The d electron plays a significant role in determining and controlling the properties of magnetic materials. However, the d electron transitions, especially d–d emission, have rarely been observed in magnetic materials due to the forbidden selection rules. Here, we report an observation of d–d emission in antiferromagnetic nickel phosphorus trisulfides (NiPS3) and its strong enhancement by stacking it with monolayer tungsten disulfide (WS2). We attribute the observation of the strong d–d emission enhancement to the charge transfer between NiPS3 and WS2 in the type-I heterostructure. The d–d emission peak splits into two peaks, D1 and D2, at low temperature below 150 K, from where an energy splitting due to the trigonal crystal field is measured as 105 meV. Moreover, we find that the d–d emissions in NiPS3 are nonpolarized lights, showing no dependence on the zigzag antiferromagnetic configuration. These results reveal rich fundamental information on the electronic and optical properties of emerging van der Waals antiferromagnetic NiPS3.
Van der Waals (vdW) magnetic materials have gained tremendous attention since the discovery of intrinsic ferromagnetic order in atomically thin chromium triiodide (CrI3) and chromium germanium telluride (Cr2Ge2Te6).1–6 Particularly, antiferromagnetic metal phosphorus trichalcogenides (MPX3, M = Mn, Fe, Ni; X = S, Se) are becoming attractive for their promising potential for next-generation spintronics.7–10 Among this family of vdW magnetic materials, nickel phosphorus trisulfide (NiPS3) has been extensively studied in recent years with many intriguing phenomena reported, such as phonon–magnon coupling,11 charge-spin correlation,12 and spin-correlated photon emission.13–15 However, although the unpaired d electrons in transition metal atoms of magnetic materials play crucial roles in determining their properties, studies on the transition between d electronic states of NiPS3, especially the d–d emission, were very limited even though they possess the promising potential to couple to the magnetic property.16,17 For example, previous studies have observed the photoluminescence (PL) due to the d–d transition in vdW magnetic materials CrX3 (X = I, Br), which is spontaneous circularly polarized light determined by the magnetization direction.18,19 However, researchers have revealed the weak absorption of the d–d transitions that are forbidden by selections rules even in bulk NiPS3,20–22 making it arduous to investigate the d–d emission.
To enhance PL intensity, researchers have invented several approaches, such as constructing dielectric or plasmonic nanostructures,23,24 using optical cavity,25,26 and applying external strain.27,28 However, these methods usually require multiple steps of lithography and the PL enhancement that usually happen in localized positions. Compared to the aforementioned methods, vdW heterostructures constructed by stacking vdW materials are used as excellent platforms to tune the properties of the materials due to their convenient fabrication and unique interlayer interactions, such as charge transfer,29–34 proximity effect,35–37 and moiré potential.38–41 Among all types of heterostructures, type-I heterostructure has a straddling band alignment formed by different materials.42 Thus, charge carriers generated in the broader-bandgap layer will transfer to the narrower-bandgap layer when the two layers are in close contact, leading to the increase in the carrier concentration and consequently PL enhancement in the narrower-bandgap layer. Utilizing this feature, the PL enhancement in the narrower-bandgap layer has been reported in many vdW stacked heterostructures, such as MoTe2/WSe2,31 MoSe2/FePS3,32 and InSe/MoS2.43
Here, we report the observation of d–d emission in NiPS3 and its enhancement in type-I vdW heterostructure between NiPS3 and monolayer tungsten disulfide (WS2). A broad emission from NiPS3 with energy of around 1.7 eV is observed. This emission is assigned to the d–d transition of NiPS3 from the 3T1g to the 3A2g state in Ni2+ ions under an octahedral crystal field. Compared to the bare NiPS3 itself, the d–d emission in the WS2/NiPS3 heterostructure is enhanced by ∼10 − 50 times under different energies of laser excitation. The excitation wavelength and power dependence of the emission are investigated, showing that the charge transfer between NiPS3 and WS2 is the origin of the enhancement. We further investigate this d–d emission to reveal the fine energy level structure of the d electrons by performing temperature dependent measurements. A splitting of the d–d emission peak into two peaks is observed below the Néel temperature (i.e., 152 K), showing an energy difference of ∼105 meV between the split 3A2g and 3Eg states due to the trigonal crystal field. Furthermore, polarization measurements on the d–d emission suggest a non-polarized property distinct from the spin-correlated emission reported previously.13–15 This work deepens our understanding about the physical properties of the emerging vdW antiferromagnetic NiPS3 and could serve as a model for the study of other antiferromagnets in the family.
The NiPS3 flakes were prepared by mechanical exfoliation of its bulk single crystals that were synthesized using the chemical vapor transport (CVT) method.44 Detailed characterizations of the bulk crystals are shown in Fig. S1. We used an all-dry transfer method to stack monolayer WS2 onto the top of the NiPS3 flake that was mechanically exfoliated on an SiO2/Si substrate.45 Figure 1(a) shows the schematic of a WS2/NiPS3 van der Waals heterostructure. Figure 1(b) shows a typical optical image of a monolayer WS2 stacked on an NiPS3 flake, showing the heterostructure region and the individual NiPS3 and WS2 regions for a comparison study. Raman spectra and atomic force microscopy (AFM) image of the heterostructure (Fig. S2) show that the WS2 flake is a monolayer, and the NiPS3 flake is about 122 nm thick. Figure 1(c) shows PL spectra under the excitation of 2.33 eV collected from three different regions of the sample at 78 K. The spectrum from monolayer WS2 shows a pronounced excitonic emission at 2.02 eV. The spectrum from the bare NiPS3 region features signals in the range of 1.3–1.5 eV, including the spin-correlated emission (peak X) reported recently.13–15 Note that a very weak emission at 1.7 eV can be observed in the bare NiPS3 down to six layers (Fig. S3), which has not been reported before. The energy of this emission at 1.7 eV matches well with one of the d–d transitions in NiPS3 reported in the literature.16 When stacking WS2 and NiPS3 together, this emission is ∼13 times enhanced under excitation of 2.33 eV in the heterostructure region compared to that in the bare NiPS3. The PL intensity color map in Fig. 1(d) shows that this enhancement occurs in the whole WS2/NiPS3 region, excluding the possibility of a localized phenomenon. The excitonic emission of WS2 is strongly quenched with an energy blue shift in the heterostructure (Fig. S4), indicating a significant interlayer interaction between WS2 and NiPS3.46 These observations clearly show that the interaction between WS2 and NiPS3 plays a key role in enhancing the d–d emission of NiPS3. In addition, we also notice that the emission in the range of 1.3–1.5 eV of NiPS3 does not show changes in the heterostructure, which needs further investigation in future studies.13–15,47
NiPS3 crystal has the layered van der Waals structure belonging to the monoclinic space group C2/m, with the honeycomb structure of Ni2+ ions distributed in the (P2S6)4− dimers (Fig. S1).48,49 The (P2S6)4− dimers create a trigonally distorted octahedral crystal field to the Ni2+ ions, leading to the splitting of the d-orbitals on Ni. Specifically, the degenerated d orbitals of the Ni2+ ion (which forms the 3F state) splits into the ground state 3A2g and the excited states 3T2g and 3T1g under the Oh crystal field. The 3T1g state will further split into 3Eg and 3A2g states because of the trigonal crystal field (D3h) formed by the distortion of the Oh crystal field [Fig. S5(a)].21,50 The electronic transitions between these splitting states, also known as the d–d transitions, are governed by the spin and Laporte selection rules with light excitation.51 Specifically, only the transitions without multiplicity change (ΔS = 0) and with parity change (Δl = ±1) are allowed to occur. The d–d transition between the 3T1g and 3A2g states in NiPS3 with the transition energy of ∼1.7 eV is spin allowed yet Laporte forbidden, resulting in a weak absorption from the room temperature (300 K) to the cryogenic temperature (4.2 K),21 which also accounts for the hardly observable d–d emission [Fig. S5(b)]. However, in the type-I WS2/NiPS3 heterostructure, the 3T1g and 3A2g states of NiPS3 locate inside between the conduction band minimum (CBM) and the valence band maximum (VBM) of WS2 [Fig. 2(a)],52 making the photo-excited electrons and holes transfer readily from the WS2 layer to the NiPS3 layer. This interlayer transfer increases the carrier population on the d states in NiPS3, leading to the enhancement of the d–d emission in NiPS3.
This hypothesis is supported by the excitation wavelength-dependent measurements at 78 K. As shown in Fig. 2(b), we compare the PL spectra from the bare NiPS3 and WS2/NiPS3 heterostructure under the excitations of 1.91, 2.18, 2.33, and 2.71 eV. When the excitation energy (e.g., 2.18, 2.33, and 2.71 eV) is larger than the WS2 bandgap energy (1.95 eV),52 a strong enhancement of the d–d emission is observed in the WS2/NiPS3 heterostructure; when the excitation energy (e.g., 1.91 eV) is lower than the WS2 bandgap energy, no enhancement is observed, where the d–d emission is as weak as that from the bare NiPS3. The reason for this phenomenon is that, only when the excitation energy is higher than the bandgap energy of WS2, the photo-excited electrons and holes can be generated and transferred to NiPS3 for the d–d emission enhancement. As discussed before, the charge transfer from WS2 to NiPS3 also leads to remarkable quenching of the WS2 excitonic emission in the heterostructure region. Figure 2(c) plots the intensity ratio (R) of WS2 emission from the heterostructure region and the bare WS2 region under excitation from 2.18 to 2.71 eV, and the corresponding d–d emission enhancement factor (EF) defined as the intensity ratio of the d–d emission from the heterostructure and the bare NiPS3 region. Note that smaller R corresponds to stronger quenching of the WS2 excitonic emission and larger degree of charge transfer at the WS2/NiPS3 interface. Figure 2(c) clearly shows the correlation between R and EF: the R value reaches the minimum at 2.5 eV where EF reaches the maximum, suggesting that larger degree of charge transfer between the layers would lead to stronger d–d emission of NiPS3.
We further investigate the excitation power dependence of the d–d emission to gain more insight into the photo-physics of the heterostructure. As shown in Fig. 3(a), we measure PL spectra of the WS2/NiPS3 region using a 2.33 eV excitation with power from 10 μW to 2 mW. The PL features of the heterostructure, including the NiPS3 emissions in the energy range of 1.3–1.5 eV, the WS2 excitonic emission at 2.02 eV, and the d–d emission at around 1.7 eV, are observed in the power range we measured. Their intensities all increase with the increase in excitation power. As a comparison, the bare NiPS3 exhibits a weaker d–d emission in the same power range shown in Fig. 3(b). Figure 3(c) plots the integrated PL intensity as a function of the excitation power, which is fitted by the power law , where is the integrated PL intensity, and is the power of the excitation laser. For near-band edge emissions of semiconductors, the dimensionless exponent k is known to be 1 < k < 2 for free-exciton or bound-exciton luminescence, and k < 1 for free-to-bound exciton or donor–acceptor pair recombination luminescence.53,54 Our results show that the exponent k for WS2 exciton in the heterostructure is 1.25, consistent with the previous reports.55,56 The k value for the d–d emission from an uncovered NiPS3 is obtained as 1.13. However, the k value for the d–d emission from the WS2/NiPS3 heterostructure region is obtained as 0.56, suggesting a different mechanism from the d–d emission in the bare NiPS3. Note that such a small k (i.e., k < 1) is a characteristic property of donor–acceptor pair recombination,57–59 which is in consistence with the charge-transfer mechanism we proposed in the type-I NiPS3/WS2 heterostructure in Fig. 2(a): since the 3A2g and 3T1g states of NiPS3 are the narrower components in the type-I band alignment, these states act as the extrinsic acceptor and donor states of the WS2 layer, respectively. As a contrast, the k values of peak X are 0.92 for both the bare NiPS3 and the WS2/NiPS3 heterostructure, matching well with the reported value of 0.916.14 In addition, no noticeable enhancement of peak X emission is observed from the heterostructure compared to the bare NiPS3. The unchanged k value and emission intensity of peak X in the heterostructure imply that the d–d emission may not be coupled to the peak X emission.
Leveraging the enhanced d–d emission in NiPS3, we further investigate the fine d-orbital splitting in NiPS3 by performing temperature-dependent measurements. Figure 4(a) shows the PL spectra of the WS2/NiPS3 heterostructure from 300 K down to 78 K under the excitation of 2.33 eV, where the fitted d–d emission peaks are shown in shadows. When the temperature is below 150 K, the d–d emission splits into two peaks, D1 and D2, with the splitting energy of ∼105 meV [Fig. 4(b)]. These two peaks show blue shift at low temperature, in accordance with the empirical Varshni equation , where is the energy at 0 K, and and are fitting constants (Table S1).60 As we propose that the d–d emission originates from the transition from the 3T1g to 3A2g states, the observation of these two peaks indicates a further splitting of the d electronic states under the trigonal crystal field as shown in Fig. 4(c). The degenerate 3T1g state will split into two states of 3Eg and 3A2g, and the energy difference between these two states has been reported to be around 110 meV by absorption spectra,20,21,61 reflection spectra,22 and calculations.50,61 Here, the D1 and D2 peak positions, as well as their energy difference match well with the transition of 3A2g → 3A2g and 3Eg → 3A2g, respectively, further indicating the d–d transition origin of the emission. Figure 4(d) plots the intensity changes of the D1 and D2 peaks, as well as their intensity ratio (D1/D2), as a function of temperature. The intensities of D1 and D2 are well fitted by the Boltzmann distribution (Table S2).62 As discussed above, these two d–d emissions are supposed to be Laporte forbidden by the selection rules. However, there are two mechanisms that can relax this selection rule. First, the d electronic states can couple with the vibrations (vibronic coupling) in NiPS3, creating an admixture between the states with even and odd parity, thereby partially lifting the parity selection rule.18,63,64 Second, the local inversion symmetry breaking of NiPS3 due to the spin structure emergence also activates the d–d transitions below the Néel temperature (TN),65 which accounts for the abrupt intensity change of the d–d emission at TN for both the bare NiPS3 and WS2/NiPS3 heterostructure as shown in Fig. S6. Note that the d-orbital 3T2g splitting of NiPS3 under the trigonal crystal field has also been reported by electronic Raman spectroscopy, showing that the d-orbital splitting exists at room temperature and does not originate from the magnetic phase transition of NiPS3.64 Thus, we attribute the invisible D2 peak above the TN to the weak signal rather than the magnetic phase transition.
Since the d electron properties play a significant role in the magnetic properties, there remains a question whether the d–d emissions in NiPS3 are correlated with the magnetic configuration. Previous reports have shown that the sharp PL emission at 1.476 eV is linearly polarized due to coupling with the ordered spin in antiferromagnetic NiPS3.13–15 Here, we investigate the polarization of the d–d emissions by detecting the signals from different collective polarizations at temperature of 78 K which is below the Néel temperature (i.e., 152 K). As shown in Fig. S7, we fix the polarization of the excitation laser and selectively allow the pass of certain polarized emission signals arrive to the detector. Figure 5(a) shows the top view of the zigzag configuration of the spin order. Figure 5(b) shows the contour color map of the polarization-resolved PL spectra of the WS2/NiPS3 heterostructure, where the peak X at ∼1.476 eV and d–d emission peaks D1 and D2 exhibit distinct dependences on the collection angles. By fitting their integrated intensities using the function: ( is the rotational angle), peak X shows a clear sinusoidal dependence on the polarization angle [Fig. 5(c)], which was attributed to the correlation with the zigzag spin order as reported in our previous work.14 However, both D1 and D2 peaks do not show polarization characteristics as shown in Fig. 5(c), indicating that they are not correlated with the zigzag spin order. This is consistent with a recent study on the spin dynamics of the same d–d transitions by time-resolved pump-probe measurements.16
In summary, we report the observation of d–d emission in NiPS3 and its strong enhancement through the charge transfer in a type-I WS2/NiPS3 heterostructure. We attribute this d–d emission to the transition between the 3T1g and 3A2g states, which are d states split by the octahedral crystal field. The charge-transfer enhancement mechanism is supported by the excitation wavelength and power dependent measurements. This observation enables in-depth investigation of the d-state properties of NiPS3. Specifically, the low-temperature measurements show that the 3T1g further splits into the 3A2g and 3Eg states with a splitting energy of ∼105 meV due to the trigonal crystal field, evidenced by the splitting of the d–d emission peak into D1 and D2 peaks. The polarization investigation of the d–d emission shows that they are not correlated with the spin configuration, which is distinct from the spin-correlated sharp peak X reported before. Our results reveal rich electronic and optical properties of vdW antiferromagnetic NiPS3, providing a new model system for the study of other antiferromagnetic metal phosphorus trisulfide like manganese phosphorus trisulfide and iron phosphorus trisulfide. The readily formed heterostructure by stacking vdW materials together also shows the convenience and flexibility for property tuning, which offers a powerful platform for both the fundamental study and the application development of antiferromagnetic materials.
Single crystals of NiPS3 were grown via the chemical vapor transport (CVT) technique.44 A stoichiometric quantity of elements (molar ratio Ni:P:S = 1:1:3, 1 g in total) and pure iodine (around 20–30 mg) were sealed into a quartz ampule under pressure pumped down to 10−4 Torr. Then, the ampule was placed in a two-zone furnace and kept at 650–600 °C for a week. The layered NiPS3 crystals were obtained in a low-temperature zone and characterized using multiple tools (Fig. S1). Bulk NiPS3 crystals were exfoliated onto SiO2(90 nm)/Si substrates using the scotch-tape method. Single crystals of WS2 were purchased from HQ Graphene. Monolayer WS2 flakes were exfoliated using a simplified gold-assisted method.45,66 WS2 crystals were first exfoliated by a tape, and then, gold was evaporated and deposited directly onto the tape. Another tape was used to assist the further exfoliation by gold. The structures of tape-gold-crystal were put onto SiO2/Si substrates and then were heated at 120 °C. After 5 min of heating, tapes were peeled off at 120 °C, and then, gold were removed by etching, leaving a large area of monolayer WS2 flakes on substrates (Fig. S8). Heterostructures were prepared by dry-transferring45 monolayer WS2 onto the top of the NiPS3 flakes.
Optical measurements were conducted on a micro-Raman spectrometer (Horiba-JY T64000) and collected by a spectrometer with a liquid nitrogen cooled CCD camera. Samples were put in a cryostat (Cryo Industries of America, Inc.) with temperature ranging from 78 to 300 K. A 532 nm laser line was generated by MSL-FN-532 (CNI), and all other lasers were generated by a Kr+/Ar+ ion laser (Coherent Innova 70C Spectrum). All Raman signals were collected with a 1800 g/mm grating, and all PL signals were detected with a 150 g/mm grating.
Polarization angle resolved PL measurements
The polarization angle resolved PL measurements were performed by adding a linear polarizer and a half-wave plate in the collective path. A 532 nm laser was used as incident excitation. Samples were kept at 78 K, and the signals were collected by a 50× long-working-distance objective and a 150 g/mm grating.
See the supplementary material for additional information and figures.
This material is based upon work supported by the National Science Foundation (NSF) under Grant No. 1945364. X.L. acknowledges the membership of the Photonics Center at Boston University. T.L. and J.C. acknowledge the support from the Department of Energy (DOE) under Award No. DE-SC0021064.
Conflict of Interest
The authors have no conflicts to disclose.
Q.T. and X.L. conceived the idea of this work. Q.T. and W.L. prepared and characterized monolayer WS2. Q.T., T.L., J.C., H.K. and X.W. prepared and characterized NiPS3 samples. Q.T. and T.L. fabricated the heterostructure by dry transfer method. Q.T. and H.K. conducted the optical measurements. Q.T. and X.L. analyzed and interpretated the data. Q.T. and X.L. wrote the manuscript with contributions from all authors.
Qishuo Tan: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal). Weijun Luo: Data curation (equal); Formal analysis (supporting); Writing – review & editing (equal). Tianshu Li: Data curation (supporting); Writing – review & editing (supporting). Jun Cao: Data curation (supporting); Writing – review & editing (supporting). Hikari Kitadai: Data curation (supporting); Formal analysis (supporting); Writing – review & editing (supporting). Xingzhi Wang: Data curation (supporting); Formal analysis (equal); Methodology (equal); Supervision (equal); Writing – review & editing (equal). Xi Ling: Conceptualization (lead); Data curation (supporting); Formal analysis (equal); Funding acquisition (lead); Investigation (equal); Methodology (equal); Project administration (lead); Resources (equal); Supervision (lead); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).
The data that support this study are available from the corresponding author upon reasonable request.