Opto-electronic and magnetic characteristics induced by noncentrosymmetry in two-dimensional organic-inorganic hybrid perovskites

概要

Introduction.
Low-dimensional (LD) materials are one of the fascinating platforms for the exploration of novel physical functionalities due to their increasing accessible void space for guest molecules/ions.1,2,3 In particular, the interplay between host and guest is noteworthy for advanced applications. On the other hand, noncentrosymmetry in materials is a key factor to provide unique physical properties, which has been demonstrated in specific properties such as ferroelectricity4 and circularly polarized luminescence.5 The objective in this thesis’s work is to generate noncentrosymmetry-induced properties by introducing chiral molecules into LD materials (Figure 1). As the materials platform for controlling noncentrosymmetry in this work, organic–inorganic hybrid perovskites (OIHPs), which are a sort of materials displaying high performance and power conversion efficiency as solar cells,6 were used.

Methods.
The introduction of noncentrosymmetry into OIHPs was rationally achieved by insertion of chiral cations compensating total charge. The class of two-dimensional (2D) OIHPs allows the intercalation of large guests in their van der Waals vacancy. Thus, the centrosymmetry breaking could be guaranteed if the inserted guests/ions possess chirality. In this work, novel noncentrosymmetry-induced properties were developed based on the functions of inorganic backbone of OIHPs, including semiconducting and ferromagnetic behavior of lead(II) halides and copper(II) halides, respectively.

Results and discussion.
By intercalating chiral (R)/(S)-β-methylphenethylammonium ((R)/(S)-MPA+) with lead(II) iodides, noncentrosymmetric ((R)/(S)-MPA)2PbI4 (R/S-1) were synthesized. Both compounds crystalize in the triclinic space group P1, which indicates the chiral and polar nature. The semiconducting behavior was confirmed by absorption spectra and photocurrent measurements under applied electric fields. In the expanded view of I−V curves, zero-bias photocurrent (I0) is clearly observable with ca. 0.1 nA when the electric polarization (P) direction is set parallel to the applied electric field (Figure 2a). This indicates the occurrence of bulk photovoltaic effect (BPVE). Notably, the sign of I0 is opposite between R-1 and S-1 which possesses not only different chirality but also opposite direction of P. To clarify the origin of BPVE, measurements with P being placed vertically to the applied field were also conducted as shown in Figure 2b, where apparently I0 vanishes in both compounds. This implies the origin of BPVE from polarity despite existence of chirality dependence.

The induction of I0 was also attempted by picking up spin-orbit coupling (SOC) in noncentrosymmetric systems. Spin-splitting of band structure is expected when both effective SOC and noncentrosymmetry exist in materials. In this condition, flowing of I0 with spin polarization is possible when the electrons are excited by circularly polarized light (CPL). Such phenomenon is known as circular photogalvanic effect (CPGE). By using (R)/(S)-1-(4- bromophenyl)ethylammnium ((R)/(S)-BrPEA+) ions, a pair of chiral but nonpolar OIHPs ((R)/(S)-BrPEA)2PbI4 (R/S-2) crystallizing in tetragonal space groups P43212/P41212 were synthesized. Band structure calculation and CD spectra confirm the spin-splitting and effective chirality in inorganic component, respectively. The results of photocurrent measurement by using CPL reveal the detection of CPGE (Figure 3). Interestingly, the opposite sign of CPGE I0 between R-2 and S-2 indicates the chirality dependence of CPGE which has never been reported so far.

Noncentrosymmetry was also introduced to magnetic systems by the insertion of (R)-MPA+ ion into copper(II) halides to yield 2D OIHPs ((R)-MPA)2CuX4 (X = Br for Br and Cl for Cl). Competition between ferromagnetic exchange interaction and Dzyaloshinskii-Moriya interaction (DMI) induced by noncentrosymmetry could produce novel magnetic structures, such as spiral spin states and magnetic skyrmion. In Br, possible spin-spiral-related magnetic phases were observed (Figure 4a). To confirm the origin of spiral-spin-related magnetic phases, a centrosymmetric OIHP compound using 4-phenylbutylammonium (PBA+) ion formulated as (PBA)2CuBr4 (PBA- Br) was synthesized for comparison. As shown in Figure 4b, PBA-Br behave as a simple ferromagnet. This result indicates the importance of centrosymmetry breaking for spiral-spin-related magnetic phases.

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