First-principles calculations of SO2 sensing with Si nanowires

High chemical reactivity and large surface-to-volume ratio have recently led to growing interest in the employment of silicon nanowires (SiNWs) in sensing applications for chemical species detection. The working principle of SiNWs sensors resides in the possibility to induce modifications in their electronic properties via molecular interaction. A detailed analysis of the interaction of Si with molecular compounds is then required to design and optimize NW-based sensors. Here we study the mechanisms of adsorption on SiNWs of SO2, an air pollutant with pernicious effects on humans. First-principles density-functional calculations are performed to calculate the electronic structure of a SO2 molecule adsorbed at a silicon surface in case of undoped substrate and in presence of substitutional subsurface and deep boron impurities. Comparing the results with the case of NO2 adsorption – a similar molecule that, nonetheless has a very different interaction with a Si surface –, we show the specific traits of SO2 interaction: formation of localized states in the band-gap and absence of reactivation of pre-existing and passivated sub-surface impurities. A connection between the modifications in the system electronic structure and the strength of the molecular interaction is discussed.