Modulation of Agglomeration of Vertical Porous Silicon Nanowires and the Effect on Gas‐Sensing Response

Abstract

Porous silicon nanowires (PSiNWs) array is a promising material for development of integrated gas sensors operating at room temperature. This work reports the fabrication of PSiNWs assembly with different structural features and its effect on gas-sensing performance. Bundling and well separating PSiNWs arrays are fabricated by MACE method, respectively, based on the effective modulation of surface wettability of the initial Si substrate. The HF pretreatment creates a hydrophobic surface favorable for deposition of irregular Ag nanoflakes and then for the formation of bundling PSiNWs array. In contrast, the PSiNWs with well lateral separation are formed based on the predeposited uniform Ag nanoparticles on a hydrophilic Si surface. The PSiNWs array featured by tip-clusters is proved to be highly effective in achieving highly sensitive and rapid response to NO 2 gas at room temperature. Satisfying dynamic characteristic and selectivity are meanwhile observed for the bundling array. The formation of the bundling or separating of PSiNWs is discussed in terms of the force balance of individual nanowire, which is further correlated with non-uniform distribution of Ag nanoclusters caused by H-termination. Meanwhile, high sensing performance of bundling nanowires is analyzed based on the structural promotion of the unique configuration of tip-cluster to sensing response.

Silicon Nanowires for Gas Sensing: A Review

“…In ambient air, the absorption of oxygen ions on NW's surface creates a hole accumulation layer (HAL) (in p-type SiNWs) or a depletion layer (in n-type SiNWs) by trapping electrons from the SiNWs [111].…”

“…It is apparent that, in some cases, both of these interactions can contribute to the sensing of the gas. This interaction, in the n-type SiNWs, can change the width of the depletion layer (and as a result the diameter of conduction channel), and in the p-type SiNWs can alter the width of the HAL and the surface potential value (V s ), and finally the conductance properties of the SiNWs [111,112]. Therefore, it is interesting to investigate the different types of the gas (oxidizing and reducing gases) and their effects on the sensing mechanism in detail.…”

“…Assuming that we have a p-type SiNW in vicinity of an oxidizing gas, this oxidizing gas extracts the electrons (which are minority carriers) from the conduction band in p-type Si and makes the HAL formed previously by oxygen ions, to become thicker. [111,112]. While the reducing gas releases the electrons trapped by O 2 − (ads) and makes the HAL to become thinner.…”

Compositional and structural engineering of inorganic nanowires toward advanced properties and applications

“…Devices based on FET and resistor configurations have been frequently fabricated, which display activity in response to various gaseous species, that is, NO 2 , NH 3 , H 2 , and H 2 O . Porous silicon nanowires, possessing significantly enhanced surface area and defect density, are promising materials for the development of integrated gas sensors operating at room temperature, benefiting from the mature chemical etching technology in terms of large‐scale fabrication, cost savings, and convenience for hybrid design . Liu et al recently reported a simple SiNW‐TiO 2 core‐shell heterojunction created by the sol‐gel and drop‐casting methods (Figure A).…”

“…165 Porous silicon nanowires, possessing significantly enhanced surface area and defect density, are promising materials for the development of integrated gas sensors operating at room temperature, benefiting from the mature chemical etching technology in terms of large-scale fabrication, cost savings, and convenience for hybrid design. 172 Liu et al recently reported a simple SiNW-TiO 2 core-shell heterojunction created by the sol-gel and drop-casting methods 173 ( Figure 14A). The fabricated chemiresistor sensor exhibited a linear response to CH 4 gas in the 30-120 ppm range, with a detection limit of 20 ppm at room temperature, benefiting from a depletion layer at the interface of SiNWs and the TiO 2 layer.…”