New high-quality chiral molecular intercalation superlattice material

On June 29, 2022, Beijing time, the Research Group of Duan Zhifeng of the University of California, Los Angeles, published a new study entitled “Chiral Molecular Intercalation Superlattices” in the international top academic journal Nature, and the research group successfully designed and prepared a new type of high-quality chiral molecular intercalation superlattice material. The material exhibits a highly ordered superlattice structure and chiral optical selection properties.

Professor Duan Zhifeng is the corresponding author of the paper, and Dr. Qian Qi and Dr. Ren Huaying are the co-first authors of the paper.

The chiral induced spin selectivity (CISS) effect directly correlates the spin of an electron with the chirality of a molecule, revealing the possibility of regulating the direction of electron spin even in an environment without an applied magnetic field, thus opening up new possibilities for the creation of new spin electronic devices. In recent years, although researchers have developed several methods of introducing CISS into solid-state material systems, these preparation methods and their material systems are often plagued by high inhomogeneity, low spin selectivity and limited stability of chiral molecular systems, and it is difficult to finally form highly selective and highly stable spin electronic devices.

In view of the above key problems that need to be solved urgently, the Duan Zhifeng Research Group of the University of California, Los Angeles, has expanded the solid material system that can be used for CISS research, and successfully designed and prepared a new type of high-quality chiral molecular intercalation superlattices (CMIS) material. The material exhibits a highly ordered superlattice structure and chiral optical selection properties. In addition, by using CMIS as an electron spin filter layer, the authors designed and processed a high-efficiency spin tunneling electronic device that achieved a spin magnetoresistance ratio of more than 300% and a spin polarization rate above 60%, far exceeding other chiral molecular tunneling devices.

Figure 1: CiSS and CMIS schematic

Two-dimensional layered crystals are formed by stacking together weak van der Waals interaction forces between adjacent atomic layers, and there are countless van der Waals gaps between layers without covalent bonds. This unique gap allows many foreign molecules to be inserted into it, while also keeping the crystal structure within the atomic layer of the two-dimensional material from being destroyed, resulting in a perfect superlattice. Through the method of chemical intercalation, the authors successfully inserted chiral molecules R-α-methylbenzylamine (RMBA) and S-α-methylbenzylamine (SMBA) and other molecules (see the main text of the article) into the two-dimensional layered crystals H-TaS2, T-TaS2 and TiS2 to form a variety of CMISs.

Figure 2: Structural characterization of CMIS

The authors characterized the superlattice structure of CMIS using XRD, AFM, STEM, and SAED. Through XRD test, it was found that the overall layer spacing of the intercalation reaction material compared with the layer spacing of the original material was significantly increased, and the increase value matched the height difference before and after the intercalation measured in situ by AFM and the change of layer spacing shown in the STEM high-resolution image, which proved that the chiral molecules have been inserted into the two-dimensional material system in an orderly manner, and a highly ordered superlattice structure has been formed. Through the high-resolution images in the surface and SAED, it can be found that the lattice arrangement of H-TaS2 has not changed after being interpolated, which is of great significance for the composite of two-dimensional materials and chiral molecules.

Figure 3: Optical properties of CMIS

Raman spectroscopy further illustrates that the crystal structure of the atomic layer of the two-dimensional material in CMIS is intact and of high quality. It is worth mentioning that for the TaS2 material of the H phase, the Raman peak has a significant displacement compared with the eigensal phase H-TaS2 material, which is consistent with the Raman peak of the single-layer H-TaS2, further proving that the MBA molecule has been inserted into the layered material layer by layer, and forms an ordered superlattice structure. In addition, circular dichroic spectroscopy detection showed that CMIS had obvious optical rotation selection characteristics, and its chirality was consistent with the direction of the molecular chiral CD signal used, while CMIS formed using racemic molecular intercalation had no chiral signal detection.

Figure 4: CMIS-based spin tunneling device

CMIS can be thought of as a multilayer spin-select device series structure that effectively enhances spin filtering and spin polarization. By using CMIS as the self-selecting filter layer and the layered magnetic material Cr3Te4 as the spin polarization layer, the author constructs an efficient and stable spin tunneling electronic device, and observes the corresponding IV curve and magnetic field scanning curve of CMIS in the opposite direction of the magnetic field, achieving a spin magnetoresistance ratio of more than 300% and a spin polarization rate of more than 60%.

Figure 5: Transport characteristic curve of a spin tunneling device over temperature

Through further research, the authors found that the spin polarization rate of cmIS-based spin tunneling devices gradually decreased with increasing temperature, becoming zero after the Curie temperature above Cr3Te4. On the other hand, the spin polarization rate is affected by two parts: the spin-dependent tunnel-through conductance and the spin-unrelated tunnel-through conductance. The spin-related tunnel conductance exhibits non-monotonic temperature characteristics due to the competition between spin polarization and chiral molecular vibrations in ferromagnets. This series of experimental results show that CMIS is a high-quality solid material system, which is very suitable for further exploration of the basic principles of CISS.

In the next step, by replacing different two-dimensional layered materials and chiral molecules, the chiral superlattice material structure is expected to expand a richer artificial chiral material system, opening up new ideas for the design of solid chiral materials, and creating new possibilities for further studying the CISS effect and capturing its wide application in new spin electronic devices. (Source: Science Network)

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