The band-edge excitons observed in few-layer NiPS3

Abstract

Band-edge excitons of few-layer nickel phosphorous trisulfide (NiPS3) are characterized via micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Prominent μTR features of the A exciton series and B are simultaneously detected near the band edge of NiPS3. The A exciton series contains two sharp A1 and A2 levels and one threshold-energy-related transition (direct gap, E∞), which are simultaneously detected at the lower energy side of NiPS3. In addition, one broadened B feature is present at the higher energy side of few-layer NiPS3. The A series excitons may correlate with majorly d-to-d transition in the Rydberg series with threshold energy of E∞ ≅ 1.511 eV at 10 K. The binding energy of A1 is about 36 meV, and the transition energy is A1 ≅ 1.366 eV at 300 K. The transition energy of B measured by μTR is about 1.894 eV at 10 K. The excitonic series A may directly transit from the top of valence band to the conduction band of NiPS3, while the B feature might originate from the spin-split-off valence band to the conduction band edge. The direct optical gap of NiPS3 is ~1.402 eV at 300 K, which is confirmed by μTR and transmittance experiments.

Spectroscopy and Structural Investigation of Iron Phosphorus Trisulfide—FePS3

“…Monotonic absorption edge blueshift upon cooling in the range covering the PM-AFM transition has already been observed for other MPX 3 materials, including MnPS 3 [51,52] and NiPS 3 . [53] This observation might be a link between the magnetic ordering and changes in the band structure of the material resulting from crystal lattice distortion, as previously reported based on temperature-dependent structural studies. [39] The d-d transition peak energy, depicted by a purple curve in Figure 6a, goes parallel with the temperature dependence of the fundamental absorption edge, ranging from 1.095 to 1.055 eV.…”

“…[ 24,59 ] Scarce information about the Fröhlich coupling can be found for MPX 3 materials, though recent analysis of temperature‐dependent absorption and modulated thermoreflectance spectra have been reported on NiPS 3 . [ 53 ] Significantly higher electron‐phonon interaction strength of (1800 ± 800) meV for one of the optical transitions has been explained by flat spin‐orbit split‐off band dispersion. Our γ LO value of (520 ± 2) meV can be explained in a similar matter taking into account the localized character of the d ‐ d transition [ 60 ] and therefore complements the physical understanding of physical processes affecting the optical activity of transition metal phosphorus trisulfides.…”

Limited Ferromagnetic Interactions in Monolayers of MPS3 (M = Mn and Ni)

“… 14 In addition, theoretical predictions point to the existence of a large binding energy of excitons in MnPS 3 , whereas the experimental reports have observed excitons in few layers of NiPS 3 strongly related to magnetic order. 13 , 15 , 16 Recent reports have demonstrated an all-optical control of the magnetic anisotropy in NiPS 3 by tuning the photon energy in resonance with an orbital transition between crystal field-split levels. 17 The aforementioned demonstrates that this family of compounds is an ideal platform to study correlation effects in the true 2D limit.…”

Electronic Raman scattering in the 2D antiferromagnet NiPS 3

“…Because of its antiferromagnetic nature, the optical properties of NiPS 3 , such as low-frequency Raman modes and optical absorption, have showed correlations to the magnetic structure and exhibit distinct behaviors with variations of temperature and external field ( 6 , 7 ). Specifically, it has been demonstrated that the excitonic emission in NiPS 3 is strongly correlated to the spin structure, which gives rise to a distinct linear polarization of the excitonic emission ( 8 – 10 , 43 ). However, the correlation between spins and d electrons remains unexplored, especially in low-dimensional structures.…”