Versatile Method for Preparing Two-Dimensional Metal Dihalides

Abstract

Ever since the ground-breaking isolation of graphene, numerous two-dimensional (2D) materials have emerged with 2D metal dihalides gaining significant attention due to their intriguing electrical and magnetic properties. In this study, we introduce an innovative approach via anhydrous solvent-induced recrystallization of bulk powders to obtain crystals of metal dihalides (MX₂, with M = Cu, Ni, Co and X = Br, Cl, I), which can be exfoliated to 2D flakes. We demonstrate the effectiveness of our method using CuBr₂ as an example, which forms large layered crystals. We investigate the structural properties of both the bulk and 2D CuBr₂ using X-ray diffraction, along with Raman scattering and optical spectroscopy, revealing its quasi-1D chain structure, which translates to distinct emission and scattering characteristics. Furthermore, microultraviolet photoemission spectroscopy and electronic transport reveal the electronic properties of CuBr₂ flakes, including their valence band structure. We extend our methodology to other metal halides and assess the stability of the metal halide flakes in controlled environments. We show that optical contrast can be used to characterize the flake thicknesses for these materials. Our findings demonstrate the versatility and potential applications of the proposed methodology for preparing and studying 2D metal halide flakes.

Keywords: Raman scattering spectroscopy; mechanical exfoliation; metal dihalides; photoemission; solvent-assisted recrystallization; two-dimensional materials.

Figure 1

Layered structure and characterization of CuBr₂. (a) Schematic representation of layered crystal CuBr₂. The Cu atom is in cyan color, and the Br atom is in orange color. (b) Digital photograph of recrystallized CuBr₂, and (c) SEM image shows the layered nature of the crystal. (d) Optical image of CuBr₂ thin flakes. (e) AFM images from a zoomed-in region (blue dashed rectangle) on the optical images. The corresponding height profiles are taken along the black dashed line on the AFM images. (f) TEM image of the CuBr₂ flake (inset: electron diffraction pattern, see also Supporting Figure S3). (g) High-resolution TEM of the flake with an inset schematic image showing the crystal orientation. (h) EDS spectra of the CuBr₂ flake with corresponding HAADF-STEM and EDS mapping images in the insets (scale bar, 5 μm). (i) XRD patterns of bulk CuBr₂ (black) and 2D flakes of CuBr₂ (red), respectively. The magnified peaks from 35° to 55° (in the inset) from bulk CuBr₂ are absent in the 2D flakes.

Figure 2

Low-temperature optical spectroscopy of bulk CuBr₂. (a) Raman spectra obtained at T = 5 K with 2.33 eV excitation. Three distinctive features, labeled as P1, P2, and P3, can be observed with energies of 66, 112, and 182 cm^–1 respectively. (b) Polar plots of the integrated intensities of the P1, P2, and P3 phonon modes. (c). Photoluminescence spectrum of bulk CuBr₂ under 1.58 eV excitation with a femtosecond laser (75.7 MHz repetition rate). (d) Power dependence measurements of emitted light for continuous wave and pulsed femtosecond laser with excitation energy ∼1.58 eV. The y = Ax^α function has been fitted to both data sets yielding linear and superlinear dependence for continuous wave and pulsed femtosecond laser excitation, respectively. Here, α and A are constants, corresponding to the exponent of the power law and the width of the scaling relationship.

Figure 3

Photoemission spectro-microscopy of 2D CuBr₂ thin flakes. (a) Schematic diagram of the encapsulated CuBr₂ with grounding electrodes used for ultraviolet photoemission measurements. The CuBr₂ 2D flakes were encapsulated in monolayer graphene to prevent degradation as well as to provide grounding. Gold strips were deposited on top of graphene layer to further eliminate the charging issue. The incoming photon source was directed onto 2D flakes, and the analyzer collected the reflected electrons to generate the pattern. (b) Optical image of CuBr₂ encapsulated with monolayer graphene, with a zoom-in on the CuBr₂ flake shown in (c). The blue dashed line in (b) is a contour of monolayer graphene. (d) AFM micrograph of the flake in a zoomed-in region (yellow dashed rectangle in image c). (e) Photoelectron microscopy image of the CuBr₂ flakes encapsulated in graphene, recorded at a photoelectron energy of 4.5 eV. (f) Spatially resolved work function map of the same flakes. The color scale shows the extracted work function in eV. (g) Spatially resolved map of the “cutoff” energy of the secondary electron spectra (color scale in eV). (h) Secondary electron spectra of the flakes and the graphene region. The inset shows the derivative of the spectra near the secondary electron cutoff. (i) UPS of CuBr₂ flakes recorded with a photon energy of 21.2 eV (He I light source). Solid lines represent smoothed data (using a Savitzky–Golay filter), while raw data are shown as symbols (Figure S9). Scale bars in panels (c)–(g) are 1 μm.

Figure 4

Simulated band structure of bulk and 2D CuBr₂ via DFT. Electronic band structure and density of states (DOS) of (a) bulk and (b) monolayer CuBr₂. Both the band structure and DOS are orbital projected to show the contribution of atomic orbitals of Cu and Br.

Figure 5

Layered structure of CuCl₂ and its optical contrast as a function of the layer thickness of a 2D CuCl₂ flake exfoliated on a 90 nm SiO₂/Si substrate. (a) Optical image (left) and AFM image (right) from the zoomed-in region (yellow dashed rectangle) on the optical image. The corresponding height profile (below) was taken along the black dashed line in the AFM images. (b) Raman spectra of both bulk and 2D CuCl₂. High-magnification optical image of CuCl₂ (c) under white light and (d) under a light filter that blocks wavelengths below 500 nm. (e) Comparison between the experimental data of the optical contrast against the layer thickness obtained by AFM imaging; the red line shows the data fit using the Fresnel function (see Supporting Section 8). The labeled dots with i–iv in panel (e) correspond to the flakes in the optical images (c) and (d).

Figure 6

Layered structure and characterization of NiI₂. (a) XRD patterns of bulk NiI₂. Inset: schematic layered crystal structures of NiI₂. The Ni atom is in gray color, and the I atom is in purple color. (b) Raman spectra of both bulk (black color) and thin flake (red color) of NiI₂. (c) Optical (top) and AFM (bottom) images from the dashed yellow rectangle shown on the optical image. The AFM superimposed height profile was taken on the direction of the yellow line. (d) AFM images showing the hydration progression of freshly exfoliated NiI₂ flakes at 4 different humidity levels (full range of RH in Figure S16). Scale bar: 1 μm. Height profiles at (e) location '1' and (f) location '4' (as indicated in the image d) with increasing RH. (g) Flake thickness vs RH for different places of the flake as marked in (d).