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Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator

Nature Nanotechnology volume 16, pages 655–660 (2021)Cite this article

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Abstract

Two-dimensional (2D) semiconductors enable the investigation of light–matter interactions in low dimensions1,2. Yet, the study of elementary photoexcitations in 2D semiconductors with intrinsic magnetic order remains a challenge due to the lack of suitable materials3,4. Here, we report the observation of excitons coupled to zigzag antiferromagnetic order in the layered antiferromagnetic insulator NiPS3. The exciton exhibits a narrow photoluminescence linewidth of roughly 350 μeV with near-unity linear polarization. When we reduce the sample thickness from five to two layers, the photoluminescence is suppressed and eventually vanishes for the monolayer. This suppression is consistent with the calculated bandgap of NiPS3, which is highly indirect for both the bilayer and the monolayer5. Furthermore, we observe strong linear dichroism (LD) over a broad spectral range. The optical anisotropy axes of LD and of photoluminescence are locked to the zigzag direction. Furthermore, their temperature dependence is reminiscent of the in-plane magnetic susceptibility anisotropy. Hence, our results indicate that LD and photoluminescence could probe the symmetry breaking magnetic order parameter of 2D magnetic materials. In addition, we observe over ten exciton-A1g-phonon bound states on the high-energy side of the exciton resonance, which we interpret as signs of a strong modulation of the ligand-to-metal charge-transfer energy by electron–lattice interactions. Our work establishes NiPS3 as a 2D platform for exploring magneto-exciton physics with strong correlations.

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Fig. 1: Anisotropic antiferromagnetic excitons in thin bulk NiPS3 crystal.
Fig. 2: Thickness-dependent exciton luminescence and electronic structure.
Fig. 3: Exciton-zigzag antiferromagnetic order coupling in a five-layer sample.
Fig. 4: Multiple exciton–phonon bound states.

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Data availability

All other data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.

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Acknowledgements

This work was mainly supported by the Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division (DE-SC0012509 and DE-SC0018171). Device fabrication and part of PL measurement were supported by Air Force Office of Scientific Research Multidisciplinary University Research Initiative program, grant no. FA9550-19-1-0390. Bulk crystal growth is supported by grant no. NSF MRSEC DMR-1719797 and the Gordon and Betty Moore Foundation’s EPiQS Initiative, grant no. GBMF6759 to J.H.C. Y.W. is supported by NSFC Projects (grant nos. 61674083 and 11604162). We also acknowledge the use of the facilities and instrumentation supported by grant no. NSF MRSEC DMR-1719797. X.X. and J.H.C. acknowledge the support from the State of Washington funded Clean Energy Institute.

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Author notes

  1. These authors contributed equally: Kyle Hwangbo, Qi Zhang.

Authors and Affiliations

  1. Department of Physics, University of Washington, Seattle, WA, USA

    Kyle Hwangbo, Qi Zhang, Qianni Jiang, Jordan Fonseca, Geoffrey M. Diederich, Jiun-Haw Chu & Xiaodong Xu

  2. School of Physics, Nankai University, Tianjin, China

    Yong Wang

  3. Department of Physics, Carnegie Mellon University, Pittsburgh, PA, USA

    Chong Wang & Di Xiao

  4. Department of Chemistry, University of Washington, Seattle, WA, USA

    Daniel R. Gamelin

  5. Department of Physics and Center of Theoretical and Computational Physics, University of Hong Kong, Hong Kong, China

    Wang Yao

  6. Department of Materials Science and Engineering, University of Washington, Seattle, WA, USA

    Xiaodong Xu

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Contributions

X.X., Q.Z. and K.H. conceived the experiment. K.H. and Q.Z. fabricated samples and performed optical measurements, assisted by J.F. and G.M.D. All authors contributed to the data analysis and interpretation. Y.W. and W.Y. performed band-structure calculation. C.W. and D.X. calculated the optical anisotropy. Q.J. and J.-H.C. synthesized and characterized the bulk crystals. K.H., Q.Z., X.X. and J.F. wrote the paper with input from all authors. All authors discussed the results and commented on the manuscript.

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Correspondence to Xiaodong Xu.

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Extended data

Extended Data Fig. 1 Polarization-resolved Raman spectroscopy, optical reflection and photoluminescence measurements of the same thin-bulk NiPS3 flake.

a, Co-linearly polarized Raman scattering (the XX channel). b, Raman spectra at 0°, 45°, and 90° of linear polarization. c, Polar plot of 178.3 cm−1, and d, 180.8 cm−1 Raman mode intensity as function of linear polarization angle. Data is taken at 15 K. e, Offset optical reflection at 633 nm as function of linear polarization angle. The positive lobes indicate the high reflection direction. f, Polar plot of photoluminescence intensity as a function of linear polarization detection angle. Near unity linearly polarized photoluminescence is observed along the vertical direction. Horizontal (H) and vertical (V) polarization direction is defined along 0° and 90° in this figure, respectively.

Extended Data Fig. 2 Photoluminescence as a function of excitation linear polarization angle.

a, Photoluminescence intensity plot as a function of photon energy and excitation polarization. Detection is unpolarized. b, Polar plot of peak intensity vs excitation polarization.

Extended Data Fig. 3 Temperature- and polarization-resolved photoluminescence from a bulk NiPS3 flake.

The excitation laser is at 633 nm and vertically polarized. Photoluminescence intensity plot with a, vertically and b, horizontally polarized detection. The main excitonic peak is strongly linearly polarized. There are low energy and very weak photoluminescence features. These states are more visible in c, temperature dependent degree of linear polarization plot. The first three features have energy separation of about 10 ~ 12 meV, implying that the two low energy features are phonon replica of the exciton. The feature at 1.44 eV has stronger photoluminescence intensity than the first two while having opposite polarization, indicating that it has a different origin from the first two. We speculate that this state may arise from the electric dipole forbidden d-d transitions localized in the Ni2+. The identification of the exact origin of these states requires future efforts.

Extended Data Fig. 4 Comparison of temperature dependent exciton photoluminescence properties with in-plane magnetic susceptibility anisotropy Δχ.

Δχ is the magnetic susceptibility difference between a and b axes of bulk crystal NiPS3, extracted from ref. 19. The photoluminescence measurements are done on the sample presented in Fig. 1e in the main text. a and b overlays Δχ (red) with degree of linear polarization (DOP, blue) and linear dichroism (LD, blue) as a function of temperature, respectively. The temperature dependent behavior of DOP and LD resembles that of Δχ, supporting the in-plane magnetic susceptibility anisotropy as their cause. Error bars for DOP were obtained from a Lorentzian fit of the PL signals. Temperature dependent c, photoluminescence intensity and d, full width at half maximum (FWHM). FWHM is not shown above 120 K as the peak shape was not suitable for fitting. Error bars in c and d signify the confidence bounds of a Lorentzian fit.

Extended Data Fig. 5 DFT calculated band structure and oscillator strength of inter-band optical transition.

a–f, layer number dependent band structure. The rectangular unit cell and Brillouin zone have been exploited during the calculations. The k path for the band is along M-Y-Γ-X, where Γ = (0,0), X = (π/a,0),Y=(0, π/b), M = (π/a,π/b). The zero energy has been set to the conduction band minimum at Y point for comparison. The highest energy valence band is near the X point, indicated in the figure. We can see that while the lowest energy conduction band is at Y point in monolayer and bilayer, an energy minimum between Γ and X, indicated by the arrow in the trilayer band structure, appears as the layer number increases to 3 and above. Therefore, NiPS3 experiences a transition from indirect in monolayer and bilayer to less indirect bandgap in trilayer and above. This transition is likely responsible for the observed layer thickness dependent exciton photoluminescence, which manifest most strongly between trilayer and bilayer. g–i, Oscillator strength along the zigzag (a) and armchair (b) direction in the vicinity of the X point. We have averaged the oscillator strength over the quasi-degenerate bands near the band edges, that is, \(\frac{1}{{32}}\mathop {\sum}\nolimits_{v,c} {|\langle ck|\hat v|vk\rangle |^2}\), where v runs over the 4 near degenerate valence bands and c runs over 8 nearly degenerate conduction bands. The unit of the averaged velocity matrix elements is (eV · Å)2. The large difference of the oscillator strength between zigzag and armchair, that is (zigzag-armchair)/(zigzag+armchair), represents highly anisotropic states due to the zigzag antiferromagnetic order. This is consistent with the observed exciton LD.

Extended Data Fig. 6 Additional measurements of atomically thin samples.

Polarization resolved photoluminescence spectra of a, bilayer and b, trilayer at 15 K. c, Comparison of the photoluminescence spectra with and without hBN encapsulation of a trilayer. There is no appreciable linewidth difference between the two. d, Temperature dependent linear dichroism of bilayer, trilayer, and thin bulk crystal. Error bars were obtained from a sinusoidal fit of the rotational LD measurements.

Extended Data Fig. 7 Comparison of temperature dependent optical properties of a five-layer sample with in-plane magnetic susceptibility anisotropy Δχ of bulk crystal NiPS3.

a, Temperature dependent energy splitting ΔP of the 180 cm−1 Raman mode. ref. 21 shows that ΔP can be used to probe the zigzag antiferromagnetic order. Here, we compare ΔP with Δχ. Remarkably, both physical quantities share similar temperature dependent behavior. This comparison shows that ΔP originates from Δχ, and Δχ in five-layer sample is similar to that of the bulk crystal. Error bars were obtained from a Lorentzian fit of the Raman peaks. b, Temperature dependent degree of linear polarization (DOP) and linear dichroism (LD) overlaid with Δχ. Both DOP and LD resemble Δχ, although DOP are limited below 100 K due to the vanishing photoluminescence above 100 K. c, Temperature dependent photoluminescence intensity and d, full width at half maximum (FWHM). Error bars in figures c and d represent the confidence bounds of a Lorentzian fit. e, Photoluminescence spectra and their respective Lorentzian fits at select temperatures.

Extended Data Fig. 8 Polarization-resolved optical reflection and its connection to linear dichroism spectrum.

a, Polarization dependent linear dichroism (LD) spectra. Black dashed line corresponds to LD presented in Fig. 4a (main text), which is obtained by the excitation along the vertical axis. b, Red and blue curves are vertically and horizontally polarized optical reflection spectra, overlaid with the LD spectrum (black curve). Two resonances are observed30. The low energy resonance near 1.4815 eV has much stronger polarization dependence than the one near 1.504 eV. The difference of these two peaks partially contribute to the observed LD lineshape.

Extended Data Fig. 9 Magnetic-field dependent photoluminescence and linear dichroism.

a, Photoluminescence spectra of a five-layer sample at selected magnetic fields. b, Photoluminescence intensity plot of the same 5L sample as a function of magnetic field and photon energy. c, LD of a bulk crystal at selected magnetic fields. d, LD intensity plot as a function of magnetic fields and photon energy. All experiment data are taken at 15 K.

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Source data for main Fig. 1d–f.

Source Data Fig. 2

Source data for main Fig. 2a–c.

Source Data Fig. 3

Source data for main Fig. 3a–d.

Source Data Fig. 4

Source data for main Fig. 4a–e.

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Hwangbo, K., Zhang, Q., Jiang, Q. et al. Highly anisotropic excitons and multiple phonon bound states in a van der Waals antiferromagnetic insulator. Nat. Nanotechnol. 16, 655–660 (2021). https://doi.org/10.1038/s41565-021-00873-9

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