Electron Spin-selective Transmission through double-stranded DNA self-assembled on gold

Doctoral examination procedure finished at: Doctoral examination procedure at University of Münster

Start date of doctoral examination procedure: 01/06/2007

End date of doctoral examination procedure: 19/10/2012

Name of the doctoral candidate: Göhler, Benjamin

Doctoral subject: Physik

Doctoral degree: Dr. rer. nat.

Awarded by: Department 11 - Physics

Monolayer of chiral dsDNA-molecules were analyzed with regard to their spin-dependent electron transmission. Therefore, photoelectrons were excited from a gold substrate covered with a self-assembled monolayer of DNA molecules. Using a Mott polarimeter the longitudinal spin polarization of those electrons was measured that were transmitted through the layer of the molecules. The experiments demonstrate that the spin of the transmitted electrons is predominantly oriented antiparallel to their direction of propagation. Spin polarization values of up to approximately - 60% were measured. These results suggest that a layer of self-assembled dsDNA-molecules acts as a spin-filter with a relatively high efficiency. Spin selective electron transmission was known prior to this work for chiral molecules in the gas phase [1] and was inferred for chiral, organic molecules that are self-assembled on a surface [2]. In the latter experiments the photoelectron yield of polycrystalline gold coated with a thin film of chiral molecules was found to depend on the helicity of the circularly polarized radiation that was used for the excitation. Since it was known that light of different helicities can excite photoelectrons of antiparallel (longitudinal) spin polarization, an electron spin-specific interaction of the monolayer was inferred. The direct measurement of the spin-polarization conducted in this work provides evidence that the monolayer of chiral dsDNA-molecules interacts with the spin of the transmitting electrons. The spin-filtering effect of dsDNA monolayers was investigated in a systematic study. In these experiments, the length of the molecules was varied between 26 base pairs and 78 base pairs. Within this range the (absolute) spin polarization of the transmitted electrons increased from approximately - 10% to - 60%, demonstrating an approximately linear rise. This observation supports the interpretation that the self-assembled monolayer acts as a spin filter as it fulfills the naive expectation that the filtering effect is enhanced the longer the electrons propagate through the filtering medium. Furthermore, the order of the molecules was recognized to have substantial influence on the spin-filtering effect, a finding that was validated in two experiments. In the first of these experiments, a monolayer of single-stranded DNA was adsorbed on a surface. Molecules of this conformation form a floppy instead of ordered layer. The spin-polarization of the transmitted electrons was substantially reduced as compared to those transmitted through a monolayer of the same DNA molecules (number and sequence of base pairs) in the double-stranded conformation. In a second confirmatory experiment, the spin-polarization of electrons transmitted through a monolayer of dsDNA was monitored while intermittently intense UV light was applied to induce single strand breaks, which destroys the double-helix structure. This reduced the spin-filtering effect and after 15 minutes of exposure to intense UV radiation, a spin polarization similar to that of electrons transmitted through single-stranded molecules was obtained. The spin-selective interaction of chiral, oriented molecules is not completely understood to date. The effect, termed electron dichroism, could be explained for randomly oriented molecules in the gas phase (containing at least one atom with a high atomic number) as being a spin-dependent elastic scattering. For layers of ordered molecules, on the other hand, initial theories underestimated the spin-dependent asymmetry (constituted of atoms with low atomic numbers only) by several orders of magnitude [3]. [1] Journal of Physics B: Atomic, Molecular and Optical Physics 29, 3497 (1996) [2] Science 283, 814 (1999) [3] The Journal of Chemical Physics 131, 014707 (2009)