1-propynylmagnesium bromide. The result is an alkylated monolayer on top of the Si NWs with Si-CH₂-CH₂-CH₃, Si-CH=CH-CH₃, and Si-C≡C-CH₃ terminals respectively. X‑ray photon spectroscopy is used to confirm the surface modification, and electrical characterization is used to evaluate the current-voltage profiles; both analyses will verify the success of the surface reaction. Our goal is to achieve complete monolayer coverage with an increase in the device's on/off ratio. We also use standardization gas mixtures (simulated cancer breath samples) to create calibration curves for our sensors and make predictions on the sensitivity and selectivity. We hypothesize that the 1‑propenylmagnesium bromide modification (double bond) will be the most stable, while the 1‑propynylmagnesium bromide modification (triple bond) will be the most sensitive.

In conclusion, alkylated silicon nanowire field effect transistors could provide a more accurate and sensitive diagnostic breath sensor for evaluating and discriminating various diseases.

References

1.            Peng G, Tisch U, Adams O, Hakim M, Shehada N, Broza YY, Billan S, Abdah-Bortnyak R, Kuten A, Haick H. Diagnosing lung cancer in exhaled breath using gold nanoparticles. Nature Nanotechnology 2009;4:669-673.

2.            Wang B, Haick H. Effect of functional groups on the sensing properties of silicon nanowires toward volatile compounds. Applied Materials & Interfaces 2013:in press.