Understanding polarity in semiconductor nanorods with linear-scaling density-functional theory simulations

Binary polar semiconductors with the wurtzite structure have been observed to exhibit large dipole moments along $[0001]$. To explore the origin of these dipole moments, we use a linear-scaling density-functional theory code [1] to perform first-principles calculations of entire wurtzite GaAs nanorods consisting of several thousand atoms. We find that both the direction and magnitude of the dipole moment of a nanorod, and the electric field, depend sensitively on how its surfaces are terminated and not strongly on the spontaneous polarization of the underlying lattice [2]. We show that our calculations can be explained in terms of a pinning of the Fermi level at the polar surfaces that fixes the potential difference across the nanorod, and that this effect can have a determining influence on the polarity of nanorods, with consequences for the way a nanorod responds to changes in its surface chemistry, the scaling of its dipole moment with its size, and the dependence of polarity on its composition [3]. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly. \\[4pt] [1] J.~Chem.~Phys. 122, 084119 (2005).\\[0pt] [2] Phys.~Rev.~B 83, 241402(R) (2011).\\[0pt] [3] Phys.~Rev.~B 85, 115404 (2012)