Discrete Electronic Subbands due to Bragg Scattering at Molecular Edges
A. Martín-Jiménez, J. M. Gallego, R. Miranda, and R. Otero
The discretization of the electronic structure of nanometer-size solid systems due to quantum confinement and the concomitant modification of their physical properties is one of the cornerstones for the development of nanoscience and nanotechnology. In this Letter we demonstrate that the Bragg scattering of Cu(111) surface-state electrons by the periodic arrangement of tetracyanoquinodimethane molecules at the edges of self-assembled molecular islands, along with the dominant contribution of backscattering processes to the electronic density of states, discretizes the possible values of the electron momentum parallel to the island edge. The electronic structure consists thus of a discrete number of subbands which occur in a nonclosed space, and therefore without quantum confinement.
(a) STM image (30.2×30.2 nm2, Vbias=1.4 V, It=150 pA) of the edge of a TCNQ island, showing the bulk and edge molecular arrangements. The rhomboidal unit cell is shown. (b) 1D Fourier transform (FT) of a scan line over the island edge. (c) 2D FT of the island bulk. Black tick marks correspond to the peaks in the 1D FT that correspond to the projection of the bulk reciprocal space over the edge direction, while red ticks mark the position of the new peaks due to the edge periodicity. (d) STM image (26.3×8.6 nm2, It=150 pA) recorded at low voltage (Vbias=100 meV), where the standing wave pattern of the Cu(111) surface state is visible.