Influence of impurities on phase transition in quasi-one-dimensional nanowires on Si surface

We investigated using low-energy electron diffraction the influence of impurity doping on the structural phase transition in an array of quasi-one dimensional In nanowires on Si(111). A clean Si(111)4$\times$1-In surface, in its pristine form, undergoes a structural phase transition into a 8$\times$2 phase below 120 K. Introducing various impurities (hydrogen, oxygen, and alkali metals) on the surface at room temperature was found to affect the 4$\times$1-to-8$\times$2 structural phase transition by changing the transition temperature (T$_c$). Adsorption of the two types of the gases affected the transition in opposite ways: hydrogen adsorption lowered the T$_c$, whereas oxygen adsorption raised the T$_c$. Dosing of different alkali metals (Na, K, and Li) all decreased the T$_c$. Usually, impurities are expected to suppress the phase transition into the symmetry-broken phase (the low-temperature phase) by acting as random fluctuations in structure. In this sense, the increase in T$_c$ by the oxygen adsorption is an exceptional case enhancing the phase transition. Possible mechanisms leading to different influences of the various impurities on the structural phase transition of this In/Si(111) will be discussed.