(left) NC AFM image of Au nano-islands on AlN(0001) ; (middle) Atomic resolution NC AFM image obtained at low temperature (5K) showing the moiré pattern formed by the Au monolayer; (right) Superposition of the model calculated by DFT (yellow : Au atom , violet : Al atom, grey : N atom) on the experimental NC-AFM image.

A key step to electrically connect a single molecule in a planar geometry is to engineer metallic nano-pads on an insulating substrate. These pads should be thin enough (a few monolayers at most) to allow the observation of the connected object by Atomic Force Microscopy (AFM). But most metals grow on the usual insulating substrates (SiO ₂ , MgO, KBr…) as tridimensional clusters. We chose to use the wide-gap semiconductor AlN (6.2 eV gap), whose surfaces are reactive, potentially favouring 2D growth of metals. Mastering the growth of AlN films by Molecular Beam Epitaxy allows us to control its surface at the atomic scale: The surface is (2x2) reconstructed, with one additional N atom per unit cell [1]. Depositing gold on this surface at 500°C results in the formation of large (a few 100 nm) monoatomic high nano-islands as shown in the non-contact AFM image on the left of the figure. A moiré pattern appears in the atomic resolution middle image, obtained at low temperature (5K). The analysis of this pattern led to the construction of a few atomic models that were tested by DFT calculations. This study shows that some of the Au atoms of the monolayer establish chemical bonds with the additional N atoms, which behave as electron acceptors, and with the Al atoms in sp2 configuration, which behave as electron donors. Moreover, the analysis of the atomic charges in this structure evidences an electron transfer from the AlN surface to the Au monolayer (-0,11 |e|/Au) as well as a lateral transfer in the unit cell. The combination of these three mechanisms is at the origin of the stabilisation of the Au monolayer on AlN(0001). Finally, the DFT-optimized atomic structure displays a distorted hexagonal structure that fits well with the experimental observation, as seen in the image on the right. Future experiments aim to connect a single molecule to these gold nano electrodes and to observe a charge transfer to the molecule.

[1] Noncontact atomic force microscopy and density functional theory studies of the (2×2) reconstructions of the polar AlN(0001) surface, Florian Chaumeton, Roberto Robles, Miguel Pruneda, Nicolás Lorente, Benoit Eydoux, Xavier Bouju, Sébastien Gauthier, and David Martrou , Phys. Rev. B 94 , 165305 – Published 17 October 2016

Reference

“Stabilization of Au Monatomic-High Islands on the (2×2)−Nad Reconstructed Surface of Wurtzite AlN(0001)”
Benoit Eydoux, Bulent Baris, Hassan Khoussa, Olivier Guillermet, Sébastien Gauthier, Xavier Bouju, and David Martrou
Phys. Rev. Applied 8, 044002 – Published 12 October 2017
DOI: https://doi.org/10.1103/PhysRevApplied.8.044002

Contact

David MARTROU , CEMES (CNRS)
dmartrou at cemes.fr