Factors influencing the distribution of charge in polar nanocrystals

We perform first-principles calculations of wurtzite GaAs nanorods to explore the factors determining charge distributions in polar nanostructures. We show that both the direction and magnitude of the dipole moment $\mathbf{d}$ of a nanorod, and its electric field, depend sensitively on how its surfaces are terminated and do not depend strongly on the spontaneous polarization of the underlying lattice. We identify two physical mechanisms by which $\mathbf{d}$ is controlled by the surface termination, and we show that the excess charge on the nanorod ends is not strongly localized. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly.

Figure 1

An unrelaxed GaAs nanorod with Ga, As, and H atoms colored red (medium gray), blue (dark gray), and green (light gray), respectively. The Ga and As terminated ends are indicated.

Figure 2

Dipole moment $d_z$ as a function of H atom coverage of the polar surfaces for a nanorod with fully H-terminated lateral surfaces.

Figure 3

(a) Charge density and (b) electrostatic potential as a function of position $z$ along the long axes of the H/H, H/B, and H/P nanorods. Both quantities are integrated over the plane perpendicular to the long axis and smoothed on the length scale of a wurtzite unit cell.

Figure 4

Local densities of states for three nanorods: (a) H/H, (b) H/B, and (c) B/H. The global Fermi level is plotted in each case. Red (light gray) curves correspond to slabs close to the polar surfaces terminated by Ga and the blue (dark gray) curves correspond to slabs close to the polar surfaces terminated by As.