(559k) 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.