Highly sensitive and reliable octyltrichlorosilane coated silicon sensors for nitrogen gas flow detection

In this article, we present highly sensitive and reliable nitrogen gas flow sensors based on surface modified n-type (n-Si) and p-type (p-Si) silicon wafers. The surfaces of n-Si and p-Si wafers were coated with octyltrichlorosilane (OTS) molecules. The surface modified n-Si and p-Si wafers were used as sensing elements. The sensing elements were kept on the 45 degrees angle inclined to the experimental mount and nitrogen gas was flown over the inclined surface at subsonic velocities. The flow induced voltage generation is caused by the interplay between the mechanisms of Bernoulli's principle and Seebeck effect. The experimental results show that the half coated and full coated n-Si and p-Si wafers generate more voltage than that of the uncoated at a given velocity. In the case of n-Si wafers, the half coated surface generated voltage was approximate to 4.80 times than that of the uncoated surface. In the case of p-Si wafers, the half coated surface generated voltage was approximate to 3.83 times than that of the full coated. The surface analysis of the half and full coated surfaces show that the OTS SAM molecules on n-Si surfaces were homogeneous and well ordered, however on p-Si surfaces, the OTS SAM molecules were heterogeneous and disordered. The enhanced voltage generations and high sensitivities are caused by an effective change in the gradient of Fermi energy (E-F) due to OTS SAM coating. The experimental observations and theoretical analysis of enhanced voltage generation show that the OTS surface modification causes synergism of different mechanisms, contributing to the cumulative increase of flow induced voltage. The effect of aging process on the reliability of the half coated OTS/n-Si and OTS/p-Si sensors were tested for 24 h, 30 days and 90 days, the experimental results confirm that the flow induced voltages are consistent and reproducible and both the OTS/n-Si and OTS/p-Si sensors are very reliable. The investigation of the effects of the coverage of the OTS SAM and the angle of inclination on the flow induced voltage shows that the 50% coverage of the OTS/n-Si, OTS/p-Si and the 45 degrees angle of inclination produced a maximum flow induced voltage at any given velocity. (C) 2018 Elsevier B.V. All rights reserved.