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[en] Nanostructure-based sensors working based on the adsorption-desorption mechanism received much attention in a wide variety of applications as a way to achieve both ultrasensitive sensing and small dimensions. In this paper, the adsorption of ammonia molecule on silicon nanowire surface with square, circular, and triangular cross-sectional shapes, which has almost similar cross-sectional area (1.2 nm2), is studied. The effects of various adsorption configurations are considered on the sensing properties, utilizing first-principles ab initio density functional theory calculations. Structural properties such as bond lengths and bond angles are obtained along with electronic properties such as band structure, charge transfer, and projected density of states for several adsorption configurations. The most stable and most sensitive structure is calculated to be the triangular nanowire considering formation enthalpy as a stability factor. For this nanowire, adsorption energy, covalent bond length, and charge transfer between ammonia and nanowire are calculated to be 1.296 eV, 3.124 Å, and 0.227 |e|, respectively. We also consider nanowires with fully hydrogen-passivated surface and nanowires with a dangling bond. It has been shown that nanowires with a dangling bond are more sensitive toward ammonia than fully passivated nanowires.