Exfoliation and Raman Spectroscopic Fingerprint of Few-Layer NiPS3 Van der Waals Crystals

Abstract

The range of mechanically cleavable Van der Waals crystals covers materials with diverse physical and chemical properties. However, very few of these materials exhibit magnetism or magnetic order, and thus the provision of cleavable magnetic compounds would supply invaluable building blocks for the design of heterostructures assembled from Van der Waals crystals. Here we report the first successful isolation of monolayer and few-layer samples of the compound nickel phosphorus trisulfide (NiPS3) by mechanical exfoliation. This material belongs to the class of transition metal phosphorus trisulfides (MPS3), several of which exhibit antiferromagnetic order at low temperature, and which have not been reported in the form of ultrathin sheets so far. We establish layer numbers by optical bright field microscopy and atomic force microscopy, and perform a detailed Raman spectroscopic characterization of bilayer and thicker NiPS3 flakes. Raman spectral features are strong functions of excitation wavelength and sample thickness, highlighting the important role of interlayer coupling. Furthermore, our observations provide a spectral fingerprint for distinct layer numbers, allowing us to establish a sensitive and convenient means for layer number determination.

Figure 1. Atomic structure and optical characterization of exfoliated NiPS₃.

(a) Schematic crystal structure. View perpendicular to layers, only top layer shown (upper schematic). The atoms contained in the section shaded grey are shown in view parallel to layers (lower schematic). Unit cell (dashed outlines), covalent bonds within (P₂S₆)⁴⁻ anions (grey lines). (b) Brightfield microscope image of cleaved bulk NiPS₃ samples. (c) Image of NiPS₃ flake exfoliated onto oxidized silicon substrate, comprising mainly thick sheets (>10 layers). 60° and 120° angles indicated. (d) Image of thin exfoliated NiPS₃ sheets, 2–7 layers indicated. (e) Optical intensity contrast of ultrathin NiPS₃ sheets (2–7 layers), evaluated for red, green, and blue color channels separately, with reference to the substrate. (Photos in panels (c) and (d) were acquired at different illumination conditions to maximize visibility of relevant features).

Figure 2. AFM characterization of exfoliated NiPS₃.

(a,b) Tapping-mode AFM topography image of ultrathin NiPS₃ sheets, (a) 3–8 layers indicated, (b) 1 and 2 layers indicated. Insets: corresponding optical photographs. (c) Height profiles along the lines shown in (a,b). (d) Apparent layer heights evaluated from the AFM scans in (a,b), from 1 to 8 layers. Step heights between consecutive layers are ~0.6 nm, base height offset with respect to bare Si/SiO₂ substrate is ~0.8 nm.

Figure 3. Raman spectra of exfoliated NiPS₃.

(a) Spectra of a thick sheet  nm), acquired at excitation wavelengths and 633 nm. These spectra are indistinguishable from those reported for bulk NiPS₃. 5 in-plane and 3 out-of-plane phonon modes are indicated. The Raman peak of silicon visible at 520 cm⁻¹ stems from the substrate. (b) Spectra of thin NiPS₃ sheets (2–7 layers), acquired at  nm, together with the spectrum of a thick sheet shown in panel (a), and the substrate spectrum. All spectra were acquired in a single pass, under identical experimental conditions, on the sample shown in Fig. 1d. Spectra have not been scaled; data are offset vertically for clarity. The spectral region dominated by the first-order Raman peak of the silicon substrate around 520 cm⁻¹ has been omitted. The spectral feature at ~300 cm⁻¹ is a second-order Raman peak of silicon.

Figure 4. Out-of-plane A_1g phonon modes, and analysis of corresponding Raman spectral peaks of exfoliated NiPS₃, for excitation at 633 nm.

(a) Schematic representation (top view, side view) of vibrational amplitudes of Ni₂P₂S₆ unit cell atoms in phonon mode. (b) Detailed view of Raman data in corresponding spectral range, for thin sheets (2–7 layers), thick sheet, and silicon substrate. Spectra are overlayed with Lorentzian line shape fits. Data of bilayer sample have been magnified by factor 5. Spectra are offset vertically for clarity. (c) Central frequencies determined by Lorentzian peak fits in (b). Error bars correspond to spread of experimental data. (d) Integrated line shape intensities (=peak area) determined by Lorentzian peak fits in (b). (e–h) Analogous information for phonon mode. Raman spectra of 4-layer, 7-layer and thick NiPS₃ sheets are composed of two distinct Lorentzian lines: individual Lorentzian fit curves (grey lines) and sum of fit curves (black lines). (i–l) Analogous information for phonon mode. Raman spectra of thin sheets (2–7 layers) contain two superimposed Lorentzian lines.

Figure 5. Raman spectral features of second order processes, at λ_exc = 633 nm.

(a) Detailed view of spectral range containing second-order processes. Spectra are described by superposition of up to 8 Lorentzian peak shapes: individual Lorentzian fit curves (grey lines) and sum of fit curves (black lines). Labels state wavelength shifts (in cm⁻¹) of individual peak centers. Data of bilayer sample and substrate have been magnified by factor 5. Spectra are offset vertically for clarity. (b) Spectral range containing second-order processes. Spectra are described by up to 5 Lorentzian line shapes. See the text for details.