Edge phonons in layered orthorhombic GeS and GeSe monochalcogenides

Germanium sulfide (GeS) and germanium selenide (GeSe) are layered orthorhombic crystals whose structure bears a strong resemblance with that of black phosphorus and, additionally, are expected to exhibit high piezoelectricity in the few layer domain. In this work, we investigate the Raman properties of exfoliated GeS and GeSe and show that their edges exhibit unusual polarized Raman features that were first observed in black phosphorus. The results include the activation in the spectra of otherwise not allowed modes at the edges of the sample, depending on the crystallographic direction of the edge and the polarization configuration used in the measurements. These features are attributed to atomic rearrangements at the crystal terminations, as well as their impact on phonon symmetries, similar to the case of black phosphorus. Our conclusions are further corroborated by using density functional theory and suggest that edge rearrangements, which will have an impact on the mechanical, electronic, and chemical properties of devices, is a general phenomenon of orthorhombic layered structures.

Figure 1

GeS and GeSe crystal structure from (a) top and (b) lateral views. (c) Typical unpolarized Raman spectra of GeS and GeSe, with the mode symmetries indicated.

Figure 2

Optical microscopy images of the measured (a) GeS and (b) GeSe flakes. The crystallographic axes are indicated in white. The red squares indicate the regions investigated via hyperspectral Raman imaging, while the yellow lines are the directions and regions where line scans were performed. (c)–(f) Polarized Raman spectra obtained with the indicated polarization configurations at the center (bottom) of the flake, and at the armchair (middle) and the zigzag (top) edges. The mode symmetries for each peak are indicated.

Figure 3

Raman line scan images obtained with the GeS flake at the (a)–(d) armchair and (e)–(h) zigzag edges, using four different polarization configurations [from top to bottom c(a,a)$\overline{\mathrm{c}}$, c(b,b)$\overline{\mathrm{c}}$, c(a,b)$\overline{\mathrm{c}}$, and c(b,a)$\overline{\mathrm{c}}$ configurations, respectively]. Yellow dashed lines delimit the flake edges. The color scale represents the spectral intensities, which were normalized by the highest value in each image. White dashed lines delimit spectral regions in which the intensity was multiplied by a constant factor (indicated next to the dashed lines) for better visualization. The mode symmetries for each peak are indicated on top of the figure.

Figure 4

Raman line scan images obtained with the GeSe flake at the (a)–(d) armchair and (e)–(h) zigzag edges, using the four different polarization configurations (from top to bottom c(a,a)$\overline{\mathrm{c}}$, c(b,b)$\overline{\mathrm{c}}$, c(a,b)$\overline{\mathrm{c}}$, and c(b,a)$\overline{\mathrm{c}}$ configurations, respectively). Yellow dashed lines delimit the flake edges. The color scale represents the spectral intensities, which were normalized by the highest value in each image. White dashed lines delimit spectral regions in which the intensity was multiplied by a factor (indicated next to the dashed lines) for better visualization. The mode symmetries for each peak are indicated on top of the figure.

Figure 5

Hyperspectral Raman images of selected modes and polarization configurations, taken within the red squares from Fig. 2 for the (a)–(d) GeS and (e)–(f) GeSe flakes. The intensities in all images are in logarithmic color scales.

Figure 6

Ball-and-stick representation of GeS edges. (a)–(d) GeS with unpassivated dangling bonds. (e)–(h) GeS with H-passivated dangling bonds. (a) Armchair edges; (b) S-terminated zigzag edges; (c) Ge-terminated zigzag edges; (d) alternating Ge-S zigzag edges; (e) H-passivated armchair edges; (f) SH-terminated zigzag edges; (g) GeH-terminated zigzag edges; (h) GeHSH-terminated zigzag edges. For (b)–(d), interlayer covalent bonds are shown in the shaded gray areas. Color scheme: Ge (green), S (yellow), and H (black).