Monolayer Assembly of Chiral Molecules on Semiconductors for Spin Filtering

Chirality-induced spin selectivity (CISS) has been observed in chiral molecules including dsDNA and oligopeptide on noble metals.¹ The effect may have profound implications in biological processes and as novel means of spin injection and detection without using any magnetic material. Here, we report on a study on the formation of self-assembled monolayer (SAM) of thiolated dsDNA and polyalanine molecules on Au and GaAs by AFM and ellipsometry. On Au, ‘blocking’ by hydrophobic alkanethiol SAM is necessary to achieve assembly of dsDNA oriented away from the substrate. The assembly on GaAs is aided by an ammonium sulfide treatment for oxide removal and surface passivation,² on which the polyalanines form oriented SAM without any ‘blocking’. The spin filtering of the electrons from a Au electrode by polyalanine SAM is ascertained using perpendicularly magnetized (Ga,Mn)As as a spin analyzer: The magnetoresistance of a (Ga,Mn)As/polyalanine/Au junction is measured in a perpendicular magnetic field. Sharp jumps in the junction resistance are observed at the coercive fields of the (Ga,Mn)As.
¹ R. Naaman and D.H. Waldeck, Annu. Rev. Phys. Chem. 66, 263 (2015).
² X. Wang, et al., Adv. Mater. 27, 8043 (2015).