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Chiral molecular intercalation superlattices

Abstract

The discovery of chiral-induced spin selectivity (CISS) opens up the possibility to manipulate spin orientation without external magnetic fields and enables new spintronic device designs1,2,3,4. Although many approaches have been explored for introducing CISS into solid-state materials and devices, the resulting systems so far are often plagued by high inhomogeneity, low spin selectivity or limited stability, and have difficulties in forming robust spintronic devices5,6,7,8. Here we report a new class of chiral molecular intercalation superlattices (CMIS) as a robust solid-state chiral material platform for exploring CISS. The CMIS were prepared by intercalating layered two-dimensional atomic crystals (2DACs) (such as TaS2 and TiS2) with selected chiral molecules (such as R-α-methylbenzylamine and S-α-methylbenzylamine). The X-ray diffraction and transmission electron microscopy studies demonstrate highly ordered superlattice structures with alternating crystalline atomic layers and self-assembled chiral molecular layers. Circular dichroism studies show clear chirality-dependent signals between right-handed (R-) and left-handed (S-) CMIS. Furthermore, by using the resulting CMIS as the spin-filtering layer, we create spin-selective tunnelling junctions with a distinct chirality-dependent tunnelling current, achieving a tunnelling magnetoresistance ratio of more than 300 per cent and a spin polarization ratio of more than 60 per cent. With a large family of 2DACs of widely tunable electronic properties and a vast selection of chiral molecules of designable structural motifs, the CMIS define a rich family of artificial chiral materials for investigating the CISS effect and capturing its potential for new spintronic devices.

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Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author on reasonable request.

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Acknowledgements

We acknowledge the helpful discussions with T. Liu, and Electron Imaging Center for NanoMachines (EICN) at California NanoSystems Institute (CNSI) and Nanoelectronics Research Facility (NRF) at UCLA for technical support. The cross-sectional STEM experiments were partly conducted using the facilities and instrumentation at the UCI Irvine Materials Research Institute (IMRI), which is supported in part by the National Science Foundation through the UCI Materials Research Science and Engineering Center (DMR-2011967). Xiangfeng Duan acknowledges partial support from the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Science and Engineering through award DE-SC0018828. Y.H. acknowledges financial support from the Office of Naval Research through award N00014-18-1-2491. X.P. acknowledges financial support from US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under grant no. DE-SC0014430. Xidong Duan acknowledges the support from the National Natural Science Foundation of China (grant number 51872086) and the Innovative Research Groups of Hunan Province (grant 2020JJ1001). Z.S. was supported by the Czech Science Foundation (GACR no. 20-16124J).

Author information

Authors

Contributions

Xiangfeng Duan conceived the research. H.R., Z.W. and Q.Q. designed the experiments. H.R. developed synthetic methods of CMIS, performed XRD, AFM, CD and Raman characterizations and analysed the data. Q.Q. and Z.W. performed device fabrication, electrical measurements and data analysis with help from P.W., and Jingyuan Zhou contributed to discussions and helped analyse the data. Jingxuan Zhou performed the TEM characterization. X.Y. and X.P. performed the cross-sectional STEM characterization. Bailing Li, Bo Li and Xidong Duan prepared Cr3Te4 nanoplates. Z.S. provided H-TaS2 and T-TaS2 crystals. J.C. performed the SEM characterization. Y.H. and Xiangfeng Duan supervised the research. Z.W., H.R., Q.Q. and Xiangfeng Duan co-wrote the manuscript with input from all of the authors. All authors discussed the results and commented on the manuscript.

Corresponding author

Correspondence to Xiangfeng Duan.

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The authors declare no competing interests.

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Nature thanks Ron Naaman and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data

is available for this paper at https://doi.org/10.1038/s41586-022-04846-3.

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Extended data figures and tables

Extended Data Fig. 1 Characterizations of CMIS layer expansion.

a, The evolution of XRD patterns of R-MBA intercalated H-TaS2 as a function of reaction time. The XRD patterns show that the original set of (00l) peaks gradually dwindle and eventually completely vanish with increasing intercalation duration, whereas a new set of (00l) peaks of the intercalation materials grows and eventually evolves into the only set of diffraction peaks after 48 h of intercalation, indicating the complete intercalation to form phase-pure intercalation superlattices. b, XRD patterns of the R-DMPEA/H-TaS2 and S-DMPEA/H-TaS2 CMIS and intrinsic H-TaS2. c, XRD patterns of the L-histidine/H-TaS2 CMIS and intrinsic H-TaS2. XRD results show that diffraction peaks of (00l) planes of DMPEA/H-TaS2 and L-histidine/H-TaS2 are considerably shifted to lower diffraction angles with an interlayer expansion of 11.6 Å and 4.5 Å, respectively, when compared with those of the intrinsic bulk H-TaS2 (b), suggesting the successful intercalation of these molecules in H-TaS2.

Extended Data Fig. 2 STEM image and SAED patterns of the intrinsic H-TaS2.

a, STEM image of the basal plane of intrinsic H-TaS2. Blue and yellow dots represent tantalum and sulfur, respectively. Scale bar, 1 nm. b, SAED patterns of intrinsic H-TaS2. Scale bar, 2 nm−1. The in-plane lattice parameter remains unchanged between H-TaS2 and MBA/TaS2 CMIS, confirming that the H-TaS2 host layers retain the original atomic structure with no notable lattice distortion after the formation of CMIS.

Extended Data Fig. 3 CD spectra of CMIS with different chiral molecule intercalations.

a, CD spectra of chiral molecules dispersed in IPA and pure IPA. All the CD spectra are acquired by using quartz cuvette. The concentrations of R-MBA and S-MBA molecules are about 0.5 mmol ml−1. b, CD spectra of R-DMPEA/H-TaS2 and S-DMPEA/H-TaS2 CMIS and intrinsic H-TaS2. c, CD spectra of L-histidine/H-TaS2 CMIS and intrinsic H-TaS2.

Extended Data Fig. 4 Further characterizations of Cr3Te4 and non-chiral junctions.

a, Temperature-dependent anomalous Hall resistance of a representative Cr3Te4 nanoplate, showing a coercive field of 0.5 T at 10 K and a Curie temperature of around 200 K. The coercive field is consistent with the abrupt changes of the conductivity observed in the STJ device. The large hysteresis loops of the anomalous Hall resistance suggest a robust out-of-plane ferromagnetic ordering at low temperature, which is required for the vertical STJ. b, Two-terminal resistance of a typical Cr3Te4 nanoplate at 10 K, showing typical resistances <3 kΩ. c, The zoomed-in plot of b between 2.4 and 2.6 kΩ, with a <100 Ω (<1% of the resistance of Cr3Te4) smooth change of the resistance around the coercive field. The resistance variation around the coercive field is about 104 times smaller than the resistance change observed in the STJ, suggesting that the tunnelling conductance across the STJ is dominated by vertical transport across the superlattices and the serial resistance of the lateral transport through the Cr3Te4 nanoplate is negligible. d,e, Magnetic-field-dependent tunnelling current measurements of an intrinsic H-TaS2/Cr3Te4 (d) and a rac-MBA/H-TaS2/Cr3Te4 device (e), respectively. Here rac-MBA is the racemic mixture of R-MBA and S-MBA and the bias voltage applied is 0.1 V. The intrinsic H-TaS2/Cr3Te4 device showed around three orders of magnitude higher current than that of the CMIS device, which is not surprising because both Cr3Te4 and TaS2 are either metal or semimetal. The device with rac-MBA/H-TaS2 intercalation superlattice showed a similar current level to that of the CMIS devices. In both cases, the tunnelling magnetoresistance experiments do not show apparent changes with magnetic field, which is clearly different from that with the CMIS devices.

Extended Data Fig. 5 Comparison between 2-terminal and 4-terminal measurements.

a,b Schematic drawing of the 2-terminal and 4-terminal measurement set-ups, respectively. c, IV measurements of the 2-terminal and 4-terminal configurations under different magnetic field directions. d, Magnetic-field-dependent tunnelling conductance determined from the 2-terminal and 4-terminal configurations at the bias of 0.1 V. 2T, 2-terminal; 4T, 4-terminal. Our 4-terminal and 2-terminal measurements essentially gave the same results, indicating negligible contribution from the contact resistance or the series resistance from the Cr3Te4 and H-TaS2. Compared with the overall resistance of the CMIS junction (typically larger than 10 MΩ at 10 K), the contribution from the contact and series resistance is estimated to be only about 0.1% of the total measured resistance in the 2-terminal measurement and, thus, would not affect our interpretation.

Extended Data Fig. 6 Bias-dependent magnetoresistance ratio measured at different temperatures.

ad, Bias-dependent IV at different temperatures. e,f, The corresponding bias-dependent magnetoresistance ratio MR%. The MR% values at the low-bias regime (<0.050 V) are omitted, which cannot be reliably determined owing to low tunnelling current.

Extended Data Fig. 7 Further temperature-dependent studies for MBA/H-TaS2 STJ devices.

a, Additional temperature-dependent curves for S-MBA/H-TaS2 STJ devices. Data were taken on the same device as shown in Figs. 4d,f and 5. b, The corresponding temperature-dependent conductance of the high current state and the low current state. c, Temperature-dependent GSI. The black dashed line is the fitting between 50 K and 300 K with the activation function of $${G}_{{\rm{SI}}}(T){{=G}_{0}{\rm{e}}}^{\frac{-{E}_{{\rm{A}}}}{{k}_{{\rm{B}}}T}}$$, in which G0 is a normalization factor and kB is the Boltzmann constant. The resulting activation energy EA = 12 meV. d, Temperature-dependent conductance of the high current state and the low current state. Data were taken on the same device as shown in Fig. 4c,e for R-MBA/H-TaS2 CMIS. e, Temperature-dependent GSI. The black dashed line is the fitting with the Arrhenius function, which yields an activation energy EA = 16 meV. The activation energy is slightly higher than the S-MBA/H-TaS2 CMIS, which is consistent with the smaller conductivity observed in this device.

Extended Data Fig. 8 Temperature-dependent polarization for different devices.

The temperature-dependent polarization shows a similar trend between different devices, in which the polarization reduces with increasing temperature. Device 1 and Device 2 are reported in Fig. 4c,d, respectively.

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Qian, Q., Ren, H., Zhou, J. et al. Chiral molecular intercalation superlattices. Nature 606, 902–908 (2022). https://doi.org/10.1038/s41586-022-04846-3

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• DOI: https://doi.org/10.1038/s41586-022-04846-3

• Two-dimensional superlattices intercalated with chiral molecules for spin selectivity

• Wen Su
• Fanglong Yuan

Science China Materials (2022)