Towards Wireless Detection of Surface Modification of Silicon Nanowires by an RF Approach

Abstract

This paper shows the possibility to detect the presence of grafted molecules on the surface of silicon nanowires with a wireless RF radar approach based on the measurement of the backscattered signal of a resonant structure on which the nanowires are deposited. The measured resonance frequency allows the determination of the intrinsic properties related to temperature and humidity variations, which can be related to the presence of the grafted molecules. Several functionalizations of nanowires have been realized and characterized. For the first time, an RF approach is used to detect significant differences related to the presence of grafted molecules on the surface of nanowires. In addition to detecting their presence, the obtained results show the potential of the radar approach to identify the type of functionalization of nanowires. A set of six different grafted molecules (including octadecyltrichlorosilane, ethynylpyrene, N3) was tested and correctly separated with the proposed approach. Various measurements of the same samples showed a good repeatability which made the approach compatible with the possibility of differentiating the molecules with each other by radar reading. Moreover, discussions about the application of such functionalizations are made to increase the sensibility of sensors using a radar approach.

Keywords: RF characterization; humidity dependency; physicochemical characterization; radar; silicon nanowires; surface functionalization; temperature dependency; wireless measurement.

Figure 1

Principle of the measurement of the scatterer’s resonance frequency used to sense temperature and humidity. The climatic chamber allows the control of both temperature and humidity values.

Figure 2

(a) Schematic of the loop-sensing resonator. The nanowires are deposited in the center of the gap since it is the spot with the highest E field. (b) Illustration in red of the E field concentrated in the loop gap at the resonance.

Figure 3

Simulated resonance frequency ($S_{21}$) of a Rogers RO4003C tag for different values of temperature T and humidity $RH$.

Figure 4

Temperature and humidity measured with an electronic sensor inside the climatic chamber during the measurements.

Figure 5

Cross-sectional SEM image of p-type SiO2-SiNWs array. TEM image of SiNW (inset). The insert is a top view picture.

Figure 6

(a) Infrared spectrum of an octadecyl-functionalized silicon nanowire; C-H region enlarged, upper right; C-H stretching vibration of OTS according to Figure 1C of [23] (inset, upper left); (b) organosilica films stretching vibration before (b) and after (a) alkyl groups grafting as shown in Figure 3 from [24].

Figure 7

Infrared spectrum of unmodified silicon nanowires (a), azide-modified silicon nanowires (b), pyrene-modified silicon nanowires (c).

Figure 8

XPS spectra recorded on silicon nanowires modified with azide (red curve) and pyrene groups (blue curve) including enlarged spectra of N1s before and after click chemistry.

Figure 9

Photo of the measurement bench and tags. (a) Setup inside the climatic chamber. (b) Copper loop resonators on Rogers RO4003C substrates used for the measurements. Resonances frequencies were 2.013, 2.121, 2.218, 2.287, 2.430 and 2.558 GHz.

Figure 10

Normalized resonance frequencies for different temperatures at a constant humidity of RH = 60% (normalized by the initial frequency at 40 °C).

Figure 11

Normalized resonance frequencies for different temperatures at a constant temperature of T = 40 °C (normalized by the initial frequency at 60%RH).

Figure 12

(a) Three-dimensional representation of the 6 samples characterized in practice using the climatic chamber and the RF approach. (b) Top-view of the Gaussian representation.

Figure 13

Likelihood representation of the 6 samples.