Chirality refers to the phenomenon that an object cannot coincide with its mirror image, and is a term that describes the asymmetric nature of molecules. Chiral molecules are molecules that are mirror symmetrical in chemical structure and cannot be completely overlapped, just like the left and right hands of human beings, and cannot overlap each other, so a pair of chiral molecules that mirror each other are called enantiomers, see Figure 1. Chirality is a unique property of molecular isomers that plays a vital role in medical and life sciences, food chemistry, and pharmaceutical manufacturing. It is a fascinating geometric property that has attracted the attention of chemists for more than 170 years, mainly because enantiomer pairs exhibit unique functions when interacting with other chiral objects. Many biomolecules essential for life exist in the form of single enantiomers, that is, they are homochiral.
Fig.1 Schematic diagram of chiral molecular structure (from Sogou)
Semiconductors are materials that conduct electricity at room temperature between conductors and insulators, and the introduction of chirality in semiconductors allows the control of charge, spin, and optoelectronics without magnetic components. Chiral semiconductor materials are divided into chiral organic semiconductors and chiral inorganic semiconductor materials.
Organic semiconductor molecules are mainly composed of basic elements such as C, H, O, N, etc. C has an atomic number of 6, and its 6 extranuclear electrons occupy 1s, 2s, and 2p atomic orbitals, respectively. The 2 electrons contained in the 1s orbital are generally regarded as atomic solids together with the nucleus, and are not affected by other atomic solids in the vicinity. When the C atom forms a compound, the four electrons on the periphery of the atom occupy 2s, 2p, 2p, and 2p orbitals, respectively, and these orbitals and their different hybridizations affect the structure and properties of organic semiconductor materials.
We know that the chirality of a large number of organic compounds comes from their asymmetric C atoms or C-H groups, and according to their different asymmetry, chiral molecular configurations can be divided into: central chiral molecules, axial chiral molecules, planar chiral molecules, and helical chiral molecules. For the central chiral molecule, the hybrid C atom is connected with four other different atoms or groups, so that the whole molecule has chiral characteristics, and the C atom in the center is the chiral center or asymmetrical center; for the axial chiral molecule, there is a chiral axis in the molecule, and the arrangement of other atoms or groups around the chiral axis is not symmetrical, and the whole molecule has chiral characteristics; for the planar chiral molecule, there is a molecular symmetry plane, and the existence of other groups destroys its symmetry, making the molecule chiral; in particular, the spiral chiral molecule has neither a chiral center nor a chiral axis, and its configuration is a molecular chain spiraling around a fixed axisWhen looking in the ascending direction, S-type and R-type molecules are distinguished according to the direction of rotation of the spiral.
It is generally believed that the chiral-related effects caused by the helical configuration of organic molecules are the most significant. However, compared with a large library of chiral organic molecules, chiral inorganic systems are not common, some natural minerals, such as quartz, are chiral, and some chiral inorganic semiconductors, such as HgS, Sn complexes and Te complexes, but compared with chiral organic semiconductor materials, chiral inorganic semiconductors are rare. In the following, we will briefly introduce the hot chiral metal halide semiconductors that are mainly studied in the field of chiral inorganic semiconductors.
01 Digitization of chiral metal halide semiconductor EMU inspection mark
Halides are compounds containing fluorine (F), chlorine (Cl), bromine (Br), iodine (I), astatine (At) halogen elements (halogens) with negative valence. It is important to note that many metals can form halides, including alkali metals, alkaline earth metals, and halides of lanthanides and actinides.
In simple terms, chiral metal halide semiconductors are organic-inorganic hybrid materials that combine chiral organic molecules or ionic ligands with extended properties of inorganic semiconductors. This hybrid material has properties of inorganic and organic sublattices, but also exhibits fascinating properties due to the unique interaction of inorganic and organic components. The self-assembly of these chiral organic molecules with inorganic subunit units provides an effective strategy for the formation of chiral organic-inorganic hybrid structures that exhibit properties and functions that are not available in individual components. Organic molecules transfer chirality into hybrid structures through interaction with inorganic components, while inorganic subunits give them optoseminational properties. Among the various organic-inorganic hybrid structures, chiral metal halide semiconductors—including but not limited to those with perovskite structures—have recently emerged as promising and interesting materials for controlling light, spin, and charge, and for constructing spin-dependent optoelectronic devices.
Fig.2. Overview of the structure of chiral metal halides
1.1 Design strategies for chiral metal halide semiconductors
In terms of chiral construction of metal halide semiconductor materials, the first method is that the structural flexibility and diversity of metal halide semiconductor materials allow multiple types of chiral cationic organic ligands to be inserted directly into the crystal lattice of metal halide semiconductors, resulting in deformation, distortion or helical arrangement of crystal structures. Thus, the molecular chirality of cationic organic ligands can be transferred to the crystal structure of metal halide components, and induced chirality is generally stronger than that of chiral quantum dots and chiral metal nanoparticles.
The second method for constructing chiral metal halide semiconductors is surface modification by chiral organic ligands. In general, chiral metal halide semiconductors can be synthesized by direct modification of chiral organic ligands present in the reaction, or they can be prepared by ligand exchange reactions after synthesis with chiral organic ligands. For example, Chen et al. used chiral α-octanide to modify CsPbBr nanocrystals to achieve upconversion circularly polarized light emission based on two-photon absorption. They attributed the chiral activity of CsPbBr nanocrystals to surface lattice distortion induced by chiral end-capping ligands.
The third strategy for the construction of chiral metal halide semiconductors is achieved through supramolecular assembly, which can also obtain chiral activity by supramolecular assembly with chiral media. For example, Shi et al. mixed achiral all-inorganic CsPbX semiconductor nanocrystals (X=Cl,Br,I) with a chiral organogelling agent with N,N'-bis(octadecyl)-L-glutamate diamide (LGAm) or its D-configuration enantiomer (DGAm) as the main component in a non-polar solvent to impart polarized emission characteristics to semiconductor nanowafers, and chiral transfer was attributed to the ordered supramolecular assembly of CsPbX semiconductor nanocrystals along chiral components.
1.2 Applications of chiral metal halide semiconductors
To date, the family of chiral metal halide semiconductors has expanded rapidly and is primarily being studied for applications in circularly polarized light detection and optical spintronics.
1.2.1 Photospintronics
Conventional optical detectors in optical spintronics applications require semiconductors coupled to optical polarizers to detect circularly polarized light (CPL), which often limits their sensitivity and resolution. In CPL detectors, direct CPL detection needs to efficiently convert circular dichroism (CD) features into a sufficiently large electrical signal and amplify the difference between chiral CPLs (anisotropy factors). Therefore, high-performance CPL photodetectors require high photocurrent amplitude, large optical responsivity, and low operating voltage. One possible solution is to use a chiral material that can essentially distinguish circularly polarized light. Chiral metal halide semiconductors have become candidates for the fabrication of circularly polarized photodetectors. Chiral low-dimensional chiral metal halide semiconductors have strong chirality and can be used to fabricate CPL photodetectors, including many two-dimensional one-dimensional materials, such as: 2D R-α-(PEA)PbI, 2D (R-Br-PEA) 2PbI, 1D (R-/S-α-PEA)PbI, and 1D (S-NEA)PbI. For example, Ma et al. constructed a CPL photodetector based on the hBN/(R-/S-MBA)PbI/MoS heterostructure, which had a photoresponsivity of 450 Ma W and a specific detection rate of 450 Ma W and 2.2 × 10 Jones, respectively.
Other metal-based chiral metal halide semiconductors such as 0D (zero-dimensional) (MBA) CuCl have also been reported to fabricate CPL photodetectors in heterostructures composed of (MBA) CuCl and single-walled carbon nanotubes (SWCNTs). Electrons excited by left/right-handed CPL in chiral 0D (MBA)CuCl can be rapidly transferred to the SWCNT layer, providing superior polarization-dependent optical responsiveness. The resulting CPL photodetector has a high optical responsivity of 452 A W, a large anisotropy factor of up to 0.21, and a low operating voltage of 0.01 V.
Fig.3 Schematic diagram of chiral transition metal halide circularly polarized luminescent materials (Figure from Sina.com)
1.2.2 Spintronic components
The unique circularly polarized emission spin properties of chiral metal halide semiconductors make them have full application potential in the construction of spin photovoltaic devices and spin valve devices. The spin properties are introduced into semiconductor devices, and the electron charge and spin are used together as the carrier of information, so as to construct an electron spin device. For example, Vardeny and colleagues report spin-dependent photovoltaic devices based on chiral 2D (R-/S-MBA) PbI.
They observed that the photocurrents of the photovoltaic devices differed by about 10% under left-handed CPL and right-handed CPL illumination, which was attributed to chiral-induced spin-selectivity effects. In contrast, achiral (rac-MBA) PbI-based PV devices do not observe a light helix-dependent PV response. In addition, photoelectric measurements show that chiral-induced circular photoelectric effects arise from spin-polarized splitting that occurs in the electron bands of these compounds. Lu et al. fabricated a spin valve with a geometrical structure ITO/(R- or S-MBAPbI/NiFe/Au) using a chiral (MBA) PbI membrane as a sandwich between a ferromagnetic electrode (NiFe) and a non-magnetic electrode (indium tin oxide (ITO)). Magnetoresistive measurements at 10K show that devices based on MHS layers with opposite chirality confirm the spin filtering effect achieved by chiral-induced spin selectivity.
Interestingly, the magnetoresistance exhibits a weak dependence on the thickness of the membrane, unlike conventional spin valve devices, for which the maximum magnetoresistive response typically decreases exponentially with thickness. The weak thickness dependence is attributed to the balance between two competing processes: spin filtering due to chiral-induced spin-selectivity effects (increasing with thickness) and spin relaxation due to spin scattering in thicker films. Therefore, chiral-induced spin-selectivity provides a new strategy for fabricating chiral metal halide semiconductor spin valve devices with potentially unique device physical properties.
02 Summary
This article briefly summarizes the chiral metal halide semiconductor materials, which are part of the large family of chiral materials, and we briefly explain their potential applications according to their most basic properties, especially in the field of spintronics, starting from its construction strategy, aiming to disseminate the concept of metal halide semiconductors to the public, promote the further development of the material, and make the chiral family more and more brilliant.
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