Efficient Spin-Selective Electron Transport at Low Voltages of Thia-Bridged Triarylamine Hetero[4]helicenes Chemisorbed Monolayer

Abstract

The Chirality Induced Spin Selectivity (CISS) effect describes the capability of chiral molecules to act as spin filters discriminating flowing electrons according to their spin state. Within molecular spintronics, efforts are focused on developing chiral-molecule-based technologies to control the injection and coherence of spin-polarized currents. Herein, for this purpose, we study spin selectivity properties of a monolayer of a thioalkyl derivative of a thia-bridged triarylamine hetero[4]helicene chemisorbed on a gold surface. A stacked device assembled by embedding a monolayer of these molecules between ferromagnetic and diamagnetic electrodes exhibits asymmetric magnetoresistance with inversion of the signal according to the handedness of molecules, in line with the presence of the CISS effect. In addition, magnetically conductive atomic force microscopy reveals efficient electron spin filtering even at unusually low potentials. Our results demonstrate that thia[4]heterohelicenes represent key candidates for the development of chiral spintronic devices.

Spin Polarization in Transport Studies of Chirality-Induced Spin Selectivity

“…In recent reports of CISS transport, presumably because j ↑(↓) in eq cannot be directly probed in transport measurements, j ↑(↓) has been widely replaced by j M ↑(↓) , the current density with different magnetization directions M ↑(↓) in the magnetic layer. ,,,− ,− The currents with M ↑ and M ↓ are two different currents and can be measured directly. Equation has been reported as (with XSP being the incorrectly determined SP): X S P C I S S = j M ↑ − j M ↓ j M ↑ + j M ↓ (to the best of our knowledge, the initial definition of XSP can be traced back to the “spin-dependent electrochemistry” section in a review article on CISS).…”

“…, MTJ), we analyze eq and extract possible physical meanings of XSP. According to the literature on CISS spin-valves, the current–voltage ( I – V ) curves of such structures are not linear over large current ranges, although various organic and inorganic chiral systems have been studied. ,,,− ,− ,− Because of nonlinearities, the conductance is bias dependent. However, in the low-bias regime, comparing the conductance at a fixed current density is common practice in the field of spintronics …”

Chiral Induced Spin Selectivity

“…Although helicenes do not contain carbon stereocenters, they possess axial chirality. CISS manifests in enantiospecific adsorption of helicenes to magnetized surfaces, in spin-filtered electron transmission via photoelectron spectroscopy, and in conduction experiments through monolayer films of helicenes. ,,,− Although several classes of helicenes have been investigated, no reports have drawn a clear correlation between the structure of a helicene and its spin-filtering power. Photoemission studies indicate some effect of the substrate on the spin polarization; however, no simple correlation was found between the spin–orbit coupling of the substrate and the size of the spin polarization in the CISS effect .…”

Spin Polarized Current in Chiral Organic Radical Monolayers

“…22 A molecular monolayer of neutral thia [4]azahelicenes provided a spin polarization percentage at room temperature above 60% detectable also at unusually low voltages. 23 Additionally, these molecules undergo a reversible one-electron oxidation providing a stable radical cation where the spin density is localized on the nitrogen atom. 24 Paramagnetic chiral molecules were previously employed in the detection of CISS, resulting in a small but sizeable increase in the spin filtering.…”

“…These values are comparable to those observed for the parent neutral species. 23 As the spin density of the radical mainly resides on the tensioned and chiral nitrogen atom, thia [4]azahelicenes appear as a promising platform to investigate CISS-based manipulation of radical spins.…”