In electron-transfer processes, spin effects normally are seen either in magnetic materials or in systems containing heavy atoms that facilitate spin-orbit coupling. We report spin-selective transmission of electrons through self-assembled monolayers of double-stranded DNA on gold. By directly measuring the spin of the transmitted electrons with a Mott polarimeter, we found spin polarizations exceeding 60% at room temperature. The spin-polarized photoelectrons were observed even when the photoelectrons were generated with unpolarized light. The observed spin selectivity at room temperature was extremely high as compared with other known spin filters. The spin filtration efficiency depended on the length of the DNA in the monolayer and its organization.

We report on the observation of chirality induced spin selectivity for electrons transmitted through monolayers of oligopeptides, both for energies above the vacuum level as well as for bound electrons and for electrons conducted through a single molecule. The dependence of the spin selectivity on the molecular length is measured in an electrochemical cell for bound electrons and in a photoemission spectrometer for photoelectrons. The length dependence and the absolute spin polarization are similar for both energy regimes. Single molecule conductance studies provide an effective charge transport barrier between the two spin channels and it is found to be on the order of 0.5 eV

The interaction of low-energy photoelectrons with well-ordered monolayers of enantiopure helical heptahelicene molecules adsorbed on metal surfaces leads to a preferential transmission of one longitudinally polarized spin component, which is strongly coupled to the helical sense of the molecules. Heptahelicene, composed of only carbon and hydrogen atoms, exhibits only a single helical turn but shows excess in longitudinal spin polarization of about P = 6 to 8% after transmission of initially balanced left- and right-handed spin polarized electrons. Insight into the electronic structure, that is, the projected density of states, and the spin-dependent electron scattering in the helicene molecule is gained by using spin-resolved density functional theory calculations and a model Hamiltonian approach, respectively. Our results support the semiclassical picture of electronic transport along a helical pathway under the influence of spin-orbit coupling induced by the electrostatic molecular potential.

Spin-dependent photoelectron transmission and spin-dependent electrochemical studies were conducted on purple membrane containing bacteriorhodopsin (bR) deposited on gold, aluminum/ aluminum-oxide, and nickel substrates. The result indicates spin selectivity in electron transmission through the membrane. Although the chiral bR occupies only about 10% of the volume of the membrane, the spin polarization found is on the order of 15%. The electrochemical studies indicate a strong dependence of the conduction on the protein's structure. Denaturation of the protein causes a sharp drop in the conduction through the membrane.electron transfer | electrochemistry | magnetic effect | chirality T he role of the electron spin in chemistry and biology has been receiving much attention because of a plausible relation to electromagnetic field effects on living organisms (1), and due to the seemingly importance of the earth's magnetic field on birds and fish navigation (2). Part of the difficulty in studying the subject arises from the lack of a physical model that can rationalize these phenomena. Recently, the chiral-induced spin selectivity (CISS) effect was observed in electron transmission and conduction through organic molecules (3). The spin selectivity was observed for photoelectron transmission through monolayers of double-stranded DNA adsorbed on gold (4). Another study discovered a spin dependence in the conduction through single molecules of double-stranded DNA. In this configuration, one end of the molecule was adsorbed on a Ni substrate, whereas the other was attached to a gold nanoparticle (5).The CISS effect may provide a novel approach for better understanding the role of electron spin in biological systems. The studies mentioned above led to several questions, including the actual role played by the gold substrate in the overall spinfiltering process. Gold exhibits a very large spin orbit coupling; hence, one may wonder whether gold itself affects the CISS phenomenon. In addition, the interface between gold and the thiol group, through which the molecules are attached to the gold, may play a role. Because many of the past studies were performed with DNA, an important question arises whether CISS is a general effect or possibly a special property of DNA. CISS was only observed for double-stranded DNA, whereas for single-stranded molecules, no spin selectivity was found. On the one hand, this was attributed to the lack of ordered monolayers (4, 6); on the other hand, a theoretical model, proposed to rationalize the CISS effect, predicted that a double-helix structure (7) was needed for CISS to occur, whereas other approaches do not emphasize this need (8). Finally, because many of the past studies were performed in vacuum or in ambient air, it is of importance to probe to what extent the effect persists in solutions, which are more relevant to biology. The present study aims at answering the above questions in an attempt to establish CISS as a general phenomenon.For the present study, we chose bacteriorhodopsin (...

for self-assembled helical molecules attached to metals, mostly on gold substrates, [ 9 ] and for chiral molecules evaporated onto a magnetic substrate. [ 10 ] Recently, it was further shown that helical poly-alanine can be used to magnetize ferromagnetic nickel at low temperatures. [ 11 ] In this Communication we present a silicon-based spin source at room temperature without magnetic layers, based only on the spin fi lter effect in dsDNA covalently bound to Si(100).The fi rst step in developing the spin source requires the immobilization of dsDNA on Si(100). A good control of the selfassembly is possible via nucleophilic-electrophilic chemistry or by thiol-ene/yne click chemistry. Both approaches enable the binding of DNA to semiconductor surfaces. [ 12 ] In the present experiment we used the nucleophilic-electrophilic amine-isothiocyanate reaction. A schematic of the preparation steps involved in this reaction is shown in Figure 1 . For this purpose 2.5% HF acid solution was used to etch-off the natural oxide layer on silicon and to form a hydrogen terminated silicon surface. Subsequent treatment with acid peroxide (piranha solution) leads to the formation of a hydroxylated surface. Water contact angles of less than 20° indicate a highly hydrophilic nature of the surface, as expected. Functionalization of the surface with a solution of 3-isothiocyanatopropyl triethoxy silane (ICPTES) linker molecules in dry toluene increases the hydrophobicity, causing an increase of the contact angle to nearly 70°. [ 13 ] This confi rms the successful formation of linker molecules on the substrate, because the decrease of hydrophilicity is a result of the aliphatic chains of the linker molecules. Afterward, dsDNA was immobilized by wetting of the surface with 10 × 10 −6 M solution of 50 base pair dsDNA dissolved in phosphate buffer at pH 8 for 72 h. The water contact angle decreases to 35° due to the hydrophilicity of the DNA's backbone.The functionalization of the silicon surface was also investigated by X-ray photoelectron spectroscopy (XPS) for binding energies in the E B = 0-650 eV range, as shown in Figure 2 . On the hydroxylated silicon surface the Si 2p and 2s features are detected at E B = 100.7 eV and E B = 152.1 eV binding energies, respectively. Additionally, the O 1s signal is clearly visible at E B = 532.1 eV binding energy. After attaching the linker molecules to the surface additional signals appear from C 1s and N 1s at about 285 and 400 eV binding energies, respectively. Further, a small contribution from S 2s at E B = 226.3 eV can be discerned. These signals are contributions from the carbon chains and the N  C  S moiety of the linker molecules. In case of the dsDNA functionalized surface the intensity of nitrogen and carbon features increase and a P 2s signal appears at 191.8 eV binding energy. These changes are contributed from the dsDNA backbone. For a better insight into the preparation high resolution XPS allows to confi rm the binding of single Controlling the electron spin orientation and spin inj...