Which chromium-sulfur compounds exist as 2D material?

TL;DR

This work establishes Cr$_{2}$S$_{3}$-2D and Cr$_{2\frac{2}{3}}S_{4}$-2D as two distinct, phase-pure chromium-sulfide monolayers grown on graphene-supported Ir substrates by molecular beam epitaxy. Using LEED, STM, and STS, the authors determine a hexagonal $a_{2D}\approx0.341$ nm lattice, with Cr$_{2}$S$_{3}$-2D at $h\approx0.79$ nm and Cr$_{2\frac{2}{3}}S_{4}$-2D at $h\approx1.02$ nm, the latter displaying a $(\sqrt{3}\times\sqrt{3})$-R$30^{\circ}$ superstructure due to a 2/3 Cr-occupancy in the middle layer. Although CrS$_{2}$-2D remains elusive under these growth conditions, first-principles calculations reveal that Cr$_{2}$S$_{3}$-2D and Cr$_{2\frac{2}{3}}S_{4}$-2D adopt NiAs-type, S-terminated stacks with significant surface relaxation; they are indirect bandgap semiconductors in the freestanding form, with $E_g$ around $0.9$–$1.0$ eV for Cr$_{2}$S$_{3}$-2D and $0.33$ eV (0 eV in the absence of $U$) for Cr$_{2\frac{2}{3}}S_{4}$-2D, increasing to ~0.62 eV at $U=2$ eV. On graphene, the bands shift toward metallic behavior due to $W_f$ differences, and both phases carry sizable magnetic moments ($M_{Cr}\approx3\mu_B$ per Cr without SOC, reduced to $1.73\mu_B$ with SOC; interlayer coupling depends on $U$). The work highlights Cr$_{2}$S$_{3}$-2D and Cr$_{2\frac{2}{3}}S_{4}$-2D as promising 2D magnetic semiconductors, with clear phase-pure growth and well-defined structural motifs, while prompting reevaluation of CrS$_{2}$ as a 2D material in this chemistry space.

Abstract

Two-dimensional (2D) chromium-sulfides are synthesized by molecular beam epitaxy using graphene as a substrate. Structure characterization by employing scanning tunneling microscopy and low energy electron diffraction indicates that there are two 2D phases, Cr$_2$S$_3$-2D and Cr$_{2\frac{2}{3}}$S$_4$-2D, which have not been reported before. Cr$_{2\frac{2}{3}}$S$_4$-2D is related to bulk Cr$_5$S$_6$, but thinner than a bulk unit cell. For Cr$_2$S$_3$-2D, an even thinner material, no bulk counterpart exists. Both 2D materials are found to be structurally stable under ambient conditions and exhibit interesting electronic properties. Extensive first-principles calculations provide further insight into the electronic structure of these systems and indicate that they should be magnetic. Although single layers of CrS$_2$ were predicted to be stable by density functional theory calculations and reported in previous experimental studies, we were unable to synthesize CrS$_2$ under our range of experimental conditions.

Figures (9)

• Figure 1: Schematic side view ball models of Cr$_x$S$_y$ materials. (a) Rhombohedral Cr$_2$S$_3$. (b) T-phase CrS$_2$. (c) Cr$_5$S$_6$. (d) Cr$_2$S$_3$-2D. (e) Cr$_{2\frac{2}{3}}$S$_4$-2D. In the ball models of bulk materials (a)-(c), a unit cell is indicated, in (d) and (e) single unit cell thick 2D materials are shown. Additional schematic top and side view ball model views visualizing the full structure are given in Figure S1 (Supporting Information).
• Figure 2: Characterization of Cr$_x$S$_y$-2D by LEED and STM. (a) Contrast-inverted 120 eV LEED pattern of Cr$_2$S$_3$-2D on Gr/Ir(110). Annealing temperature 750 K. First-order Ir and Gr reflections are encircled magenta and green, respectively. Reciprocal Ir and Gr primitive translations are indicated. Modulated diffraction ring of radius $a_\mathrm{2D}^*$ is due to Cr$_2$S$_3$-2D. (b) STM topograph corresponding to sample in (a). Inset displays atomically resolved lattice of Cr$_2$S$_3$-2D. Contrast-enhanced box makes grain boundaries (dark lines) and $\approx 3.3$ nm Gr/Ir(110) moiré periodicity and its imprint on Cr$_2$S$_3$-2D visible. The lower panel shows the height profile along the white line in the topography. (c) Contrast-inverted 120 eV LEED pattern corresponding to sample in (a) and (b), but after additional annealing to 950 K. The diffraction ring has fragmented into elongated spots. (d) STM topograph corresponding to the sample of (c). Green arrow points to Cr$_{2\frac{2}{3}}$S$_4$-2D island, which displays $(\sqrt{3} \times \sqrt{3})$-R30$^{\circ}$ superstructure as visible in atomically resolved inset. Note the presence of a single Ir(110) substrate step covered by Gr in the lower right corner. Lower panel shows height profile along the white line in the topograph. Compared to Cr$_{2}$S$_3$-2D, the apparent height of Cr$_{2\frac{2}{3}}$S$_4$-2D is larger. STM topographs are acquired with $V_\mathrm{b} = 1.0$ V and $I_\mathrm{t} = 50$ pA at 1.7 K. Insets atomically resolved STM insets obtained with (b) -750 mV, $I_\mathrm{t} = 100$ pA and (d) 50 mV and $I_\mathrm{t} = 200$ pA. STM topographs are 300 nm $\times$ 200 nm and insets are 4 nm $\times$ 4 nm.
• Figure 3: (a) STM topograph of pseudomorphic Cr islands on Ir(111) after 210 s of Cr deposition. An atomic step of Ir(111) is crossing the image. (b) Cr$_2$S$_3$-2D islands on Gr/Ir(110) grown by 210 s of Cr deposition in S background pressure and additional annealing to 750 K. The same Cr evaporator with identical settings and identical evaporator-sample geometry was used. STM images obtained at 1.7 K with (a) $V_\mathrm{b} = 1.0$ V and $I_\mathrm{t} = 20$ pA and (b) $V_\mathrm{b} = 1.0$ V and $I_\mathrm{t} = 50$ pA. Image sizes are 200 nm $\times$ 150 nm
• Figure 4: Phase-pure Cr$_2$S$_3$-2D and Cr$_{2\frac{2}{3}}$S$_4$-2D. (a) STM topograph of a coalesced layer of Cr$_2$S$_3$-2D with a coverage of 0.85 and deposition time 360 s. Lower panel displays the height profile along white line in STM topograph. (b) Constrast-inverted 130 eV LEED patterns of Cr$_2$S$_3$-2D sample. Few first-order Ir and Gr reflections are encircled magenta and green, respectively. Diffraction ring is due to randomly oriented Cr$_2$S$_3$-2D islands. (c) STM topograph of a coalesced layer of Cr$_{2\frac{2}{3}}$S$_4$-2D with a coverage of 0.92 after additional growth step on sample in (a) involving 150 s Cr deposition. Lower panel displays height profile along the white line in the STM topograph. (d) Constrast-inverted 130 eV LEED pattern of Cr$_{2\frac{2}{3}}$S$_4$-2D sample in (c). The blue arrow indicates diffraction ring resulting from the ($\sqrt3\times\sqrt3$)-R30$^{\circ}$ superstructure. See text. STM images are acquired at room temperature with $V_\mathrm{b} = 1.0$ V, $I_\mathrm{t} = 100$ pA. Image sizes are 200 nm $\times$ 150 nm.
• Figure 5: Moving of Cr$_2$S$_3$-2D islands and their air stability. (a) Cr$_2$S$_3$-2D island on Gr/Ir(111). (b) After interaction with STM tip (see debris at upper tip of island). Dashed blue line indicates initial position of island, as in (a). (c) Cr$_2$S$_3$-2D on Gr/Ir(110) after exposure for 3 hours to ambient conditions and subsequent pump-down without annealing. STM images (a-c) are taken at 1.7 K with $V_\mathrm{b}$ = 1 V and $I_\mathrm{t}$ = 50 pA. Image sizes of (a) and (b) are 100 nm $\times$ 100 nm, and (c) is 200 nm $\times$ 100 nm.