Highly Efficient and Tunable Filtering of Electrons' Spin by Supramolecular Chirality of Nanofiber‐Based Materials

Abstract

Organic semiconductors and organic–inorganic hybrids are promising materials for spintronic‐based memory devices. Recently, an alternative route to organic spintronic based on chiral‐induced spin selectivity (CISS) is suggested. In the CISS effect, the chirality of the molecular system itself acts as a spin filter, thus avoiding the use of magnets for spin injection. Here, spin filtering in excess of 85% in helical π‐conjugated materials based on supramolecular nanofibers at room temperature is reported. The high spin‐filtering efficiency can even be observed in nanofibers assembled from mixtures of chiral and achiral molecules through chiral amplification effect. Furthermore and most excitingly, it is shown that both “up” and “down” orientations of filtered spins can be obtained in a single enantiopure system via the temperature‐dependent helicity (P and M) inversion of supramolecular nanofibers. The findings showcase that materials based on helical noncovalently assembled systems are modular platforms with an emerging structure–property relationship for spintronic applications.

Detecting Chirality in Two-Terminal Electronic Nanodevices

“…Such a bias-even MR is indeed in agreement with most experimental observations. 5 , 6 , 8 − 12 The experimental conditions concerning the vanishing MR in the linear response regime are however unclear, since sometimes the currents are too low to be measured. 5 , 6 , 9 For experiments where a linear response regime can be identified, some do show a strongly suppressed or vanishing MR, 8 , 12 while some do not.…”

“…In two-terminal (2T) electronic nanodevices that contain a chiral component and a single ferromagnet (FM), CISS is reported as a change of electrical resistance upon magnetization reversal. 5 − 12 This magnetoresistance (MR) has been interpreted in analogy to that of a conventional spin valve, based on the understanding that both the FM and the chiral component act as spin–charge converters. 13 − 18 However, this interpretation overlooks the fundamental distinction between their underlying mechanisms—magnetism breaks time reversal symmetry, while CISS, as a spin–orbit effect, does not.…”

Spin Filtering Along Chiral Polymers

“…Past studies on chiral polymers were conducted either on polythiophene [8] or on helical supramolecular p-conjugated nanofibers. [10] In the two cases the polymers were lying on the surface.I nt he case of polythiophene,t he spin polarization was measured to be about 34 %a nd the magnetoresistance measured was less than 1%.Inthe case of the supramolecular nanofibers,v ery high spin polarization of about 85 %w as measured. Thed ifference is attributed to conduction of the current along the polymer backbone,a si nt he present case, whereas for the supramolecular wires the conduction is across the diameter, and the process involves sequential hopping between the molecules.H owever,t he magnetoresistance in this case was around one order of magnitude lower than in the current study.T he low values of the magnetoresistance observed in the past are due to the ill-defined organization of the polymers on the surfaces that allows pin-holes.While in the present study the organization is closely packed and well defined.…”

“…Theresults also confirm that the spin polarization does not involve spin flipping,a nd they are consistent with recent findings that the spin polarization is ar esult of suppression of back scattering for the favored spin, while the other spin is scattered back. [10] Finally,wesee that the common CISS effect is going hand to hand with the spin dependent charge reorganization (SDCR) and with asymmetric MR effects.H ence well controlled chiral polymers are good candidates for organicspintronic CISS-based applications.…”

“…[9] In ar ecent study,t he effect of secondary chiral structures of supramolecular systems on spin-selective transmission was investigated and correlated with circular dichroism (CD) intensities for nanofiber-based materials. [10] Whereas for most chiral molecules,t he spin selectivity is measured when the molecules are adsorbed on as olid substrate and the measurements are performed along the main molecular axis (the longest dimension), perpendicular to the substrate plane,f or polymers this configuration is problematic.T herefore,t ypically in the case of chiral polymers and wires,t he measurements are conducted when the long axis of the molecules are in plane and the spin polarization is measured perpendicular to the molecular main axis ( Figure 1A). These measurements were necessarily lacking precision regarding the paths of the electrons in the film studied and they did not probe the effect of the polymer backbone on the spin polarization.…”