Intercalation offers a promising way to alter the physical properties of two-dimensional (2D) layered materials. Here, we investigate the electronic and vibrational properties of 2D layered MoSe2 intercalated with atomic manganese at ambient and high pressure up to 7 GPa by Raman scattering and electronic structure calculations. The behavior of optical phonons is studied experimentally with a diamond anvil cell and computationally through density functional theory calculations. Experiment and theory show excellent agreement in optical phonon behavior. The previously Raman inactive A2u mode is activated and enhanced with intercalation and pressure, and a new Raman mode appears upon decompression, indicating a possible onset of a localized structural transition, involving the bonding or trapping of the intercalant in 2D layered materials. Density functional theory calculations reveal a shift of the Fermi level into the conduction band and spin polarization in MnxMoSe2 that increases at low Mn concentrations and low pressure. Our results suggest that intercalation and pressurization of van der Waals materials may allow one to obtain dilute magnetic semiconductors with controllable properties, providing a viable route for the development of new materials for spintronic applications.

Manganese incorporation in two-dimensional layered and semiconducting materials has received increasing attention as it shows promise for revolutionizing advancements in spintronics1–4 and nanostructures with ferromagnetic ordering5–7 and tunable functionalities.7–9 These perspectives have inspired doping investigations that revealed a plethora of unique magnetic and opto-electronic behaviors. Theory and experiment have shown that Mn-doping can lead to ferromagnetic ordering in 2D materials.6,7,9 A recent study of the 2D dilute magnetic semiconductor Mn-doped MoS2 predicts that MoS2 doped to 10%–15% manganese is ferromagnetic at room temperature.5 Intrinsic ferromagnetism in MnxMo1−xS2 nanosheets, doped by supercritical hydrothermal methods, was reported.10 Mn-doping in MoSe2 has also been shown to promote additional active sites for hydrogen evolution reactions.11 It was also reported that MnBi2Te4, an intrinsic magnetic topological insulator, is an ideal platform to realize high-temperature quantum anomalous Hall insulator states.12 

In layered materials, Mn-intercalation offers a unique alternative to Mn-doping. Intercalation, the insertion of an atom or molecule into the van der Waals gap, provides a chemical handle to tune physical properties including electronic structure and phonon propagation without disturbing the host lattice.13–15 Intercalation in layered materials has demonstrated an enormous realm of physical and chemical tunability in both current and historical research.13,15–19 Through intercalation, it is possible to adjust the superconducting temperature,15,20 enhance transparency and conductivity,21–24 and reversibly alter optoelectronic behaviors including color and photoluminescence.22,23,25 Recently, a wet chemical route was achieved to intercalate zero-valent manganese, post-growth, into 2D layered materials,13 opening a new avenue for the experimental study of manganese incorporated two-dimensional materials beyond that of doping.

Molybdenum diselenide (MoSe2) is a heavily investigated layered n-type indirect bandgap semiconductor (Eg = 1.1 eV) that shows a transition to a direct bandgap with reduction in the number of layers.26–28 In a recent report, spin states protected from intrinsic electron–phonon coupling were demonstrated in monolayer MoSe2, reaching 100 ns lifetimes at room temperature.29 High-pressure investigations have shown that MoSe2 does not undergo any phase transitions up to 30 GPa.30 Above 40 GPa, a possible semiconductor–metal phase transition has been identified.31,32 The effect of Mn-intercalation on the pressure-induced metallization of MoSe2 is a point of interest, as the intercalated metal may alter the metallization behavior of van der Waals systems.33 Whether pressure favors magnetic ordering at ambient temperature is another point of interest as an analogous mechanism was observed with a high concentration of substitutional Mn in monolayer MoS2.5 Pressure results in greater wavefunction overlap that could lead to a stronger coupling between the isolated Mn intercalant and the MoSe2 semiconductor electron density, enhancing spin polarization effects. Understanding how structure and bonding in this material system change at pressure can provide crucial insight into its fundamental nature.

Here, through both experiment and first-principles calculations, we show that both the intercalation of manganese into MoSe2 and pressurization can alter the host structure and its optical phonon frequencies, giving rise to new Raman-active vibrational modes and modifying the electronic band structure. Pressure-dependent Raman scattering, investigated up to 7 GPa under hydrostatic conditions in a diamond anvil cell, suggests the formation of pressure-induced bonding between selenium and the manganese intercalant. First-principles calculations exhibit Raman shifts in agreement with experiments and shed light on the changes of the magnetic and vibrational properties of intercalated MoSe2 with a low Mn content as a function of pressure. In addition, electronic structure calculations provide predictions for the structural and electronic properties of MoSe2 intercalated with higher amounts of Mn. Spin-polarized band structure calculations unravel the conditions at which Mn-intercalated MoSe2 can sustain significant spin currents, making it a suitable dilute magnetic semiconductor.

Molybdenum diselenide (MoSe2) was prepared by deposition from the as-delivered powder containing large single-crystal platelets onto fused silica substrates followed by drop-casting ethanol onto the substrate to adhere the layered material and prevent loss in solution during intercalation. MoSe2 single-crystal platelets were on the order of 1 μm–100 μm with varied thicknesses ranging from tens of nanometers to micrometers.

Zero-valent manganese was intercalated through the decomposition of dimanganese decacarbonyl (C10O10Mn2) in dilute acetone under inert atmosphere.13 This route was shown to successfully intercalate manganese into hosts. For completeness, the MoSe2 coated substrates were placed in a 25ml–50 ml round bottom flask with a reflux condenser attached to a Schlenk line, evacuated, and flushed with N2 gas. Extra-dry acetone (5 ml) was added to the flask and heated to 48 °C. A 10 mM solution of the carbonyl in 5 ml of acetone was added to the flask dropwise over the course of 1.5 h and kept at 48 °C for an additional ∼1 h. Substrates were then removed from the solution and rinsed with acetone. All chemicals and powders were obtained from Sigma-Aldrich.

High pressures were generated using an Alamax EasyLab Mini-Bragg diamond anvil cell (DAC) with Boehler anvils with 0.6 mm culets. Spring steel gaskets were pre-indented to 80 μm–100 μm and drilled with a 250 μm hole. Ruby spheres (Alamax) were used as a pressure calibrant. In the DAC, a solution of 4:1 v/v methanol:ethanol was used as a pressure transmitting fluid. Pressures up to 7 GPa were measured to avoid all phase transitions and to remain at relatively hydrostatic pressures of the pressure transmitting fluid. Single-crystal platelets were identified optically.

Raman spectra and ruby fluorescence were measured using a home-built system with a λ = 532 nm Coherent Sapphire operating with <1 mW on the sample, a Leica DMi8 inverted microscope, a Princeton Instruments Isoplane SC320, and a Princeton Instruments Pixis CCD camera. For Raman spectroscopy, a Semrock laser-edge filter and dichroic with an edge cutoff of ∼38 cm−1 was used with the same apparatus. Acquisition times were on the order of 5 s–15 s with 10–20 averaged spectra. All spectra were acquired using an 1800 groove/mm grating. Scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX) were acquired on 5–10 isolated flakes with a FEI SCIOS Dual-Beam FIB/SEM using an Oxford EDX detector with 10 keV–20 keV accelerating voltage. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were acquired using a JEOL 2100Fac operating at 200 keV. X-ray diffraction (XRD) data were acquired using a Bruker D8 Advance Eco with copper k-edge (λ = 1.54 Å) x rays. Rietveld refinement was performed with GSAS to determine lattice constants.34 

Density functional theory (DFT) calculations were performed within the local spin density approximation (LSDA) of the exchange and correlation functional35 by using the Quantum-Espresso package.36 Core electrons are treated implicitly through projector augmented wave (PAW) pseudopotentials,37,38 and the valence electronic wavefunctions are expanded in a plane-wave basis set with a kinetic energy cutoff of 49 Ry. The charge density is integrated on 16 × 16 × 4 Monkhorst–Pack meshes of k-points for pristine MoSe2. Structural and cell relaxations are performed using a quasi-Newton optimization algorithm with a convergence criterion of 10−4 Ry/bohr for the maximum residual force component. The strong correlation effect of transition metal d-electrons is considered using the LSDA+U method39,40 by introducing a Hubbard type interaction. We use a moderate Ueff = 4 eV for both Mo and Mn.41–45 Other Ueff values, for example, 2 eV and 6 eV, were tested, and consistent results were obtained. With LSDA+U, the calculated bandgap of pure MoSe2 increases from 0.7 eV to 0.8 eV with respect to LSDA results. The frequencies of the phonon modes at the Brillouin zone center (Raman shift) as a function of pressure were calculated using density-functional perturbation theory (DFPT).46 The threshold for the iterative calculation of the perturbed Kohn–Sham wavefunctions was set to 10−16 Ry. This approach is well suited to predict Raman shift in monolayer and few-layer transition metal dichalcogenides (TMDs) upon strain47,48 as well as in bulk.49 

To model Mn-intercalation of MoSe2 with different Mn concentrations and to account for the possible structural changes induced by intercalation, we considered both 2H and 1T′ MoSe2 initial structures,50 with Mn atom(s) intercalated in the vdW gap (details are provided in Table S1 of the supplementary material). The total and spin-polarized carrier concentrations were calculated from DFT spin-polarized Kohn–Sham states integrating the first Brillouin zone on the uniform k-point meshes at least 27 times denser than those k-point meshes used in the self-consistent calculations using the BoltzTrap software.51 

The 2H stable phase of MoSe2 has a hexagonal crystal structure [Fig. 1(a); space group: P63/mmc]. The overall host structure is maintained with intercalation as confirmed through SAED [Fig. 1(b)] and XRD [Fig. 1(c)], which show that Mn–MoSe2 is also hexagonal. An example SEM image with the EDX elemental mapping of Mn-intercalated MoSe2 [Fig. 1(d)] shows that the intercalant is distributed throughout the plates at concentrations of ∼1 at. % to 2 at. % (Mn0.02MoSe2). Nevertheless, XRD would not be suitable to detect local lattice distortions at the intercalation sites.

FIG. 1.

(a) Schematic crystal structure of Mn–MoSe2. Mn likely occupies octahedral sites. (b) SAED of Mn–MoSe2 confirms the hexagonal structure with intercalation. (c) XRD shows the expansion of the MoSe2 host with Mn-intercalation. (d) SEM–EDX of Mn–MoSe2 shows Mn throughout.

FIG. 1.

(a) Schematic crystal structure of Mn–MoSe2. Mn likely occupies octahedral sites. (b) SAED of Mn–MoSe2 confirms the hexagonal structure with intercalation. (c) XRD shows the expansion of the MoSe2 host with Mn-intercalation. (d) SEM–EDX of Mn–MoSe2 shows Mn throughout.

Close modal

The intercalation energy per Mn atom, estimated by total energy calculations, depends on the Mn concentration, and it is defined as

Eintercalation=EMnxMoSe2EMoSe2naEMnna,
(1)

where EMnxMoSe2 is the total energy of a MoSe2 supercell intercalated with an atomic concentration x of Mn, EMoSe2 is the total energy of a 2H-MoSe2 supercell with the same number of formula units as the intercalated system, EMn is the energy per atom of bulk bcc Mn, and na is the number of Mn atoms intercalated. The intercalation energy per Mn atom decreases with increasing Mn concentration from 2.39 eV/atom at 3% Mn to 1.96 eV/atom at 25% Mn. This trend and the values are similar to copper and silver intercalation in MoS2.52 Although the intercalation energy per Mn atom decreases slightly with increasing Mn concentration, the total intercalation energy per MoSe2 unit actually increases as a function of the Mn content from 0.149 eV at 3% Mn to 0.980 eV per unit cell at 25% Mn, indicating that intercalation becomes energetically less favorable the larger the Mn concentration.53 

In the hypothetical case of x = 1, we find that the most stable phase of Mn1MoSe2 would be a 1T′ with AA stacking (see Fig. S2 of the supplementary material for the phonon dispersion curves—the absence of the imaginary frequency throughout the Brillouin zone indicates the structural stability), as opposed to the AB stacking of the 2H phase. This intercalation-induced structural transition is analogous to that observed in other metal-intercalated transition metal dichalcogenides, e.g., Li:MoS2.54,55 For x = 0.25, the transition to 1T′ lowers the total energy by 0.178 eV per MoSe2 formula unit. Conversely, at lower Mn concentrations, for example, x = 0.125, the 2H phase has lower energy than the 1T′ phase by 0.103 eV per MoSe2 formula unit (formation energies per MoSe2 formula unit are summarized in Table S1 of the supplementary material). These calculations predict the 1T′ phase to be stable at higher Mn concentrations (x ≥ 0.25) and the 2H phase to remain stable at low Mn concentrations (x ≤ 0.125). Under ambient conditions, the system would then stay in the 2H phase upon intercalation at the low Mn concentrations of the experiment.

Successful intercalation of manganese is confirmed by XRD, which shows an expansion of the host lattice constants and the unit cell volume [Table I; Fig. 1(c)]. This expansion is associated with the insertion of atoms into the van der Waals gap.13,56 Expansion of the unit cell volume is measurable, with an almost 3% increase even at low Mn concentrations. The volume change calculated using DFT is similar to ∼1%–6% expansion, depending upon the Mn-intercalation concentration. Experiments with very low Mn concentrations show a mild ∼0.05 Å expansion of the in-plane lattice parameter (a) upon intercalation and a ∼0.02 Å expansion of the cross-plane lattice parameter (c). DFT calculations for low concentrations of Mn in the 2H phase would predict an expansion of the c-axis only, while a expands at higher concentrations of Mn. In contrast, if intercalation is accompanied by a transition to the 1T′ structure, a expands and c contracts. While a complete transition to 1T′ cannot occur at such low Mn concentrations, we argue that the a expansion observed in experiments stems from local distortions around the intercalation sites, which also disrupt the long range crystalline order of the system, as suggested by the disappearance of the high order peaks in XRD [Fig. 1(c)].

TABLE I.

Equilibrium lattice parameters of pristine MoSe2 and Mn-intercalated MoSe2 from experiments and DFT calculations. In parentheses, it is indicated whether Mn is interstitial within the MoSe2 layer or in the vdW gap between two layers.

a (Å)c (Å)Vol. (Å3)
MoSe2 (Expt.) 3.285(3) 12.921(3) 120.8(2) 
2H-MoSe2 (DFT) 3.260 12.718 117.0 
Mn0.02MoSe2 (Expt.) 3.336(5) (+1.5%) 12.940(5) 124.7(3) 
2H-Mn0.03MoSe2 (DFT, vdW gap) 3.260 12.801 117.9 
2H-Mn0.06MoSe2 (DFT, vdW gap) 3.261 12.879 118.6 
2H-Mn0.125MoSe2 (DFT, vdW gap) 3.261 13.065 120.3 
2H-Mn0.25MoSe2 (DFT, vdW gap) 3.273 13.423 124.3 
1T′-Mn0.06MoSe2 (DFT, vdW gap) 3.392 12.227 118.9 
1T′-Mn0.25MoSe2 (DFT, vdW gap) 3.428 12.021 121.0 
1T′-Mn1MoSe2 (DFT, vdW gap) 3.501 12.094 124.9 
2H-Mn0.03MoSe2 (DFT, interstitial) 3.283 12.707 118.6 
2H-Mn0.06MoSe2 (DFT, interstitial) 3.299 12.710 119.9 
2H-Mn0.125MoSe2 (DFT, interstitial) 3.353 12.728 123.9 
a (Å)c (Å)Vol. (Å3)
MoSe2 (Expt.) 3.285(3) 12.921(3) 120.8(2) 
2H-MoSe2 (DFT) 3.260 12.718 117.0 
Mn0.02MoSe2 (Expt.) 3.336(5) (+1.5%) 12.940(5) 124.7(3) 
2H-Mn0.03MoSe2 (DFT, vdW gap) 3.260 12.801 117.9 
2H-Mn0.06MoSe2 (DFT, vdW gap) 3.261 12.879 118.6 
2H-Mn0.125MoSe2 (DFT, vdW gap) 3.261 13.065 120.3 
2H-Mn0.25MoSe2 (DFT, vdW gap) 3.273 13.423 124.3 
1T′-Mn0.06MoSe2 (DFT, vdW gap) 3.392 12.227 118.9 
1T′-Mn0.25MoSe2 (DFT, vdW gap) 3.428 12.021 121.0 
1T′-Mn1MoSe2 (DFT, vdW gap) 3.501 12.094 124.9 
2H-Mn0.03MoSe2 (DFT, interstitial) 3.283 12.707 118.6 
2H-Mn0.06MoSe2 (DFT, interstitial) 3.299 12.710 119.9 
2H-Mn0.125MoSe2 (DFT, interstitial) 3.353 12.728 123.9 

Raman spectra of MoSe2 and Mn–MoSe2 as a function of pressure are presented in Fig. 2. The observed in-plane modes are E1g2 at 168 cm−1 and E2g1 at 286 cm−1. The only Raman active out-of-plane mode is the A1g mode, which is initially at 242 cm−1.30 A peak at 354 cm−1 is seen at pressures above 2.89 GPa and can be assigned as A2u.26 It is an infrared active phonon that is forbidden in Raman scattering,57 as confirmed by DFT calculations. Previous studies observed this peak at higher excitation energies under ambient conditions and reason that resonance effects allow this peak to be observable.26,58 This mode has also been observed at higher pressures.31,32 Intercalation of manganese and application of pressure may result in symmetry breaking, allowing the forbidden mode to appear in Raman spectra. Symmetry breaking may also be responsible for the appearance of the E2g1 mode at higher pressures. With increasing pressure, both modes increase in scattering intensity.30–32 Symmetry breaking with pressure is common and expected. These additional peaks may have been observed in these high pressure studies over other investigations because of the high resolution grating used.

FIG. 2.

(a) Raman spectra under pressure of pristine MoSe2 and (b) Mn-intercalated MoSe2 (black and dark blue lines for increasing pressure, gray and cyan lines for decreasing pressure). The asterisk, described in the text, coincides with the Raman frequency of a Mn–Se bond. (c) Force vectors for the mode at ∼250 cm−1 in 1T′-Mn0.06MoSe2 from DFT calculations, showing a collective optical mode that involves displacements of Mn, Mo, and Se atoms, possibly corresponding to the * Mn–Se peak in the experiment. (d) Force vectors for the Mn–Se collective mode at ∼250 cm−1 in 2H-Mn0.06MoSe2 with Mn at the interstitial site from DFT calculations, which might also correspond to the * Mn–Se peak in the experiment.

FIG. 2.

(a) Raman spectra under pressure of pristine MoSe2 and (b) Mn-intercalated MoSe2 (black and dark blue lines for increasing pressure, gray and cyan lines for decreasing pressure). The asterisk, described in the text, coincides with the Raman frequency of a Mn–Se bond. (c) Force vectors for the mode at ∼250 cm−1 in 1T′-Mn0.06MoSe2 from DFT calculations, showing a collective optical mode that involves displacements of Mn, Mo, and Se atoms, possibly corresponding to the * Mn–Se peak in the experiment. (d) Force vectors for the Mn–Se collective mode at ∼250 cm−1 in 2H-Mn0.06MoSe2 with Mn at the interstitial site from DFT calculations, which might also correspond to the * Mn–Se peak in the experiment.

Close modal

An additional feature not observed in MoSe2 appears post decompression at ∼250 cm−1 in Mn–MoSe2 [labeled * in Fig. 2(a)]. The frequency of this mode matches a calculated longitudinal optical phonon mode in wurtzite MnSe, suggesting bonding between the Mn intercalant and the host with released pressure.59 Through DFT calculations, we found that this mode is related to the formation of a bond between the host structure and the intercalant guest. This may, however, happen in different ways: either through a transition to the metastable 1T′ structure or through the interstitial embedding of Mn into a MoSe2 layer, which was recently proposed in MoSe2 monolayers on the basis of DFT calculations.60 

In fact, the 250 cm−1 peak appears in the calculation of 1T′ Mn-intercalated MoSe2 at any given concentration of the intercalant from 0.06 to 0.25. The frequency corresponds to a collective optical mode that involves displacements of Mn, Mo, and Se atoms [Fig. 2(c) and Figs. S3 and S4]. Since Mn atoms into interstitial sites might be another possible explanation for the onset of the new mode observed in the experiment, we computed the phonon frequencies of Mn–MoSe2 with Mn at the interstitial site at 12.5%, 6%, and 3% Mn concentrations. With Mn at the interstitial site, the calculated in-plane lattice expansion rates are 2.9%, 1.2%, and 0.7% for Mn concentrations at 12.5%, 6%, and 3%, respectively (see Table I for the optimized lattice parameters). The calculated in-plane expansion rates for Mn at interstitial are in good agreement with the in-plane lattice expansion rate 1.5% measured in the experiment at low Mn concentrations of 1%–2%. However, the frequency of the Mn–Se collective mode is sensitive to the Mn concentration. At 12.5% of Mn, the calculated Mn–Se collective mode appears at ∼240 cm−1 (see Fig. S7), which is lower than the new mode observed at 250 cm−1 in the experiment. At 6% Mn concentration, the calculated Mn–Se collective mode also appears at around 250 cm−1, in very good agreement with the measured value [see Fig. 2(d) and Fig. S8]. However, at 3% Mn concentration, the Mn–Se collective vibrational mode blueshifts to around 256 cm−1 (see Fig. S9 of the supplementary material). Given the linear shift of the frequency of the Mn–Se mode with the Mn concentration, one could not conclude that the new mode at 250 cm−1 observed in the experiment is solely due to the interstitial trapping of Mn. Since the 250 cm−1 peak appears in the calculation of 1T′ Mn-intercalated MoSe2 at any given concentration, we would conjecture that both (i) Mn bound in the vdW gap and (ii) Mn bound in interstitials are possible. However, it remains open whether Mn binds more in the vdW gap or in interstitials and there is no simple experimental route to directly address this hypothesis. These results suggest that Mn-intercalation combined with compression and decompression processes may provide possible new routes to Mn interstitial doping of layered materials.60,61

Figure 3 shows the measured [Fig. 3(b)] and calculated [Fig. 3(c)] vibrational frequencies for each Raman mode as a function of pressure. Figure 3(a) shows the schematic of each vibrational mode. In Fig. 3(c), the calculated Raman shifts are plotted alongside the linear fit of the experimental shifts in solid black and blue lines for MoSe2 and Mn–MoSe2, respectively. DFT calculated Raman shifts are in very good agreement with the experimental results. Although experimental data do not show significant Raman shifts upon Mn-intercalation with low concentrations of the Mn intercalant of ∼1 at. % to at. %, DFT calculations suggest that the frequency of the Raman shift would decrease with higher Mn concentrations at ambient and relatively low pressure (see Fig. 3, and Fig. S1 of the supplementary material).

FIG. 3.

(a) Mode schematics. (b) Experimentally determined Raman shifts for pristine MoSe2 and Mn0.02MoSe2 under pressure (black and dark blue full circles for increasing pressure, gray and cyan full circles for decreasing pressure). For pristine MoSe2, A2u, A2uE2g2, and E2g1 modes appear after 2.89 GPa as indicated with a red vertical dashed line. (c) DFT calculated Raman shifts for pristine MoSe2 and Mn0.06MoSe2. The linear fit of the experimental shifts are solid lines for reference.

FIG. 3.

(a) Mode schematics. (b) Experimentally determined Raman shifts for pristine MoSe2 and Mn0.02MoSe2 under pressure (black and dark blue full circles for increasing pressure, gray and cyan full circles for decreasing pressure). For pristine MoSe2, A2u, A2uE2g2, and E2g1 modes appear after 2.89 GPa as indicated with a red vertical dashed line. (c) DFT calculated Raman shifts for pristine MoSe2 and Mn0.06MoSe2. The linear fit of the experimental shifts are solid lines for reference.

Close modal

Table II provides the initial Raman frequency, ωo, at ambient pressure and the change in frequency with pressure (dωdP). Despite a detectable change in the host unit cell volume (Table I), with low concentrations of Mn intercalant of ∼1 at. % to 2 at. %, the experimentally measured Raman frequency shift of each mode in MoSe2 does not change significantly upon Mn-intercalation (Table II). This is not unusual. Experimentally, shifts of the Raman modes with intercalation are complex.62 Optical phonons can exhibit stiffening, softening, or no change with intercalation affected by the acceptor or donor nature of the intercalant as well as the associated change in the host volume with intercalation.62 

TABLE II.

Ambient pressure Raman frequencies (cm−1), Raman shift pressure-derivative, dωdP (cm1GPa), and isothermal mode Grüneisen (γT) for the observed modes calculated from Eq. (2).

E1gA1g1
ωo (cm−1)dωdPcm1GPaγTωo (cm−1)dωdPcm1GPaγT
MoSe2 (Expt.) 167.8(6) 1.68(8) 0.46(2) 240.6(6) 2.9(1) 0.54(2) 
MoSe2 (DFT) 171.02 1.15(2) 0.307(5) 246.24 2.24(4) 0.416(7) 
Mn0.02MoSe2 (Expt.) 167.6(6) 1.67(6) 0.46(2) 241.3(6) 2.67(4) 0.506(8) 
Mn0.06MoSe2 (DFT) 168.20 0.96(7) 0.261(2) 244.44 2.6(4) 0.486(3) 
Mn0.125MoSe2 (DFT) 165.52 0.90(3) 0.248(8) 241.45 1.88(5) 0.55(1) 
Mn0.25MoSe2 (DFT) 158.16 0.5(2) 0.144(1) 207.21 2.1(2) 0.463(3) 
 E2g1 A2u 
 ωo (cm−1dωdPcm1GPa γT ωo (cm−1dωdPcm1GPa γT 
MoSe2 (Expt.) 286(1) 1.3(2) 0.21(3) 354.2(2) 1.5(4) 0.20(5) 
MoSe2 (DFT) 289 1.08 0.171(1) 352.8 1.31 0.215(1) 
Mn0.02MoSe2 (Expt.) 285.2(5) 1.5(1) 0.23(2) 351.5(5) 2.0(1) 0.26(1) 
Mn0.06MoSe2 (DFT) 285.48 1.8(1) 0.288(2) 342.45 1.45(3) 0.194(1) 
Mn0.125MoSe2 (DFT) 279 1.29 0.240(2) 331 0.899 0.1200(8) 
Mn0.25MoSe2 (DFT) 269.76 1.4(2) 0.237(2) 316.01 0.1(4) 0.0145(1) 
E1gA1g1
ωo (cm−1)dωdPcm1GPaγTωo (cm−1)dωdPcm1GPaγT
MoSe2 (Expt.) 167.8(6) 1.68(8) 0.46(2) 240.6(6) 2.9(1) 0.54(2) 
MoSe2 (DFT) 171.02 1.15(2) 0.307(5) 246.24 2.24(4) 0.416(7) 
Mn0.02MoSe2 (Expt.) 167.6(6) 1.67(6) 0.46(2) 241.3(6) 2.67(4) 0.506(8) 
Mn0.06MoSe2 (DFT) 168.20 0.96(7) 0.261(2) 244.44 2.6(4) 0.486(3) 
Mn0.125MoSe2 (DFT) 165.52 0.90(3) 0.248(8) 241.45 1.88(5) 0.55(1) 
Mn0.25MoSe2 (DFT) 158.16 0.5(2) 0.144(1) 207.21 2.1(2) 0.463(3) 
 E2g1 A2u 
 ωo (cm−1dωdPcm1GPa γT ωo (cm−1dωdPcm1GPa γT 
MoSe2 (Expt.) 286(1) 1.3(2) 0.21(3) 354.2(2) 1.5(4) 0.20(5) 
MoSe2 (DFT) 289 1.08 0.171(1) 352.8 1.31 0.215(1) 
Mn0.02MoSe2 (Expt.) 285.2(5) 1.5(1) 0.23(2) 351.5(5) 2.0(1) 0.26(1) 
Mn0.06MoSe2 (DFT) 285.48 1.8(1) 0.288(2) 342.45 1.45(3) 0.194(1) 
Mn0.125MoSe2 (DFT) 279 1.29 0.240(2) 331 0.899 0.1200(8) 
Mn0.25MoSe2 (DFT) 269.76 1.4(2) 0.237(2) 316.01 0.1(4) 0.0145(1) 

From the experimental data, the frequency of the Raman shift of each mode tends to decrease slightly, except for the A1g mode. Although the frequency of the Raman shift of the A1g mode seems to increase by about 0.7 cm−1 (Table II), the measurement error is comparable to the size of the change. From DFT calculations, with higher Mn concentrations, it looks more likely that the frequency of the Raman shift of each mode would decrease upon Mn-intercalation at ambient and relatively low pressure (see Fig. 3, and Fig. S1 of the supplementary material). The experimental Raman shift of Mn–MoSe2 and MoSe2 modes shows a very similar linear pressure-dependent slope (dωdP) (Fig. 3, Table II). For pristine MoSe2, the E2g1 and A2u modes do not show up until a pressure of ∼2.89 GPa and remain with decompression.57,63 An anomalous mode shows up around 320 cm−1. DFT calculations suggest that this might be ascribed to the A2uE2g2 combination band. This mode does not appear in MoSe2 until about ∼2.89 GPa, as indicated by a red dashed line in Fig. 3. This mode persists with decreasing pressure, which is consistent with the appearance of the A2u mode. It occurs in Mn–MoSe2 at around ∼1 GPa. This peak shows no change with pressure as Raman shifts of both the E2g2 and the A2u increase. Thus, there is no significant pressure-derivative of this mode. All modes show phonon stiffening, increasing linearly with pressure, except the overtone mode A2uE2g2 at around 320 cm−1, as discussed above.

Compressibility of MoSe2 and MnMoSe2 can be described using the isothermal mode Grüneisen parameter (γT),

γT=dlnωdlnVT=BTω0dωdPT,
(2)

where BT is the isothermal bulk modulus. Using the third-order Birch–Murnaghan equation of state to fit in situ high-pressure MoSe2 x-ray diffraction data, Aksoy et al.64 calculated BT as 45.7 ± 0.3 GPa. DFT calculations here find a bulk modulus of 47.9 GPa for MoSe2, close to experiments,64 and 51.3 GPa for Mn0.125MoSe2. Mn-intercalation should yield a notable decrease in the isothermal compressibility. With pressure, the empty van der Waals gap should compress first. By adding more atoms to the gap, Mn-intercalation subsumes space otherwise available for compression. Thus, intercalation makes the material less compressible. Using these values of BT, along with the relevant values of ω0 and dωdP, the mode Grüneisen is calculated from Eq. (2) for all modes (Table II). The calculated isothermal mode Grüneisen parameters in most cases exhibit similar trends as experiments, however with large uncertainties, mostly due to the shortcoming related to the approximated density functional. However, the overall agreement between Raman measurements and DFT calculations suggests that the adopted level of theory accounts well for the charge redistribution upon intercalation, and DFT calculations can be used to predict the electronic structure of Mn-intercalated MoSe2.

Figure 4 shows the electronic band structures of pristine MoSe2, Mn0.03MoSe2, and Mn0.125MoSe2 at 0 GPa and ∼7 GPa. For pristine MoSe2, with the increase in pressure, the bandgap narrows from 0.8 eV at 0 GPa to 0.4 eV at 6.66 GPa [Fig. 4(d)]. Upon intercalation of Mn, the overall host structure is retained [Figs. 4(b) and 4(c)]. At higher concentrations of the Mn intercalant [Mn0.125MoSe2, Fig. 4(c)], the Fermi level lies deeper in the conduction band. While pressure tends to close the gap between the valence and conduction bands also in intercalated systems, the position of the Fermi level and the carrier concentration are determined by doping and do not change significantly upon compression [Fig. 5(a)]. The transition to the 1T′ phase would lead to the metallization of the system, with a substantial change of the band structures and spin-polarized density of states (see Fig. S5 of the supplementary material for the spin-polarized electronic band structure for 1T′ phase Mn-intercalated MoSe2). Conversely, when Mn gets trapped in the interstitial of the MoSe2 layer, the system remains semiconducting with a localized spin state in the gap, below the Fermi level (Fig. S10).

FIG. 4.

(a) Brillouin zone schematic of MoSe2. (b) Mn0.03MoSe2 structure and (c) Mn0.125MoSe2 structure at 0 GPa with spin density and polarization; red and blue isosurfaces represent positive and negative spin density, respectively. (d) Electronic band structure of MoSe2 at 0 GPa (top) and 6.66 GPa (bottom). The projected density of states are given for either pressure to the right of the band structure. The band structure and PDOS of (e) Mn0.03MoSe2 and (f) Mn0.125MoSe2 show that Mn-intercalation metallizes the material moving the Fermi level into the conduction band. Manganese states are spin-polarized in the conduction band, and the population of states increases with pressure as the bandgap narrows.

FIG. 4.

(a) Brillouin zone schematic of MoSe2. (b) Mn0.03MoSe2 structure and (c) Mn0.125MoSe2 structure at 0 GPa with spin density and polarization; red and blue isosurfaces represent positive and negative spin density, respectively. (d) Electronic band structure of MoSe2 at 0 GPa (top) and 6.66 GPa (bottom). The projected density of states are given for either pressure to the right of the band structure. The band structure and PDOS of (e) Mn0.03MoSe2 and (f) Mn0.125MoSe2 show that Mn-intercalation metallizes the material moving the Fermi level into the conduction band. Manganese states are spin-polarized in the conduction band, and the population of states increases with pressure as the bandgap narrows.

Close modal
FIG. 5.

(a) Spin-up and spin-down carrier concentrations as a function of Mn concentration at 0 GPa and 6.83 GPa. (b) Total spin-polarized carrier concentration (the difference between spin-up and spin-down carrier concentrations).

FIG. 5.

(a) Spin-up and spin-down carrier concentrations as a function of Mn concentration at 0 GPa and 6.83 GPa. (b) Total spin-polarized carrier concentration (the difference between spin-up and spin-down carrier concentrations).

Close modal

The calculated projected density of states (PDOS) are shown in Fig. 4 for pristine and Mn-intercalated MoSe2 at 0 GPa and at ∼7 GPa. Mn-intercalation in MoSe2 shows clear signatures of spin polarization. At low concentrations of Mn (Mn0.03MoSe2), there is clear separation of the density of spin-up and spin-down electrons, which suggests that the spin-polarized current is possible in Mn-intercalated MoSe2. With pressure, the spin separation between the spin-up and spin-down states is reduced. At 0 GPa, the overall magnetic moment of the Mn0.03MoSe2 and Mn0.125MoSe2 supercell is 5.00 μB. With pressure (at 6.83 GPa), the overall magnetic moments reduce slightly to 4.61 μB and 4.54 μB for Mn0.03MoSe2 and Mn0.125MoSe2, respectively.

The calculated total (n-type) carrier concentrations increase with increasing Mn, proportional to Mn concentration, and do not shift significantly with pressure [Fig. 5(a)]. 2D semiconductors with dilute magnetic manganese impurities should have conduction completely ruled by spin polarization.1,5 One would expect that with increasing manganese concentration and increasing pressure, the concentration of spin-polarized carriers would also change with the competing effects due to spin overlap. The spin interaction should increase as the manganese concentration increases [Fig. 4(c)], thus decreasing the spin-polarized carrier concentration. As pressure is increased, one would expect a similar effect, as the spin overlap and spin interaction also increase. Calculations of the spin-polarized carrier concentration as functions of pressure and manganese concentrations [Fig. 5(b)] reveal that the concentration of net spin-polarized carriers depends on both pressure and Mn concentrations. Small concentrations of Mn show the greatest amount of spin-polarized carriers, with 3 at. % (close to that achieved by the experiment) as the maximum. It is interesting that at a low concentration of Mn (3%), pressure reduces the spin-polarized carrier concentration, while at a higher concentration (above ∼6%), pressure increases the spin-polarized carrier concentration [Fig. 5(b)]. The local structural transition, suggested by the appearance of the 250 cm−1 peak in the Raman spectrum upon decompression, would substantially affect the electronic structure of the intercalated material and spin separation. (See Fig. S5 for the band structure and spin-polarized density of states of the 1T′ phase of Mn-intercalated MoSe2 and Fig. S6 for the spin-polarized carrier concentration. It is interesting that both 2H and 1T′ phases show similar trends in spin polarization as a function of Mn concentration, and the spin-polarized carrier concentration for the 1T′ phase of Mn-intercalated MoSe2 could reach up to ∼1 · 1020 cm−3 at around 11% of Mn concentration.) These competing effects reveal the chemical and thermodynamic tunability of MoSe2 spin-polarized carriers. The predicted concentration of spin-polarized carriers, up to ∼2 · 1020 cm−3 in the 2H phase and ∼1 · 1020 cm−3 in the 1T′ phase, could possibly be observed by Hall measurements and is significantly high to enable spintronic applications.

This work illustrates the ability to adjust the phonon frequencies and the electronic band structure with Mn-intercalation and pressure. The appearance of a new phase is found associated with the Mn guest bonding with the host MoSe2. These results suggest that intercalation systems under high pressure can lead to unique bonding environments and, thus, new materials. These findings set precedent for further investigation into Mn-intercalation of 2D layered materials as an alternative to dilute magnetic doping. The robustness of this system is demonstrated by both intercalated and non-intercalated pressure-dependent phonon frequencies. DFT calculations show that Mn-intercalation causes the Fermi level to shift into the conduction band, rendering the system an n-type semiconductor or nearly metallic. Manganese intercalated MoSe2 retains a total magnetic moment that corresponds to that of single Mn atoms. Unpaired spins contribute to the density of states near the Fermi level, thus potentially enabling spin currents. Pressure reduces the spin carrier density at low Mn concentrations, but it slightly increases it at higher Mn concentrations. The spin-polarized behavior predicted in intercalated Mn–MoSe2 here has the potential to surpass that of doped systems, with the advantage that transition metal atoms may be intercalated post-growth. These results provide insights into how concentration limitations in dilute manganese doped MoSe2 may be bypassed by exploiting the van der Waals gap of a layered material through intercalation and high pressure. We expect that resistivity studies as functions of pressure and magnetic field may further elucidate the nature of Mn spin-polarized carriers in this host.

See the supplementary material for DFT calculated Raman shifts with higher Mn concentrations shown as functions of pressure and Mn concentration, in comparison with the experimentally determined Raman shifts; detailed information for constructing Mn-intercalated MoSe2 structures for DFT calculations with various Mn concentrations in 2H and 1T′ phases with Mn in the vdW gap, as well as the 2H phase with Mn at the interstitial site of MoSe2, and the corresponding Monkhorst–Pack k-point meshes and formation energy; phonon dispersion curves for 1T′-Mn1MoSe2; force vectors of the mode at ∼250 cm−1 in 1T′-Mn0.06MoSe2 and 1T′-Mn0.25MoSe2; spin-polarized electronic band structure and PDOS of the 1T′ phase of Mn-intercalated MoSe2 at 0 GPa; total spin-polarized carrier concentration for the 1T′ phase of Mn-intercalated MoSe2 as a function of Mn concentration at 0 GPa and 6.83 GPa; force vectors of the Mn–Se collective mode in 2H-Mn0.125MoSe2, 2H-Mn0.06MoSe2, and 2H-Mn0.03MoSe2 with Mn at the intersitial site of MoSe2; and spin-polarized electronic band structure and PDOS of 2H-Mn0.03MoSe2 with Mn at the intersitial site of MoSe2.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

We thank Bryan P. Moser and Daniel R. Williams for XRD patterns and SEM images, respectively. This work was supported by the Office of Naval Research (Grant No. N00014-16-1-3161).

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Supplementary Material