Few Layer MnPS₃

February 20, 2023 – Manganese phosphorus trisulfide (MnPS₃) is a particularly interesting material in the context of magnetism in a system with reduced dimensionality. A team of researchers from Ulm University and the Helmholtz-Zentrum Dresden-Rossendorf in Germany has presented an experimental and theoretical study on modifying the properties of free-standing quasi-two-dimensional (2D) MnPS₃ by local structural transformations via electron irradiation and thermal annealing, which may open opportunities to build new devices for the storage of quantum information.

Material design is an attractive idea that is very difficult to implement in practice. Combining the best of different ingredients to create the ultimate material is a task^[1] that calls for a tool able to change the structure of a material at the atomic level^[2]. The ability to characterize and change materials atom by atom is as important for progress in nanotechnology^[3]^,[4] as precise control over their growth. Aberration-corrected (AC) high-resolution (HR) in situ transmission electron microscopy (TEM)^[5]^,[6], along with scanning transmission electron microscopy (STEM)^[7], has proven especially useful here, because it delivers information about the atomic rearrangement in a sample under electron irradiation in real time^[2]^,[8]. Depending on the chosen electron beam size, (S)TEM techniques can make structural changes locally, down to the atomic level^[9], as a result of the interaction of the highly accelerated electrons with the sample^[10]^,[11].

The modification of ultrathin nanosheets has recently attracted considerable attention, because the structure, and thus the properties, of two-dimensional (2D) materials can be altered more easily than in bulk systems owing to their geometry. In particular, the crystallographic, electronic and magnetic properties of 2D materials^[12] can be tuned by creating single^[13] and extended defects^[14]^–[16] and by adsorbing atoms^[17]. In addition, (S)TEM can be used to drive spatially confined phase transformations in crystalline materials with few layers or monolayers, thereby significantly changing their local properties. Controllable structural transformations have been reported in transition metal dichalcogenides (TMDs)^[13]^,[18]^–[20], alumina^[21], silica^[21] and nanowires^[22]^,[23]. The advantage of electron-beam-mediated modification over conventional methods such as heating^[24], electrostatic gating^[25] or strain engineering^[26] is the real-time control of the transformation process down to the atomic scale^[19]^,[22]. (S)TEM is thus an excellent tool for locally controlling the properties of 2D materials.

Within the class of 2D materials, the transition metal phosphorus trisulfides (TMPTs) have recently come into the focus of researchers, and manganese phosphorus trisulfide (MnPS₃) has received the most attention among them. It is a layered antiferromagnet with a wide band gap whose properties can be manipulated in many ways; its structure and the routes to tune it are summarized in the first box.

Ball-and-stick model of MnPS3 in side and top view and an HRTEM image along [103]

Fig. 1: Structural model of MnPS3.

▸ MnPS₃: a magnetic layered host and its tunability

The authors had found earlier that few-layer TMPTs lose predominantly sulfur through the interaction with the incident electrons, which in turn leads to strong changes in the original material^[41]. In the new study they turned to the properties of the free-standing MnPS₃/MnS systems that emerge from electron-beam-induced phase transformations and from annealing, starting with what the beam does to the lattice.

HRTEM image sequence of MnPS3 under electron irradiation with growing islands of a new phase

Fig. 2: Phase islands growing under the beam.

Fig. 2(a1–a3) shows an image sequence of 80-kV Cc/Cs-corrected HRTEM of few-layer MnPS₃ under the electron beam at a constant dose rate of 0.5 × 10⁶ e⁻/(s nm²). Fig. 2(a1) shows the pristine material in the normal incidence direction [103] of MnPS₃, with an enlarged region on the right. After irradiation, changes in the material become evident, as Fig. 2(a2–a3) shows: islands with an altered structure grow continuously within the host matrix, and their area increases linearly with the accumulated dose (Fig. 2b). As the authors' previous study had shown, the primary removal of sulfur (S) and phosphorus (P) atoms by 80-keV electrons proceeds via direct electron–nucleus collisions^[41]. This leads to an S deficiency in the exposed regions of MnPS₃ and can therefore drive the growth of new structures. Where in the stack the new phase forms is explained in the second box.

▸ Layer by layer: where the new phase grows

$Electron diffraction pattern, line profile, 3D reciprocal space reconstruction and structure models of the two new phases$
Fig. 3: 3D electron diffraction of the new phases.

Three-dimensional electron diffraction (3D-ED) was used to determine the lattice parameters of the newly formed phases^[43]. Few-layer MnPS₃ was irradiated until additional Bragg reflections of the new phases were clearly visible in the ED data (Fig. 3a,b). Two different new phases, labelled phase 1 and phase 2, appear; the reflections of phase 1 are more intense and correspond to the embedded phase seen in Fig. 2. The reconstructed diffraction volume^[43] in Fig. 3c shows an overlay of two distinct crystalline lattices, the MnPS₃ host (blue) and the new phase 1 (red). Phase 1 turned out to be a cubic, NaCl-type lattice, closely related to the mineral alabandite, α-MnS, and oriented with its [111] direction along the [103] normal of the host; the authors assign it to α-MnS_1−xP_x (Fig. 3d). Phase 2, which appears at later stages of the transformation, was identified as the hexagonal γ-MnS_1−xP_x with a wurtzite structure^[44]. The measured cell parameters and the reasoning behind the assignment are given in the third box.

▸ 3D electron diffraction: the unit cells of the new phases

The irradiated crystal was tilted in the TEM over a range of ±60° with a tilt step of 1°, a diffraction pattern was recorded at each tilt position, and the tilt series was reconstructed into a 3D reciprocal space (Fig. 3c). The extracted positions of the red reflections align well with a single lattice, which suggests that all inclusions have a similar orientation within the matrix. This new lattice is cubic with a = 2.47 Å, b = 2.56 Å, c = 2.67 Å, α = 90.1°, β = 89.4° and γ = 90.3°, and its lattice parameters correspond to the Mn–S bond length in MnPS₃ of 2.61 Å. Phase 1 is therefore a NaCl-type, F-centred lattice with a cell parameter of 5.21 Å. The most stable compound in the Mn–P–S system with this structure type is alabandite, α-MnS, with a lattice constant of 5.22 Å^[45], space group Fm-3m and bivalent Mn. Because the composition of the new phase may deviate from strict MnS stoichiometry, it is assigned to α-MnS_1−xP_x. The peak at 1.84 Å in Fig. 3b can then be attributed to its 002 reflection, and the lattice is aligned with the [111] direction along the [103] reflection of the host, exactly the orientation seen in the HRTEM experiments of Fig. 2.

3D-ED cannot give the exact S:P ratio of the new phase or the Mn valence. A spatially resolved EELS analysis, however, showed that predominantly S is present and that Mn is predominantly 2+, so the combined experiments point to Mn^II,IIIS_1−xP_x with the NaCl structure, a lattice parameter of 5.21 Å, a high S and a low P concentration.

The weak reflections of phase 2 appeared later, as small arcs and sometimes as parts of diffraction rings, which the authors attribute to a partial loss of its orientation relationship with the host. The search for a phase candidate yielded the hexagonal, wurtzite-type γ-MnS_1−xP_x^[44]. Its characterization was complicated by the presence of the two other phases, but the reflection positions match the data well: the peak at 1.98 Å in Fig. 3b, for example, corresponds to the 2-1-10 reflection of γ-MnS_1−xP_x, and ultrathin patches of phase 2 were also found in HRTEM images at a later transformation stage.

HRTEM images of three orientations of MnS1−xPx with simulations and convex-hull plots

Fig. 4: HRTEM of MnS1−xPx crystals and convex hulls.

The experiments thus indicate that at the beginning of the irradiation the electron beam transforms the host into cubic α-MnS_1−xP_x oriented with its [111] direction parallel to the normal incidence [103] of the MnPS₃ matrix. With increasing imaging time, that is, a larger accumulated dose, free-standing patches of the newly formed phases could be imaged after the host material had completely degraded. Fig. 4a shows an experimental HRTEM image of a freshly grown, isolated α-MnS_1−xP_x crystal, with a magnified section in the red frame; an overlay of the simulated image with the atomic model of the cubic phase along [111] highlights the atomic column contrast, and the lattice parameters agree with those found by 3D-ED (Fig. 3). At this stage, different orientations of α-MnS_1−xP_x occur as free-standing structures as well: Fig. 4b shows a patch viewed along [001], with a magnified portion in the green outlined area, in good agreement with the simulated HRTEM image for this orientation. Fig. 4c shows a region of γ-MnS_1−xP_x in the [0001] direction, magnified in blue, and again the observed contrast matches the corresponding image simulation.

Are such phosphorus-alloyed MnS phases stable at all? The energetics of the α- and γ-MnS_1−xP_x phases formed at room temperature were studied with first-principles calculations, using the convex hull of the formation enthalpy, a standard approach in materials science to evaluate the relative stability of materials with different stoichiometries. The result, plotted in the right-hand panels of Fig. 4, is that several mixed compounds lie on or within a few meV of the hull, so that the mixing entropy stabilizes them at room temperature; the details are given in the fourth box.

▸ Energetics of MnS_1−xP_x: convex hull, mixing entropy and the simulations behind the images

$HAADF, EDX maps, EDX spectrum, EELS, diffraction profile and HRTEM of annealed MnPS3$
Fig. 5: Analytical TEM of annealed MnPS3.

Heat offers a second route to the same destination. Pristine MnPS₃ flakes of different thicknesses were heated in situ in steps of about 10 °C, and a structural transformation was observed at 490 °C. Afterwards the sample looked completely different: no large MnPS₃ flakes remained, but clustered polycrystalline nanocrystals had emerged, which indicates a complete structural transformation. To analyze the composition of a structure annealed at 600 °C, STEM HAADF images and EDX elemental maps were acquired at 80 kV from different sample positions (Fig. 5a); the maps of Mn and S prove the presence of these elements, and the EDX spectrum and quantification in Fig. 5b show mainly Mn and S, with small amounts of C, O and residual P. Core-loss EELS of the Mn L_2,3 edge (Fig. 5c) gave a white-line ratio^[47] of L₃ to L₂ of 4.50, which indicates that the structure contains mainly Mn²⁺^[41]^,[47]. Electron diffraction of the annealed sample, azimuthally integrated in Fig. 5d, agrees well with a simulated powder pattern of alabandite, the face-centred cubic Mn²⁺S²⁻, and HRTEM confirmed it: a typical grain (Fig. 5e,f) shows a cubic lattice along [100] with atomic column distances of 2.60 and 2.58 Å, in good agreement with the expected 2.62 Å^[45] for alabandite projected along [100]. Protocol, contaminants and the diffraction numbers are collected in the fifth box.

▸ Annealing in numbers: protocol, contaminants and alabandite

Specimens for annealing were prepared by stamping a PMMA-supported flake onto a microelectromechanical (MEMS) heating chip and removing the polymer in acetone and water. The transition temperature was measured directly in the TEM holder under vacuum (7 × 10⁻⁹ bar): the temperature was ramped to 250 °C and then raised stepwise by about 10 °C until the phase transformation occurred, and in a second step to 900 °C in steps of about 50 °C. For the structural investigation the sample was heated to 600 °C for 40 min under vacuum (1 × 10⁻⁵ bar) and analyzed afterwards at room temperature. The flakes themselves were obtained by polymer-assisted mechanical exfoliation of bulk crystals, pressed onto a PVA-coated Si substrate and transferred with distilled water to a Quantifoil grid.

The EDX quantification (Brown–Powell model) in Fig. 5b indicates carbon contamination as well as residual oxidized Mn, P and S, and core-loss EELS showed that oxidized phosphorus forms next to the pristine alabandite. In the integrated diffraction pattern of Fig. 5d, an additional peak at 0.45 Å⁻¹, corresponding to 2.2 Å, is probably related to graphitic impurities on the surface. The FFT of the HRTEM image (Fig. 5g) gives reciprocal distances of 0.38, 0.54 and 0.77 Å⁻¹, in good agreement with the azimuthally integrated pattern. At higher annealing temperatures the polycrystalline clusters reassemble into larger alabandite nanocrystals, which are stable as well. The authors concluded that annealing transforms MnPS₃ completely into alabandite, with graphitic contamination at the surface and oxidized residuals.

Instruments: HRTEM with a Cs-corrected FEI Titan 80-300 and the Cc/Cs-corrected SALVE microscope (20–80 kV); EDX with a Talos F200X G2 STEM; electron diffraction with the Titan at 300 kV. The EELS energy resolution was 0.6 eV, backgrounds were treated with a power-law model, and crystal thicknesses were obtained from EELS by the log-ratio method.

Projected density of states of phosphorus-alloyed MnS monolayers and a charge density plot

Fig. 6: Phosphorus states in γ-MnS.

What are the new MnS polymorphs good for? The researchers investigated their electronic and magnetic properties with DFT calculations and determined the energetically preferred magnetic order for single- and bilayer systems. In the α-MnS phase, the [001] facet is more stable in the antiferromagnetic (AFM) than in the ferromagnetic (FM) state, while the [111] facet prefers to remain ferromagnetic; in the γ-MnS phase, the thickness of the slab changes the order from ferromagnetic to antiferromagnetic. The electronic structure of bulk MnS is significantly changed in the new polymorphs: for the bulk, DFT+U gives a band gap of 1.43 eV between the valence band maximum at the M point and the conduction band minimum at the Γ point, whereas the slabs can be semiconducting or metallic depending on the orientation and thickness of the facets. Because the PBE functional generally underestimates band gaps (see, for example, Ref.^[48] and references therein), the team also computed them with DFT+U and the Heyd–Scuseria–Ernzerhof (HSE) hybrid functional; the values, and the effect of phosphorus alloying (Fig. 6), are given in the sixth box.

▸ Band gaps of the MnS facets and the effect of phosphorus

"Our in situ Cc/Cs-corrected HRTEM experiments provided insights into the annealing and electron-beam-induced structure transformations of MnPS₃," the authors write. The phase transformations can be controlled locally by the size of the electron beam and by the total electron dose, and mapped on the atomic scale at the same time: suitable dose rates allow the controlled growth of α-MnS-type phases embedded in the host, with the crystals oriented along the direction of the parent MnPS₃, while in situ annealing under vacuum converts the whole sample into pure α-MnS. For the MnS structures produced in this way, the ab initio calculations showed that the electronic and magnetic properties depend strongly on the in-plane orientation and the thickness of the crystallites: an α-[001] MnS monolayer is an antiferromagnetic semiconductor with an indirect band gap, a γ-[0001] MnS monolayer a ferromagnetic semiconductor with a direct gap, and the confinement effects in the γ-[0001] facets are strong enough to switch the gap from direct to indirect. Alloying with phosphorus (MnS_1−xP_x, 0 ≤ x < 1) proved to be a powerful way to tailor the electronic and optical properties of the facets, and the mixed phases of both α- and γ-type can be stable for certain phosphorus concentrations.

The broader message of the study is that a few-layer antiferromagnetic host can be transformed on the nanoscale into other stable phases, down to ultrathin films, either wholesale by heat or locally by a focused electron beam, and that the products come with a palette of magnetic and electronic behaviours that depend on how they are cut. "Our electron beam irradiation and annealing experiments show the possibility of a nanoscale transformation of a few-layer antiferromagnetic host into other stable phases, even down to ultrathin films," the authors conclude. The combination of aberration-corrected low-voltage HRTEM, 3D electron diffraction and ab initio theory that made this possible gives a route to writing magnetic and semiconducting MnS patterns into an insulating MnPS₃ sheet, one layer at a time.

Resource:
Storm, A., Köster, J., Ghorbani-Asl, M., Kretschmer, S., Gorelik, T. E., Kinyanjui, M. K., Krasheninnikov, A. V., Kaiser, U. (2023).
Electron-Beam- and Thermal-Annealing-Induced Structural Transformations in Few-Layer MnPS₃.
ACS Nano 17(5), 4250–4260.
https://doi.org/10.1021/acsnano.2c05895

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Few Layer MnPS3

Few Layer MnPS₃